Cognitive Neuroscience Review

Dynamic sensitivity of area V4 neurons during saccade preparation.

2009-07-24T18:41:00.000+04:30

Han X, Xian SX, Moore T.
Proc Natl Acad Sci U S A. 2009 Jul 21.
http://www.pnas.org/content/early/2009/07/20/0902412106.long

During the preparation of saccadic eye movements, visual attention is confined to the target of intended fixation and there is a corresponding diminution of visual sensitivity at nontarget locations. Neurons within the macaque visual cortex exhibit correlates of these perceptual changes, such as in area V4, where neuronal responses are enhanced during the preparation of saccades to stimuli within the receptive field (RF), and responses are suppressed during the preparation of saccades to other locations. Both the perceptual and neurophysiological effects suggest that the sensitivity of visual cortical neurons to input is dynamic during saccade preparation. We probed the contrast sensitivity of area V4 neurons to nontarget stimuli at varying times during the preparation of saccades to locations outside of the neuron's receptive field. We found that the contrast sensitivity of many neurons is profoundly altered within 50 ms of saccade onset. The luminance or color contrast sensitivity of individual V4 neurons could increase, decrease, or remain unchanged before saccade onset. For luminance contrast sensitivity, decreases in sensitivity were more frequent and larger in magnitude, resulting in an overall decrement in sensitivity across the population. For color contrast, the effects were smaller and more heterogeneous, resulting in little or no overall change in sensitivity across the population. Our results demonstrate the dynamic influence that saccade preparation has on the sensitivity of visual cortical neurons and suggest a basis for the changes in perception known to occur during saccade preparation.

PMID: 19622736

"Referred visual sensations": rapid perceptual elongation after visual cortical deprivation.

2009-07-19T09:42:00.000+04:30

Dilks DD, Baker CI, Liu Y, Kanwisher N.
J Neurosci. 2009 Jul 15;29(28):8960-4.
http://www.jneurosci.org/cgi/content/full/29/28/8960

Visual perceptual distortion (i.e., elongation) has been demonstrated in a single case study after several months of cortical deprivation after a stroke. Here we asked whether similar perceptual elongation can be observed in healthy participants after deprivation and, crucially, how soon after deprivation this elongation occurs. To answer this question, we patched one eye, thus noninvasively and reversibly depriving bottom-up input to the region of primary visual cortex (V1) corresponding to the blind spot (BS) in the unpatched eye, and tested whether and how quickly elongation occurs after the onset of deprivation. Within seconds of eye patching, participants perceived rectangles adjacent to the BS to be elongated toward the BS. We attribute this perceptual elongation to rapid receptive field expansion within the deprived V1 as reported in electrophysiological studies after retinal lesions and refer to it as "referred visual sensations" (RVS). This RVS is too fast to be the result of structural changes in the cortex (e.g., the growth of new connections), instead implicating unmasking of preexisting connections as the underlying neural mechanism. These findings may shed light on other reported perceptual distortions, as well as the phenomena of "filling-in."

PMID: 19605633

Millisecond-timescale optical control of neural dynamics in the nonhuman primate brain.

2009-07-16T21:00:00.000+04:30

Han X, Qian X, Bernstein JG, Zhou HH, Franzesi GT, Stern P, Bronson RT, Graybiel AM, Desimone R, Boyden ES.
Neuron. 2009 Apr 30;62(2):191-8.
http://www.cell.com/neuron/retrieve/pii/S0896627309002104

To understand how brain states and behaviors are generated by neural circuits, it would be useful to be able to perturb precisely the activity of specific cell types and pathways in the nonhuman primate nervous system. We used lentivirus to target the light-activated cation channel channelrhodopsin-2 (ChR2) specifically to excitatory neurons of the macaque frontal cortex. Using a laser-coupled optical fiber in conjunction with a recording microelectrode, we showed that activation of excitatory neurons resulted in well-timed excitatory and suppressive influences on neocortical neural networks. ChR2 was safely expressed, and could mediate optical neuromodulation, in primate neocortex over many months. These findings highlight a methodology for investigating the causal role of specific cell types in nonhuman primate neural computation, cognition, and behavior, and open up the possibility of a new generation of ultraprecise neurological and psychiatric therapeutics via cell-type-specific optical neural control prosthetics.

PMID: 19409264

The effect of microsaccades on the correlation between neural activity and behavior in middle temporal, ventral intraparietal, and lateral intraparietal areas.

2009-07-16T20:54:00.000+04:30

Herrington TM, Masse NY, Hachmeh KJ, Smith JE, Assad JA, Cook EP.
J Neurosci. 2009 May 6;29(18):5793-805.
http://www.jneurosci.org/cgi/content/full/29/18/5793

It is widely reported that the activity of single neurons in visual cortex is correlated with the perceptual decision of the subject. The strength of this correlation has implications for the neuronal populations generating the percepts. Here we asked whether microsaccades, which are small, involuntary eye movements, contribute to the correlation between neural activity and behavior. We analyzed data from three different visual detection experiments, with neural recordings from the middle temporal (MT), lateral intraparietal (LIP), and ventral intraparietal (VIP) areas. All three experiments used random dot motion stimuli, with the animals required to detect a transient or sustained change in the speed or strength of motion. We found that microsaccades suppressed neural activity and inhibited detection of the motion stimulus, contributing to the correlation between neural activity and detection behavior. Microsaccades accounted for as much as 19% of the correlation for area MT, 21% for area LIP, and 17% for VIP. While microsaccades only explain part of the correlation between neural activity and behavior, their effect has implications when considering the neuronal populations underlying perceptual decisions.

PMID: 19420247

Central V4 receptive fields are scaled by the V1 cortical magnification and correspond to a constant-sized sampling of the V1 surface.

2009-07-16T20:48:00.000+04:30

Motter BC.
J Neurosci. 2009 May 6;29(18):5749-57.
http://www.jneurosci.org/cgi/content/full/29/18/5749

The mapping of the topographic representation of the visual field onto cortical areas changes throughout the hierarchy of cortical visual areas. The changes are believed to reflect the establishment of modules with different spatial processing emphasis. The receptive fields (RFs) of neurons within these modules, however, may not be governed by the same spatial topographic map parameters. Here it is shown that the RFs of area V4 neurons (centered 1-12 degrees in eccentricity) are based on a circularly symmetric sampling of the primary visual cortical retinotopic map. No eccentricity dependent magnification beyond that observed in V1 is apparent in the V4 neurons. The size and shape of V4 RFs can be explained by a simple, constant sized, two-dimensional Gaussian sample of visual input from the retinotopic map laid out across the surface of V1. Inferences about the spatial scale of interactions within the receptive fields of neurons cannot be based on a visual area's apparent cortical magnification derived from topographic mapping.

PMID: 19420243

Representation of confidence associated with a decision by neurons in the parietal cortex.

2009-07-16T20:34:00.000+04:30

Kiani R, Shadlen MN.
Science. 2009 May 8;324(5928):759-64.
http://www.sciencemag.org/cgi/content/full/324/5928/759

The degree of confidence in a decision provides a graded and probabilistic assessment of expected outcome. Although neural mechanisms of perceptual decisions have been studied extensively in primates, little is known about the mechanisms underlying choice certainty. We have shown that the same neurons that represent formation of a decision encode certainty about the decision. Rhesus monkeys made decisions about the direction of moving random dots, spanning a range of difficulties. They were rewarded for correct decisions. On some trials, after viewing the stimulus, the monkeys could opt out of the direction decision for a small but certain reward. Monkeys exercised this option in a manner that revealed their degree of certainty. Neurons in parietal cortex represented formation of the direction decision and the degree of certainty underlying the decision to opt out.

PMID: 19423820

A rodent model for the study of invariant visual object recognition.

2009-07-16T20:29:00.000+04:30

Zoccolan D, Oertelt N, DiCarlo JJ, Cox DD.
Proc Natl Acad Sci U S A. 2009 May 26;106(21):8748-53.
http://www.pnas.org/content/106/21/8748.long

The human visual system is able to recognize objects despite tremendous variation in their appearance on the retina resulting from variation in view, size, lighting, etc. This ability--known as "invariant" object recognition--is central to visual perception, yet its computational underpinnings are poorly understood. Traditionally, nonhuman primates have been the animal model-of-choice for investigating the neuronal substrates of invariant recognition, because their visual systems closely mirror our own. Meanwhile, simpler and more accessible animal models such as rodents have been largely overlooked as possible models of higher-level visual functions, because their brains are often assumed to lack advanced visual processing machinery. As a result, little is known about rodents' ability to process complex visual stimuli in the face of real-world image variation. In the present work, we show that rats possess more advanced visual abilities than previously appreciated. Specifically, we trained pigmented rats to perform a visual task that required them to recognize objects despite substantial variation in their appearance, due to changes in size, view, and lighting. Critically, rats were able to spontaneously generalize to previously unseen transformations of learned objects. These results provide the first systematic evidence for invariant object recognition in rats and argue for an increased focus on rodents as models for studying high-level visual processing.

PMID: 19429704

Dissociable Neural Effects of Long-term Stimulus-Reward Pairing in Macaque Visual Cortex.

2009-07-16T20:26:00.000+04:30

Frankó E, Seitz AR, Vogels R.
J Cogn Neurosci. 2009 Jul 6
http://www.mitpressjournals.org/doi/pdfplus/10.1162/jocn.2009.21288

It has been proposed that perceptual learning may occur through a reinforcement process, in which consistently pairing stimuli with reward is sufficient for learning. We tested whether stimulus-reward pairing is sufficient to increase the sensorial representation of a stimulus by recording local field potentials (LFPs) in macaque extrastriate area V4 with chronically implanted electrodes. Two oriented gratings were repeatedly presented; one was paired with a fluid reward, whereas no reward was given at any other time. During the course of conditioning the LFP increased for the rewarded compared to the unrewarded orientation. The time course of the effect of stimulus-reward pairing and its reversal differed between an early and late interval of the LFP response: a fast change in the later part of the neural response that was dissociated from a slower change in the early part of the response. The fast change of the late interval LFP suggests that this late LFP change is related to enhanced attention during the presentation of the rewarded stimulus. The slower time course of the early interval response suggests an effect of sensorial learning. Thus, simple stimulus-reward pairing is sufficient to strengthen stimulus representations in visual cortex and does this by means of two dissociable mechanisms.

PMID: 19580385

Dissociable perceptual effects of visual adaptation.

2009-07-16T20:23:00.000+04:30

Müller KM, Schillinger F, Do DH, Leopold DA.
PLoS One. 2009 Jul 10;4(7):e6183.
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0006183

Neurons in the visual cortex are responsive to the presentation of oriented and curved line segments, which are thought to act as primitives for the visual processing of shapes and objects. Prolonged adaptation to such stimuli gives rise to two related perceptual effects: a slow change in the appearance of the adapting stimulus (perceptual drift), and the distortion of subsequently presented test stimuli (adaptational aftereffects). Here we used a psychophysical nulling technique to dissociate and quantify these two classical observations in order to examine their underlying mechanisms and their relationship to one another. In agreement with previous work, we found that during adaptation horizontal and vertical straight lines serve as attractors for perceived orientation and curvature. However, the rate of perceptual drift for different stimuli was not predictive of the corresponding aftereffect magnitudes, indicating that the two perceptual effects are governed by distinct neural processes. Finally, the rate of perceptual drift for curved line segments did not depend on the spatial scale of the stimulus, suggesting that its mechanisms lie outside strictly retinotopic processing stages. These findings provide new evidence that the visual system relies on statistically salient intrinsic reference stimuli for the processing of visual patterns, and point to perceptual drift as an experimental window for studying the mechanisms of visual perception.

PMID: 19593384

Relating neuronal firing patterns to functional differentiation of cerebral cortex.

2009-07-16T20:18:00.000+04:30

Shinomoto S, Kim H, Shimokawa T, Matsuno N, Funahashi S, Shima K, Fujita I, Tamura H, Doi T, Kawano K, Inaba N, Fukushima K, Kurkin S, Kurata K, Taira M, Tsutsui K, Komatsu H, Ogawa T, Koida K, Tanji J, Toyama K.
PLoS Comput Biol. 2009 Jul;5(7):e1000433.
http://www.ploscompbiol.org/article/info%3Adoi%2F10.1371%2Fjournal.pcbi.1000433

It has been empirically established that the cerebral cortical areas defined by Brodmann one hundred years ago solely on the basis of cellular organization are closely correlated to their function, such as sensation, association, and motion. Cytoarchitectonically distinct cortical areas have different densities and types of neurons. Thus, signaling patterns may also vary among cytoarchitectonically unique cortical areas. To examine how neuronal signaling patterns are related to innate cortical functions, we detected intrinsic features of cortical firing by devising a metric that efficiently isolates non-Poisson irregular characteristics, independent of spike rate fluctuations that are caused extrinsically by ever-changing behavioral conditions. Using the new metric, we analyzed spike trains from over 1,000 neurons in 15 cortical areas sampled by eight independent neurophysiological laboratories. Analysis of firing-pattern dissimilarities across cortical areas revealed a gradient of firing regularity that corresponded closely to the functional category of the cortical area; neuronal spiking patterns are regular in motor areas, random in the visual areas, and bursty in the prefrontal area. Thus, signaling patterns may play an important role in function-specific cerebral cortical computation.

PMID: 19593378

Decoding reveals the contents of visual working memory in early visual areas

2009-07-08T11:04:00.000+04:30

Harrison SA, Tong F.
Nature. 2009 Apr 2;458(7238):632-5.
http://www.nature.com/nature/journal/v458/n7238/full/nature07832.html

Visual working memory provides an essential link between perception and higher cognitive functions, allowing for the active maintenance of information about stimuli no longer in view. Research suggests that sustained activity in higher-order prefrontal, parietal, inferotemporal and lateral occipital areas supports visual maintenance, and may account for the limited capacity of working memory to hold up to 3-4 items. Because higher-order areas lack the visual selectivity of early sensory areas, it has remained unclear how observers can remember specific visual features, such as the precise orientation of a grating, with minimal decay in performance over delays of many seconds. One proposal is that sensory areas serve to maintain fine-tuned feature information, but early visual areas show little to no sustained activity over prolonged delays. Here we show that orientations held in working memory can be decoded from activity patterns in the human visual cortex, even when overall levels of activity are low. Using functional magnetic resonance imaging and pattern classification methods, we found that activity patterns in visual areas V1-V4 could predict which of two oriented gratings was held in memory with mean accuracy levels upwards of 80%, even in participants whose activity fell to baseline levels after a prolonged delay. These orientation-selective activity patterns were sustained throughout the delay period, evident in individual visual areas, and similar to the responses evoked by unattended, task-irrelevant gratings. Our results demonstrate that early visual areas can retain specific information about visual features held in working memory, over periods of many seconds when no physical stimulus is present.

PMID: 19225460

Comparing face patch systems in macaques and humans.

2009-07-08T10:50:00.002+04:30

Tsao DY, Moeller S, Freiwald WA.
Proc Natl Acad Sci U S A. 2008 Dec 9;105(49):19514-9
http://www.pnas.org/content/105/49/19514.long

Face recognition is of central importance for primate social behavior. In both humans and macaques, the visual analysis of faces is supported by a set of specialized face areas. The precise organization of these areas and the correspondence between individual macaque and human face-selective areas are debated. Here, we examined the organization of face-selective regions across the temporal lobe in a large number of macaque and human subjects. Macaques showed 6 regions of face-selective cortex arranged in a stereotypical pattern along the temporal lobe. Human subjects showed, in addition to 3 reported face areas (the occipital, fusiform, and superior temporal sulcus face areas), a face-selective area located anterior to the fusiform face area, in the anterior collateral sulcus. These results suggest a closer anatomical correspondence between macaque and human face-processing systems than previously realized.

PMID: 19033466

Linearly additive shape and color signals in monkey inferotemporal cortex

2009-07-08T10:50:00.000+04:30

McMahon DB, Olson CR.
J Neurophysiol. 2009 Apr;101(4):1867-75.
http://jn.physiology.org/cgi/content/full/101/4/1867

How does the brain represent a red circle? One possibility is that there is a specialized and possibly time-consuming process whereby the attributes of shape and color, carried by separate populations of neurons in low-order visual cortex, are bound together into a unitary neural representation. Another possibility is that neurons in high-order visual cortex are selective, by virtue of their bottom-up input from low-order visual areas, for particular conjunctions of shape and color. A third possibility is that they simply sum shape and color signals linearly. We tested these ideas by measuring the responses of inferotemporal cortex neurons to sets of stimuli in which two attributes-shape and color-varied independently. We find that a few neurons exhibit conjunction selectivity but that in most neurons the influences of shape and color sum linearly. Contrary to the idea of conjunction coding, few neurons respond selectively to a particular combination of shape and color. Contrary to the idea that binding requires time, conjunction signals, when present, occur as early as feature signals. We argue that neither conjunction selectivity nor a specialized feature binding process is necessary for the effective representation of shape-color combinations.

PMID: 19144745

Visually driven activation in macaque areas V2 and V3 without input from the primary visual cortex.

2009-07-08T10:22:00.000+04:30

Schmid MC, Panagiotaropoulos T, Augath MA, Logothetis NK, Smirnakis SM.
PLoS One. 2009;4(5):e5527. Epub 2009 May 13.Click here to read
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0005527

Creating focal lesions in primary visual cortex (V1) provides an opportunity to study the role of extra-geniculo-striate pathways for activating extrastriate visual cortex. Previous studies have shown that more than 95% of neurons in macaque area V2 and V3 stop firing after reversibly cooling V1. However, no studies on long term recovery in areas V2, V3 following permanent V1 lesions have been reported in the macaque. Here we use macaque fMRI to study area V2, V3 activity patterns from 1 to 22 months after lesioning area V1. We find that visually driven BOLD responses persist inside the V1-lesion projection zones (LPZ) of areas V2 and V3, but are reduced in strength by approximately 70%, on average, compared to pre-lesion levels. Monitoring the LPZ activity over time starting one month following the V1 lesion did not reveal systematic changes in BOLD signal amplitude. Surprisingly, the retinotopic organization inside the LPZ of areas V2, V3 remained similar to that of the non-lesioned hemisphere, suggesting that LPZ activation in V2, V3 is not the result of input arising from nearby (non-lesioned) V1 cortex. Electrophysiology recordings of multi-unit activity corroborated the BOLD observations: visually driven multi-unit responses could be elicited inside the V2 LPZ, even when the visual stimulus was entirely contained within the scotoma induced by the V1 lesion. Restricting the stimulus to the intact visual hemi-field produced no significant BOLD modulation inside the V2, V3 LPZs. We conclude that the observed activity patterns are largely mediated by parallel, V1-bypassing, subcortical pathways that can activate areas V2 and V3 in the absence of V1 input. Such pathways may contribute to the behavioral phenomenon of blindsight.

PMID: 19436733

What response properties do individual neurons need to underlie position and clutter "invariant" object recognition?

2009-07-08T10:18:00.000+04:30

Li N, Cox DD, Zoccolan D, Dicarlo JJ.
J Neurophysiol. 2009 Jul;102(1):360-76.
http://jn.physiology.org/cgi/content/full/102/1/360

Primates can easily identify visual objects over large changes in retinal position-a property commonly referred to as position "invariance." This ability is widely assumed to depend on neurons in inferior temporal cortex (IT) that can respond selectively to isolated visual objects over similarly large ranges of retinal position. However, in the real world, objects rarely appear in isolation, and the interplay between position invariance and the representation of multiple objects (i.e., clutter) remains unresolved. At the heart of this issue is the intuition that the representations of nearby objects can interfere with one another and that the large receptive fields needed for position invariance can exacerbate this problem by increasing the range over which interference acts. Indeed, most IT neurons' responses are strongly affected by the presence of clutter. While external mechanisms (such as attention) are often invoked as a way out of the problem, we show (using recorded neuronal data and simulations) that the intrinsic properties of IT population responses, by themselves, can support object recognition in the face of limited clutter. Furthermore, we carried out extensive simulations of hypothetical neuronal populations to identify the essential individual-neuron ingredients of a good population representation. These simulations show that the crucial neuronal property to support recognition in clutter is not preservation of response magnitude, but preservation of each neuron's rank-order object preference under identity-preserving image transformations (e.g., clutter). Because IT neuronal responses often exhibit that response property, while neurons in earlier visual areas (e.g., V1) do not, we suggest that preserving the rank-order object preference regardless of clutter, rather than the response magnitude, more precisely describes the goal of individual neurons at the top of the ventral visual stream.

PMID: 19439676

Cortical Connections to Area TE in Monkey: Hybrid Modular and Distributed Organization

2009-07-08T10:12:00.000+04:30

Borra E, Ichinohe N, Sato T, Tanifuji M, Rockland KS.
Cereb Cortex. 2009 May 23
http://cercor.oxfordjournals.org/cgi/content/full/bhp096v2

To investigate the fine anatomical organization of cortical inputs to visual association area TE, 2-3 small injections of retrograde tracers were made in macaque monkeys. Injections were made as a terminal procedure, after optical imaging and electrophysiological recording, and targeted to patches physiologically identified as object-selective. Retrogradely labeled neurons occurred in several unimodal visual areas, the superior temporal sulcus, intraparietal sulcus (IPS), and prefrontal cortex (PFC), consistent with previous studies. Despite the small injection size (<0.5 mm wide), the projection foci in visual areas, but not in IPS or PFC, were spatially widespread (4-6 mm in extent), and predominantly consisted of neurons labeled by only one of the injections. This can be seen as a quasi-modular organization. In addition, within each projection focus, there were scattered neurons projecting to one of the other injections, together with some double-labeled (DL) neurons, in a more distributed pattern. Finally, projection foci included smaller "hotspots," consisting of intermixed neurons, single-labeled by the different injections, and DL neurons. DL neurons are likely the result of axons having extended, spatially separated terminal arbors, as demonstrated by anterograde experiments. These results suggest a complex, hybrid connectivity architecture, with both modular and distributed components.

PMID: 19443621

Beyond Poisson: increased spike-time regularity across primate parietal cortex

2009-07-08T10:09:00.000+04:30

Maimon G, Assad JA.
Neuron. 2009 May 14;62(3):426-40
http://www.cell.com/neuron/retrieve/pii/S0896627309002463

Cortical areas differ in their patterns of connectivity, cellular composition, and functional architecture. Spike trains, on the other hand, are commonly assumed to follow similarly irregular dynamics across neocortex. We examined spike-time statistics in four parietal areas using a method that accounts for nonstationarities in firing rate. We found that, whereas neurons in visual areas fire irregularly, many cells in association and motor-like parietal regions show increasingly regular spike trains by comparison. Regularity was evident both in the shape of interspike interval distributions and in spike-count variability across trials. Thus, Poisson-like randomness is not a universal feature of neocortex. Rather, many parietal cells have reduced trial-to-trial variability in spike counts that could provide for more reliable firing-rate signals. These results suggest that spiking dynamics may play different roles in different cortical areas and should not be assumed to arise from fundamentally irreducible noise sources.

PMID: 19447097

Can monkeys choose optimally when faced with noisy stimuli and unequal rewards?

2009-07-08T10:04:00.001+04:30

Samuel Feng, Philip Holmes, Alan Rorie, William T. Newsome
PLoS Computational Biology 5(2):e1000284doi:10.1371/journal.pcbi.1000284
http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000284

We review the leaky competing accumulator model for two-alternative forced-choice decisions with cued responses, and propose extensions to account for the influence of unequal rewards. Assuming that stimulus information is integrated until the cue to respond arrives and that firing rates of stimulus-selective neurons remain well within physiological bounds, the model reduces to an Ornstein-Uhlenbeck (OU) process that yields explicit expressions for the psychometric function that describes accuracy. From these we compute strategies that optimize the rewards expected over blocks of trials administered with mixed difficulty and reward contingencies. The psychometric function is characterized by two parameters: its midpoint slope, which quantifies a subject’s ability to extract signal from noise, and its shift, which measures the bias applied to account for unequal rewards. We fit these to data from two monkeys performing the moving dots task with mixed coherences and reward schedules. We find that their behaviors averaged over multiple sessions are close to optimal, with shifts erring in the direction of smaller penalties. We propose two methods for biasing the OU process to produce such shifts.

Estimates of the contribution of single neurons to perception depend on timescale and noise correlation

2009-07-08T10:01:00.000+04:30

Cohen MR, Newsome WT.
J Neurosci. 2009 May 20;29(20):6635-48
http://www.jneurosci.org/cgi/content/full/29/20/6635

The sensitivity of a population of neurons, and therefore the amount of sensory information available to an animal, is limited by the sensitivity of single neurons in the population and by noise correlation between neurons. For decades, therefore, neurophysiologists have devised increasingly clever and rigorous ways to measure these critical variables (Parker and Newsome, 1998). Previous studies examining the relationship between the responses of single middle temporal (MT) neurons and direction-discrimination performance uncovered an apparent paradox. Sensitivity measurements from single neurons suggested that small numbers of neurons may account for a monkey's psychophysical performance (Britten et al., 1992), but trial-to-trial variability in activity of single MT neurons are only weakly correlated with the monkey's behavior, suggesting that the monkey's decision must be based on the responses of many neurons (Britten et al., 1996). We suggest that the resolution to this paradox lies (1) in the long stimulus duration used in the original studies, which led to an overestimate of neural sensitivity relative to psychophysical sensitivity, and (2) mistaken assumptions (because no data were available) about the level of noise correlation in MT columns with opposite preferred directions. We therefore made new physiological and psychophysical measurements in a reaction time version of the direction-discrimination task that matches neural measurements to the actual decision time of the animals. These new data, considered together with our recent data on noise correlation in MT (Cohen and Newsome, 2008), provide a substantially improved account of psychometric performance in the direction-discrimination task.

PMID: 19458234

Chronic electrical stimulation of the contralesional lateral cerebellar nucleus enhances recovery of motor function after cerebral ischemia in rats

2009-07-05T15:33:00.000+04:30

Machado AG, Baker KB, Schuster D, Butler RS, Rezai A.
Brain Res. 2009 Jul 14;1280:107-16.
Science Direct

Novel neurorehabilitative strategies are needed to improve motor outcomes following stroke. Based on the disynaptic excitatory projections of the dentatothalamocortical pathway to the motor cortex as well as to anterior and posterior cortical areas, we hypothesize that chronic electrical stimulation of the contralesional dentate (lateral cerebellar) nucleus output can enhance motor recovery after ischemia via augmentation of perilesional cortical excitability. Seventy-five Wistar rats were pre-trained in the Montoya staircase task and subsequently underwent left cerebral ischemia with the 3-vessel occlusion model. All survivors underwent stereotactic right lateral cerebellar nucleus (LCN) implantation of bipolar electrodes. Rats were then randomized to 4 groups: LCN stimulation at 10 pps, 20 pps, 50 pps or sham stimulation, which was delivered for a period of 6 weeks. Performance on the Montoya staircase task was re-assessed over the last 4 weeks of the stimulation period. On the right (contralesional) side, motor performance of the groups undergoing sham, 10 pps, 20 pps and 50 pps stimulation was, respectively, 2.5+/-2.7; 2.1+/-2.5; 6.0+/-3.9 (p<0.01) and 4.5+/-3.5 pellets. There was no difference on the left (ipsilesional) side motor performance among the sham or stimulation groups, varying from 15.9+/-6.7 to 17.2+/-2.1 pellets. We conclude that contralesional chronic electrical stimulation of the lateral cerebellar nucleus at 20 pps but not at 10 or 50 pps improves motor recovery in rats following ischemic strokes. This effect is likely to be mediated by increased perilesional cortical excitability via chronic activation of the dentatothalamocortical pathway.

PMID: 19445910

Neural activity in the visual thalamus reflects perceptual suppression

2009-07-05T15:27:00.000+04:30

Wilke M, Mueller KM, Leopold DA
Proc Natl Acad Sci U S A. 2009 Jun 9;106(23):9465-70
http://www.pnas.org/content/106/23/9465.long

To examine the role of the visual thalamus in perception, we recorded neural activity in the lateral geniculate nucleus (LGN) and pulvinar of 2 macaque monkeys during a visual illusion that induced the intermittent perceptual suppression of a bright luminance patch. Neural responses were sorted on the basis of the trial-to-trial visibility of the stimulus, as reported by the animals. We found that neurons in the dorsal and ventral pulvinar, but not the LGN, showed changes in spiking rate according to stimulus visibility. Passive viewing control sessions showed such modulation to be independent of the monkeys' active report. Perceptual suppression was also accompanied by a marked drop in low-frequency power (9-30 Hz) of the local field potential (LFP) throughout the visual thalamus, but this modulation was not observed during passive viewing. Our findings demonstrate that visual responses of pulvinar neurons reflect the perceptual awareness of a stimulus, while those of LGN neurons do not.

PMID: 19458249

Essential role for a long-term depression mechanism in ocular dominance plasticity.

2009-07-05T15:23:00.000+04:30

Yoon BJ, Smith GB, Heynen AJ, Neve RL, Bear MF.
Proc Natl Acad Sci U S A. 2009 Jun 16;106(24):9860-5. Epub 2009 May 22
http://www.pnas.org/content/106/24/9860.long

The classic example of experience-dependent cortical plasticity is the ocular dominance (OD) shift in visual cortex after monocular deprivation (MD). The experimental model of homosynaptic long-term depression (LTD) was originally introduced to study the mechanisms that could account for deprivation-induced loss of visual responsiveness. One established LTD mechanism is a loss of sensitivity to the neurotransmitter glutamate caused by internalization of postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs). Although it has been shown that MD similarly causes a loss of AMPARs from visual cortical synapses, the contribution of this change to the OD shift has not been established. Using an herpes simplex virus (HSV) vector, we expressed in visual cortical neurons a peptide (G2CT) designed to block AMPAR internalization by hindering the association of the C-terminal tail of the AMPAR GluR2 subunit with the AP2 clathrin adaptor complex. We found that G2CT expression interferes with NMDA receptor (NMDAR)-dependent AMPAR endocytosis and LTD, without affecting baseline synaptic transmission. When expressed in vivo, G2CT completely blocked the OD shift and depression of deprived-eye responses after MD without affecting baseline visual responsiveness or experience-dependent response potentiation in layer 4 of visual cortex. These data suggest that AMPAR internalization is essential for the loss of synaptic strength caused by sensory deprivation in visual cortex.

PMID: 19470483

High-frequency, long-range coupling between prefrontal and visual cortex during attention.

2009-07-05T15:18:00.000+04:30

Gregoriou GG, Gotts SJ, Zhou H, Desimone R.
Science. 2009 May 29;324(5931):1207-10
http://www.sciencemag.org/cgi/content/full/324/5931/1207

Electrical recordings in humans and monkeys show attentional enhancement of evoked responses and gamma synchrony in ventral stream cortical areas. Does this synchrony result from intrinsic activity in visual cortex or from inputs from other structures? Using paired recordings in the frontal eye field (FEF) and area V4, we found that attention to a stimulus in their joint receptive field leads to enhanced oscillatory coupling between the two areas, particularly at gamma frequencies. This coupling appeared to be initiated by FEF and was time-shifted by about 8 to 13 milliseconds across a range of frequencies. Considering the expected conduction and synaptic delays between the areas, this time-shifted coupling at gamma frequencies may optimize the postsynaptic impact of spikes from one area upon the other, improving cross-area communication with attention.

PMID: 19478185

Color vision, cones, and color-coding in the cortex.

2009-06-30T17:33:00.000+04:30

Conway BR.
Neuroscientist. 2009 Jun;15(3):274-90.
http://nro.sagepub.com/cgi/reprint/15/3/274

Color processing begins with the absorption of light by cone photoreceptors, and progresses through a series of hierarchical stages: Retinal signals carrying color information are transmitted through the lateral geniculate nucleus of the thalamus (LGN) up to the primary visual cortex (V1). From V1, the signals are processed by the second visual area (V2); then by cells located in subcompartments ("globs") within the posterior inferior temporal (PIT) cortex, a brain region that encompasses area V4 and brain regions immediately anterior to V4. Color signals are then processed by regions deep within the inferior temporal (IT) cortex including area TE. As a heuristic, one can consider each of these stages to be involved in constructing a distinct aspect of the color percept. The three cone types are the basis for trichromacy; retinal ganglion cells that respond in an opponent fashion to activation of different cone classes are the basis for color opponency (these "cone-opponent" cells increase their firing rate above baseline to activation of one cone class and decrease their firing rate below baseline to activation of a different cone class); double-opponent neurons in the V1 generate local color contrast and are the building blocks for color constancy; glob cells elaborate the perception of hue; and IT integrates color perception in the context of behavior. Finally, though nothing is known, these signals presumably interface with motor programs and emotional centers of the brain to mediate the widely acknowledged emotional salience of color.

PMID: 19436076

Acute Treatment of Intractable Migraine With Sphenopalatine Ganglion Electrical Stimulation.

2009-06-30T17:14:00.000+04:30

Tepper SJ, Rezai A, Narouze S, Steiner C, Mohajer P, Ansarinia M.
Headache. 2009 May 26.
http://www3.interscience.wiley.com/journal/122407812/abstract?CRETRY=1&SRETRY=0

Background.- We report preliminary results of a novel acute treatment for intractable migraine. The sphenopalatine ganglion (SPG) has sensorimotor and autonomic components and is involved in migraine pathophysiology. Methods.- In 11 patients with medically refractory migraine, the sphenopalatine fossa was accessed with a 20-gauge needle using the standard infrazygomatic transcoronoid approach under fluoroscopy. Patients underwent temporary unilateral electric stimulation of the SPG with a Medtronic 3057 test stimulation lead after induction of full-blown migraine. Both sham and active stimulations with different settings were carried out for
PMID: 19486173

Directed Interactions Between Auditory and Superior Temporal Cortices and their Role in Sensory Integration.

2009-06-30T17:09:00.002+04:30

Kayser C, Logothetis NK.
Front Integr Neurosci. 2009;3:7. Epub 2009 May 4
http://www.frontiersin.org/integrativeneuroscience/paper/10.3389/neuro.07/007.2009/

Recent studies using functional imaging and electrophysiology demonstrate that processes related to sensory integration are not restricted to higher association cortices but already occur in early sensory cortices, such as primary auditory cortex. While anatomical studies suggest the superior temporal sulcus (STS) as likely source of visual input to auditory cortex, little evidence exists to support this notion at the functional level. Here we tested this hypothesis by simultaneously recording from sites in auditory cortex and STS in alert animals stimulated with dynamic naturalistic audio-visual scenes. Using Granger causality and directed transfer functions we first quantified causal interactions at the level of field potentials, and subsequently determined those frequency bands that show effective interactions, i.e. interactions that are relevant for influencing neuronal firing at the target site. We found that effective interactions from auditory cortex to STS prevail below 20 Hz, while interactions from STS to auditory cortex prevail above 20 Hz. In addition, we found that directed interactions from STS to auditory cortex make a significant contribution to multisensory influences in auditory cortex: Sites in auditory cortex showing multisensory enhancement received stronger feed-back from STS during audio-visual than during auditory stimulation, while sites with multisensory suppression received weaker feed-back. These findings suggest that beta frequencies might be important for inter-areal coupling in the temporal lobe and demonstrate that superior temporal regions indeed provide one major source of visual influences to auditory cortex.

PMID: 19503750

Where Are the Human Speech and Voice Regions, and Do Other Animals Have Anything Like Them?

2009-06-30T17:07:00.000+04:30

Petkov CI, Logothetis NK, Obleser J.
Neuroscientist. 2009 Jun 10.
http://nro.sagepub.com/cgi/rapidpdf/1073858408326430v1

Modern lesion and imaging work in humans has been clarifying which brain regions are involved in the processing of speech and language. Concurrently, some of this work has aimed to bridge the gap to the seemingly incompatible evidence for multiple brain-processing pathways that first accumulated in nonhuman primates. For instance, the idea of a posterior temporal-parietal "Wernicke's" territory, which is thought to be instrumental for speech comprehension, conflicts with this region of the brain belonging to a spatial "where" pathway. At the same time a posterior speech-comprehension region ignores the anterior temporal lobe and its "what" pathway for evaluating the complex features of sensory input. Recent language models confirm that the posterior or dorsal stream has an important role in human communication, by a reconceptualization of the "where" into a "how-to" pathway with a connection to the motor system for speech comprehension. Others have tried to directly implicate the "what" pathway for speech comprehension, relying on the growing evidence in humans for anterior-temporal involvement in speech and voice processing. Coming full circle, we find that the recent imaging of vocalization and voice preferring regions in nonhuman primates allows us to make direct links to the human imaging data involving the anterior-temporal regions. The authors describe how comparison of the structure and function of the vocal communication system of humans and other animals is clarifying evolutionary relationships and the extent to which different species can model human brain function.

V4 activity predicts the strength of visual short-term memory representations.

2009-06-30T17:04:00.000+04:30

Sligte IG, Scholte HS, Lamme VA.
J Neurosci. 2009 Jun 10;29(23):7432-8
http://www.jneurosci.org/cgi/content/full/29/23/7432

Recent studies have shown the existence of a form of visual memory that lies intermediate of iconic memory and visual short-term memory (VSTM), in terms of both capacity (up to 15 items) and the duration of the memory trace (up to 4 s). Because new visual objects readily overwrite this intermediate visual store, we believe that it reflects a weak form of VSTM with high capacity that exists alongside a strong but capacity-limited form of VSTM. In the present study, we isolated brain activity related to weak and strong VSTM representations using functional magnetic resonance imaging. We found that activity in visual cortical area V4 predicted the strength of VSTM representations; activity was low when there was no VSTM, medium when there was a weak VSTM representation regardless of whether this weak representation was available for report or not, and high when there was a strong VSTM representation. Altogether, this study suggests that the high capacity yet weak VSTM store is represented in visual parts of the brain. Allegedly, only some of these VSTM traces are amplified by parietal and frontal regions and as a consequence reside in traditional or strong VSTM. The additional weak VSTM representations remain available for conscious access and report when attention is redirected to them yet are overwritten as soon as new visual stimuli hit the eyes.

Representing the forest before the trees: a global advantage effect in monkey inferotemporal cortex.

2009-06-30T16:54:00.000+04:30

Sripati AP, Olson CR.
J Neurosci. 2009 Jun 17;29(24):7788-96
Fulltext: http://www.jneurosci.org/cgi/content/full/29/24/7788

Hierarchical stimuli (large shapes composed of small shapes) have long been used to study how humans perceive the global and the local content of a scene--the forest and the trees. Studies using these stimuli have revealed a global advantage effect: humans consistently report global shape faster than local shape. The neuronal underpinnings of this effect remain unclear. Here we demonstrate a correlate and possible mechanism in monkey inferotemporal cortex (IT). Inferotemporal neurons signal the global content of a hierarchical display approximately 30 ms before they signal its local content. This is a specific expression of a general principle, related to spatial scale or spatial frequency rather than to hierarchical level, whereby the representation of a large shape develops in IT before that of a small shape. These findings provide support for a coarse-to-fine model of visual scene representation.

Seeing the future: Natural image sequences produce "anticipatory" neuronal activity and bias perceptual report.

2009-06-30T16:51:00.000+04:30

Perrett DI, Xiao D, Barraclough NE, Keysers C, Oram MW.
Q J Exp Psychol (Colchester). 2009 Jun 23:1-24.
Fulltext: http://www.informaworld.com/smpp/content~db=all?content=10.1080/17470210902959279

This paper relates human perception to the functioning of cells in the temporal cortex that are engaged in high-level pattern processing. We review historical developments concerning (a) the functional organization of cells processing faces and (b) the selectivity for faces in cell responses. We then focus on (c) the comparison of perception and cell responses to images of faces presented in sequences of unrelated images. Specifically the paper concerns the cell function and perception in circumstances where meaningful patterns occur momentarily in the context of a naturally or unnaturally changing visual environment. Experience of visual sequences allows anticipation, yet one sensory stimulus also "masks" perception and neural processing of subsequent stimuli. To understand this paradox we compared cell responses in monkey temporal cortex to body images presented individually, in pairs and in action sequences. Responses to one image suppressed responses to similar images for approximately 500 ms. This suppression led to responses peaking 100 ms earlier to image sequences than to isolated images (e.g., during head rotation, face-selective activity peaks before the face confronts the observer). Thus forward masking has unrecognized benefits for perception because it can transform neuronal activity to make it predictive during natural change.

Traditional waveform based spike sorting yields biased rate code estimates

2009-06-30T16:42:00.000+04:30

Valérie Ventura
PNAS April 28, 2009 vol. 106 no. 17 6921–6926
Free Fulltext: http://www.pnas.org/cgi/doi/10.1073/pnas.0901771106

Much of neuroscience has to do with relating neural activity and
behavior or environment. One common measure of this relationship
is the firing rates of neurons as functions of behavioral or environmental
parameters, often called tuning functions and receptive
fields. Firing rates are estimated from the spike trains of neurons
recorded by electrodes implanted in the brain. Individual neurons’
spike trains are not typically readily available, because the signal
collected at an electrode is often a mixture of activities from different
neurons and noise. Extracting individual neurons’ spike trains
from voltage signals, which is known as spike sorting, is one of the
most important data analysis problems in neuroscience, because it
has to be undertaken prior to any analysis of neurophysiological
data in which more than one neuron is believed to be recorded
on a single electrode. All current spike-sorting methods consist
of clustering the characteristic spike waveforms of neurons. The
sequence of first spike sorting based on waveforms, then estimating
tuning functions, has long been the accepted way to proceed.
Here, we argue that the covariates that modulate tuning functions
also contain information about spike identities, and that if tuning
information is ignored for spike sorting, the resulting tuning function
estimates are biased and inconsistent, unless spikes can be
classified with perfect accuracy. This means, for example, that the
commonly used peristimulus time histogram is a biased estimate of
the firing rate of a neuron that is not perfectly isolated.We further
argue that the correct conceptual way to view the problem out is to
note that spike sorting provides information about rate estimation
and vice versa, so that the two relationships should be considered
simultaneously rather than sequentially. Indeed we show that
when spike sorting and tuning-curve estimation are performed in
parallel, unbiased estimates of tuning curves can be recovered even
from imperfectly sorted neurons.

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2009-06-30T16:39:00.000+04:30

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Relationship between color discrimination and neural responses in the inferior temporal cortex of the monkey

2008-10-20T09:55:00.000+03:30

Matsumora T, Koida K, Komatsu H.
J Neurophysiol. 2008 Oct 15.

Earlier studies suggest that the inferior temporal (IT) cortex of the monkey plays a key role in color discrimination. Here, we examined the quantitative relationship between color judgment in monkeys and the responses of color-selective neurons in the anterior part of the IT cortex (area TE) by comparing neuronal activity and behavior recorded simultaneously while the monkeys performed a color judgment task. We first compared the abilities of single neurons and monkeys to discriminate color. To calculate a neuron's ability to discriminate color, we computed a neurometric function using receiver-operating-characteristics analysis. We then compared the neural and behavioral thresholds for color discrimination and found that, in general, the neural threshold was higher than the behavioral threshold, though occasionally the reverse was true. Variation in the neural threshold across the color space corresponded well with that of the behavioral threshold. We then calculated the Choice Probability (CP), which is a measure of the correlation between the trial-to-trial fluctuations in neuronal responses and the monkeys' color judgment. On average, CPs were slightly but significantly larger than 0.5, indicating the activities of these TE neurons correlate positively with the monkeys' color judgment. This suggests that individual color-selective TE neurons only weakly contribute to color discrimination and that a large population of color-selective TE neurons contribute to the performance of color discrimination.

PMID: 18922950

Full text: http://jn.physiology.org/cgi/reprint/90551.2008v1

Stimulus similarity-contingent neural adaptation can be time and cortical area dependent.

2008-10-20T09:07:00.000+03:30

Verhoef BE, Kayaert G, Franko E, Vangeneugden J, Vogels R.
J Neurosci. 2008 Oct 15;28(42):10631-40.

Repetition of a stimulus results in decreased responses in many cortical areas. This so-called adaptation or repetition suppression has been used in several human functional magnetic resonance imaging studies to deduce the stimulus selectivity of neuronal populations. We tested in macaque monkeys whether the degree of neural adaptation depends on the similarity between the adapter and test stimulus. To manipulate similarity, we varied stimulus size. We recorded the responses of single neurons to different-sized shapes in inferior temporal (IT) and prefrontal cortical (PFC) areas while the animals were engaged in a size or shape discrimination task. The degree of response adaptation in IT decreased with increasing size differences between the adapter and the test stimuli in both tasks, but the dependence of adaptation on the degree of similarity between the adapter and test stimuli was limited mainly to the early phase of the neural response in IT. PFC neurons showed only weak size-contingent repetition effects, despite strong size selectivity observed with the same stimuli. Thus, based on the repetition effects in PFC, one would have erroneously concluded that PFC shows weak or no size selectivity in such tasks. These findings are relevant for the interpretation of functional magnetic resonance adaptation data: they support the conjecture that the degree of adaptation scales with the similarity between adapter and test stimuli. However, they also show that the temporal evolution of adaptation during the course of the response, and differences in the way individual regions react to stimulus repetition, may complicate the inference of neuronal tuning from functional magnetic resonance adaptation.

PMID: 18923039

Full text: http://www.jneurosci.org/chttp://www.jneurosci.org/cgi/content/full/28/42/10631gi/content/full/28/42/10631

Spatial updating: how the brain keeps track of changing object locations during observer motion

2008-09-30T08:32:00.000+03:30

Thomas Wolbers, Mary Hegarty, Christian Büchel & Jack M Loomis
Nature Neuroscience 11, 1223 - 1230 (2008)

As you move through an environment, the positions of surrounding objects relative to your body constantly change. Updating these locations is a central feature of situational awareness and readiness to act. Here, we used functional magnetic resonance imaging and a virtual environment to test how the human brain uses optic flow to monitor changing object coordinates. Only activation profiles in the precuneus and the dorsal premotor cortex (PMd) were indicative of an updating process operating on a memorized egocentric map of space. A subsequent eye movement study argued against the alternative explanation that activation in PMd could be driven by oculomotor signals. Finally, introducing a verbal response mode revealed a dissociation between the two regions, with the PMd only showing updating-related responses when participants responded by pointing. We conclude that visual spatial updating relies on the construction of updated representations in the precuneus and the context-dependent planning of motor actions in PMd.

Fulltext:http://www.nature.com.proxy.kib.ki.se/neuro/journal/v11/n10/full/nn.2189.html

Improved visual sensitivity during smooth pursuit eye movements

2008-09-30T08:18:00.000+03:30

Alexander C Schütz, Doris I Braun, Dirk Kerzel & Karl R Gegenfurtner1
Nature Neuroscience 11, 1211 - 1216 (2008)

When we view the world around us, we constantly move our eyes. This brings objects of interest into the fovea and keeps them there, but visual sensitivity has been shown to deteriorate while the eyes are moving. Here we show that human sensitivity for some visual stimuli is improved during smooth pursuit eye movements. Detection thresholds for briefly flashed, colored stimuli were 16% lower during pursuit than during fixation. Similarly, detection thresholds for luminance-defined stimuli of high spatial frequency were lowered. These findings suggest that the pursuit-induced sensitivity increase may have its neuronal origin in the parvocellular retino-thalamic system. This implies that the visual system not only uses feedback connections to improve processing for locations and objects being attended to, but that a whole processing subsystem can be boosted. During pursuit, facilitation of the parvocellular system may reduce motion blur for stationary objects and increase sensitivity to speed changes of the tracked object.

Fulltext:http://www.nature.com.proxy.kib.ki.se/neuro/journal/v11/n10/full/nn.2194.html

Unsupervised Natural Experience Rapidly Alters Invariant Object Representation in Visual Cortex.

2008-09-15T20:28:00.000+04:30

Li N, Dicarlo JJ.
Science. 2008 Sep 12;321(5895):1502-1507

Object recognition is challenging because each object produces myriad retinal images. Responses of neurons from the inferior temporal cortex (IT) are selective to different objects, yet tolerant ("invariant") to changes in object position, scale, and pose. How does the brain construct this neuronal tolerance? We report a form of neuronal learning that suggests the underlying solution. Targeted alteration of the natural temporal contiguity of visual experience caused specific changes in IT position tolerance. This unsupervised temporal slowness learning (UTL) was substantial, increased with experience, and was significant in single IT neurons after just 1 hour. Together with previous theoretical work and human object perception experiments, we speculate that UTL may reflect the mechanism by which the visual stream builds and maintains tolerant object representations.

PMID: 18787171

Full text: http://www.sciencemag.org/cgi/reprint/321/5895/1502.pdf

Effects of category learning on the stimulus selectivity of macaque inferior temporal neurons

2008-09-13T16:31:00.000+04:30

De Baene W, Ons B, Wagemans J, Vogels R.
Learn Mem. 2008 Aug 26;15(9):717-27.

Primates can learn to categorize complex shapes, but as yet it is unclear how this categorization learning affects the representation of shape in visual cortex. Previous studies that have examined the effect of categorization learning on shape representation in the macaque inferior temporal (IT) cortex have produced diverse and conflicting results that are difficult to interpret owing to inadequacies in design. The present study overcomes these issues by recording IT responses before and after categorization learning. We used parameterized shapes that varied along two shape dimensions. Monkeys were extensively trained to categorize the shapes along one of the two dimensions. Unlike previous studies, our paradigm counterbalanced the relevant categorization dimension across animals. We found that categorization learning increased selectivity specifically for the category-relevant stimulus dimension (i.e., an expanded representation of the trained dimension), and that the ratio of within-category response similarities to between-category response similarities increased for the relevant dimension (i.e., category tuning). These small effects were only evident when the learned category-related effects were disentangled from the prelearned stimulus selectivity. These results suggest that shape-categorization learning can induce minor category-related changes in the shape tuning of IT neurons in adults, suggesting that learned, category-related changes in neuronal response mainly occur downstream from IT.

PMID: 18772261

Full text: http://learnmem.cshlp.org/cgi/content/full/15/9/717

Emergence of binocular functional properties in a monocular neural circuit

2008-08-28T08:23:00.000+04:30

Pavan Ramdya, Florian Engert
Nature Neuroscience 11, 1083 - 1090 (2008)

Sensory circuits frequently integrate converging inputs while maintaining precise functional relationships between them. For example, in mammals with stereopsis, neurons at the first stages of binocular visual processing show a close alignment of receptive-field properties for each eye. Still, basic questions about the global wiring mechanisms that enable this functional alignment remain unanswered, including whether the addition of a second retinal input to an otherwise monocular neural circuit is sufficient for the emergence of these binocular properties. We addressed this question by inducing a de novo binocular retinal projection to the larval zebrafish optic tectum and examining recipient neuronal populations using in vivo two-photon calcium imaging. Notably, neurons in rewired tecta were predominantly binocular and showed matching direction selectivity for each eye. We found that a model based on local inhibitory circuitry that computes direction selectivity using the topographic structure of both retinal inputs can account for the emergence of this binocular feature.

Full text: http://www.nature.com/neuro/journal/v11/n9/pdf/nn.2166.pdf

Direction of Visual Apparent Motion Driven Solely by Timing of a Static Sound

2008-08-28T08:19:00.000+04:30

Elliot Freeman, and Jon Driver
Current Biology, Vol 18, 1262-1266, 26 August 2008

In temporal ventriloquism, auditory events can illusorily attract perceived timing of a visual onset [1, 2, 3]. We investigated whether timing of a static sound can also influence spatio-temporal processing of visual apparent motion, induced here by visual bars alternating between opposite hemifields. Perceived direction typically depends on the relative interval in timing between visual left-right and right-left flashes (e.g., rightwards motion dominating when left-to-right interflash intervals are shortest [4]). In our new multisensory condition, interflash intervals were equal, but auditory beeps could slightly lag the right flash, yet slightly lead the left flash, or vice versa. This auditory timing strongly influenced perceived visual motion direction, despite providing no spatial auditory motion signal whatsoever. Moreover, prolonged adaptation to such auditorily driven apparent motion produced a robust visual motion aftereffect in the opposite direction, when measured in subsequent silence. Control experiments argued against accounts in terms of possible auditory grouping, or possible attention capture. We suggest that the motion arises because the sounds change perceived visual timing, as we separately confirmed. Our results provide a new demonstration of multisensory influences on sensory-specific perception [5], with timing of a static sound influencing spatio-temporal processing of visual motion direction.

Full text: http://download.current-biology.com/pdfs/0960-9822/PIIS0960982208009755.pdf

Believing is seeing: expectations alter visual awareness

2008-08-28T08:18:00.000+04:30

Philipp Sterzer, Chris Frith, and Predrag Petrovic
Current Biology, Vol 18, R697-R698, 26 August 2008

Expectations have been shown to be powerful modulators of pain [1] and emotion [2] in placebo studies. In such experiments, expectations are induced by instructions combined with manipulation of the sensory experience that is unknown to the subjects. After an expectation learning phase where a painful stimulation is surreptitiously lowered following placebo application, the placebo effectively reduces subjective pain intensity in a subsequent test phase [3]. The strength of this placebo effect is closely related to the induced expectation [4]. Here, we asked whether this powerful cognitive bias reflects a general property of sensory information processing and tested whether the contents of visual awareness could be altered by a placebo-like expectation manipulation. We found a dramatic effect of experimentally induced expectations on the perception of an ambiguous visual motion stimulus. This shows that expectations have a strong and general influence on our experience of the sensory input independently of its specific type and content.

Full text: http://download.current-biology.com/pdfs/0960-9822/PIIS0960982208007422.pdf

Neural basis for unique hues

2008-08-28T08:15:00.000+04:30

Cleo M. Stoughton and Bevil R. Conway
Current Biology, 2008, 18:16:R700-R702

All colors can be described in terms of four non-reducible ‘unique’ hues: red, green, yellow, and blue [1]. These four hues are also the most common ‘focal’ colors — the best examples of color terms in language [2]. The significance of the unique hues has been recognized since at least the 14th century [3] and is universal [4, 5], although there is some individual variation [6, 7]. Psychophysical linking hypotheses predict an explicit neural representation of unique hues at some stage of the visual system, but no such representation has been described [8]. The special status of the unique hues “remains one of the central mysteries of color science” [9]. Here we report that a population of recently identified cells in posterior inferior temporal cortex of macaque monkey contains an explicit representation of unique hues.

Full text: http://download.current-biology.com/pdfs/0960-9822/PIIS0960982208008191.pdf

The Orientation Selectivity of Color-Responsive Neurons in Macaque V1

2008-08-09T06:25:00.000+04:30

Elizabeth N. Johnson, Michael J. Hawken, Robert Shapley
The Journal of Neuroscience, August 6, 2008, 28(32):8096-8106; doi:10.1523/JNEUROSCI.1404-08.2008

Form has a strong influence on color perception. We investigated the neural basis of the form–color link in macaque primary visual cortex (V1) by studying orientation selectivity of single V1 cells for pure color patterns. Neurons that responded to color were classified, based on cone inputs and spatial selectivity, into chromatically single-opponent and double-opponent groups. Single-opponent cells responded well to color but weakly to luminance contrast; they were not orientation selective for color patterns. Most double-opponent cells were orientation selective to pure color stimuli as well as to achromatic patterns. We also found non-opponent cells that responded weakly or not at all to pure color; most were orientation selective for luminance patterns. Double-opponent and non-opponent cells' orientation selectivities were not contrast invariant; selectivity usually increased with contrast. Double-opponent cells were approximately equally orientation selective for luminance and equiluminant color stimuli when stimuli were matched in average cone contrast. V1 double-opponent cells could be the neural basis of the influence of form on color perception. The combined activities of single- and double-opponent cells in V1 are needed for the full repertoire of color perception.

Full text: http://www.jneurosci.org/cgi/reprint/28/32/8096

Converging Neuronal Activity in Inferior Temporal Cortex during the Classification of Morphed Stimuli.

2008-08-06T10:29:00.000+04:30

Akrami A, Liu Y, Treves A, Jagadeesh B.
Cereb Cortex. 2008 Jul 31.

How does the brain dynamically convert incoming sensory data into a representation useful for classification? Neurons in inferior temporal (IT) cortex are selective for complex visual stimuli, but their response dynamics during perceptual classification is not well understood. We studied IT dynamics in monkeys performing a classification task. The monkeys were shown visual stimuli that were morphed (interpolated) between pairs of familiar images. Their ability to classify the morphed images depended systematically on the degree of morph. IT neurons were selected that responded more strongly to one of the 2 familiar images (the effective image). The responses tended to peak approximately 120 ms following stimulus onset with an amplitude that depended almost linearly on the degree of morph. The responses then declined, but remained above baseline for several hundred ms. This sustained component remained linearly dependent on morph level for stimuli more similar to the ineffective image but progressively converged to a single response profile, independent of morph level, for stimuli more similar to the effective image. Thus, these neurons represented the dynamic conversion of graded sensory information into a task-relevant classification. Computational models suggest that these dynamics could be produced by attractor states and firing rate adaptation within the population of IT neurons.

PMID: 18669590

Free full text: http://cercor.oxfordjournals.org/cgi/reprint/bhn125v1

fMRI and its interpretations: an illustration on directional selectivity in area V5/MT

2008-08-06T10:24:00.000+04:30

Bartels A, Logothetis NK, Moutoussis K.
Trends Neurosci. 2008 Aug 2.

fMRI is a tool to study brain function noninvasively that can reliably identify sites of neural involvement for a given task. However, to what extent can fMRI signals be related to measures obtained in electrophysiology? Can the blood-oxygen-level-dependent signal be interpreted as spatially pooled spiking activity? Here we combine knowledge from neurovascular coupling, functional imaging and neurophysiology to discuss whether fMRI has succeeded in demonstrating one of the most established functional properties in the visual brain, namely directional selectivity in the motion-processing region V5/MT+. We also discuss differences of fMRI and electrophysiology in their sensitivity to distinct physiological processes. We conclude that fMRI constitutes a complement, not a poor-resolution substitute, to invasive techniques, and that it deserves interpretations that acknowledge its stand as a separate signal.

PMID: 18676033

Neural repetition suppression reflects fulfilled perceptual expectations.

2008-08-06T10:19:00.000+04:30

Summerfield C, Trittschuh EH, Monti JM, Mesulam MM, Egner T.
Nat Neurosci. 2008 Aug 1.

Stimulus-evoked neural activity is attenuated on stimulus repetition (repetition suppression), a phenomenon that is attributed to largely automatic processes in sensory neurons. By manipulating the likelihood of stimulus repetition, we found that repetition suppression in the human brain was reduced when stimulus repetitions were improbable (and thus, unexpected). Our data suggest that repetition suppression reflects a relative reduction in top-down perceptual 'prediction error' when processing an expected, compared with an unexpected, stimulus.

PMID: 18677308

Full text: http://www.nature.com/neuro/journal/vaop/ncurrent/full/nn.2163.html

2008-08-06T08:07:00.001+04:30

Source Event Podcast at Nature Jobs

2008-08-05T11:25:00.000+04:30

http://www.nature.com/naturejobs/magazine/podcast/index.html

Multivariate patterns in object-selective cortex dissociate perceptual and physical shape similarity.

2008-08-02T01:05:00.000+04:30

Haushofer J, Livingstone MS, Kanwisher N.
PLoS Biol. 2008 Jul 29;6(7):e187.

Prior research has identified the lateral occipital complex (LOC) as a critical cortical region for the representation of object shape in humans. However, little is known about the nature of the representations contained in the LOC and their relationship to the perceptual experience of shape. We used human functional MRI to measure the physical, behavioral, and neural similarity between pairs of novel shapes to ask whether the representations of shape contained in subregions of the LOC more closely reflect the physical stimuli themselves, or the perceptual experience of those stimuli. Perceptual similarity measures for each pair of shapes were obtained from a psychophysical same-different task; physical similarity measures were based on stimulus parameters; and neural similarity measures were obtained from multivoxel pattern analysis methods applied to anterior LOC (pFs) and posterior LOC (LO). We found that the pattern of pairwise shape similarities in LO most closely matched physical shape similarities, whereas shape similarities in pFs most closely matched perceptual shape similarities. Further, shape representations were similar across participants in LO but highly variable across participants in pFs. Together, these findings indicate that activation patterns in subregions of object-selective cortex encode objects according to a hierarchy, with stimulus-based representations in posterior regions and subjective and observer-specific representations in anterior regions.

PMID: 18666833

Full text: http://biology.plosjournals.org/perlserv/?request=get-pdf&file=10.1371_journal.pbio.0060187-L.pdf

Influence of Reward Delays on Responses of Dopamine Neurons

2008-08-01T14:58:00.000+04:30

Shunsuke Kobayashi and Wolfram Schultz
The Journal of Neuroscience, July 30, 2008, 28(31):7837-7846; doi:10.1523/JNEUROSCI.1600-08.2008

Psychological and microeconomic studies have shown that outcome values are discounted by imposed delays. The effect, called temporal discounting, is demonstrated typically by choice preferences for sooner smaller rewards over later larger rewards. However, it is unclear whether temporal discounting occurs during the decision process when differently delayed reward outcomes are compared or during predictions of reward delays by pavlovian conditioned stimuli without choice. To address this issue, we investigated the temporal discounting behavior in a choice situation and studied the effects of reward delay on the value signals of dopamine neurons. The choice behavior confirmed hyperbolic discounting of reward value by delays on the order of seconds. Reward delay reduced the responses of dopamine neurons to pavlovian conditioned stimuli according to a hyperbolic decay function similar to that observed in choice behavior. Moreover, the stimulus responses increased with larger reward magnitudes, suggesting that both delay and magnitude constituted viable components of dopamine value signals. In contrast, dopamine responses to the reward itself increased with longer delays, possibly reflecting temporal uncertainty and partial learning. These dopamine reward value signals might serve as useful inputs for brain mechanisms involved in economic choices between delayed rewards.

Full text: http://www.jneurosci.org/cgi/reprint/28/31/7837

Complementary Contributions of Prefrontal Neuron Classes in Abstract Numerical Categorization

2008-08-01T14:20:00.001+04:30

Ilka Diester and Andreas Nieder
The Journal of Neuroscience, July 30, 2008, 28(31):7737-7747; doi:10.1523/JNEUROSCI.1347-08.2008

The primate prefrontal cortex (PFC) plays a cardinal role in forming abstract categories and concepts. However, it remains elusive how this is accomplished and to what extent the interaction of functionally distinct neuron classes underlies this representation. Here, we inferred the major cortical cell types, putative pyramidal cells, and interneurons by characterizing the waveforms of action potentials recorded in monkeys performing a cognitively demanding numerosity categorization task. Putative interneurons responded much faster than cells classified as pyramidal neurons and exhibited a higher reliability of category discrimination, whereas putative pyramidal cells showed a higher degree of category selectivity. An analysis of the numerosity tuning profiles and the temporal interactions of adjacent neurons indicated that inhibitory input by putative interneurons shapes the tuning to numerical categories of putative PFC pyramidal cells. These findings favor feedforward mechanisms subserving cognitive categorization and help to clarify cellular interactions in PFC microcircuits.

Full text: http://www.jneurosci.org/cgi/reprint/28/31/7737

Task difficulty modulates the activity of specific neuronal populations in primary visual cortex

2008-08-01T10:48:00.000+04:30

Yao Chen, Susana Martinez-Conde, Stephen L Macknik, Yulia Bereshpolova, Harvey A Swadlow, Jose-Manuel Alonso

Nature Neuroscience 11, 974 - 982 (2008) | doi:10.1038/nn.2147

Spatial attention enhances our ability to detect stimuli at restricted regions of the visual field. This enhancement is thought to depend on the difficulty of the task being performed, but the underlying neuronal mechanisms for this dependency remain largely unknown. We found that task difficulty modulates neuronal firing rate at the earliest stages of cortical visual processing (area V1) in monkey (Macaca mulatta). These modulations were spatially specific: increasing task difficulty enhanced V1 neuronal firing rate at the focus of attention and suppressed it in regions surrounding the focus. Moreover, we found that response enhancement and suppression are mediated by distinct populations of neurons that differ in direction selectivity, spike width, interspike-interval distribution and contrast sensitivity. Our results provide strong support for center-surround models of spatial attention and suggest that task difficulty modulates the activity of specific populations of neurons in the primary visual cortex.

Full text: http://www.nature.com/neuro/journal/v11/n8/pdf/nn.2147.pdf

The temporal precision of reward prediction in dopamine neurons.

2008-08-01T10:38:00.000+04:30

Christopher D Fiorillo, William T Newsome, Wolfram Schultz
Nature Neuroscience 11, 966 - 973 (2008) Published online: 27 July 2008 doi:10.1038/nn.2159

Midbrain dopamine neurons are activated when reward is greater than predicted, and this error signal could teach target neurons both the value of reward and when it will occur. We used the dopamine error signal to measure how the expectation of reward was distributed over time. Animals were trained with fixed-duration intervals of 1–16 s between conditioned stimulus onset and reward. In contrast to the weak responses that have been observed after short intervals (1–2 s), activations to reward increased steeply and linearly with the logarithm of the interval. Results with varied stimulus-reward intervals suggest that the neural expectation was substantial after just half an interval had elapsed. Thus, the neural expectation of reward in these experiments was not highly precise and the precision declined sharply with interval duration. The neural precision of expectation appeared to be at least qualitatively similar to the precision of anticipatory licking behavior.

PMID: 18660807

Full text: http://www.nature.com/neuro/journal/v11/n8/pdf/nn.2159.pdf

Recovery from monocular deprivation using binocular deprivation: Experimental observations and theoretical analysis.

2008-07-28T07:27:00.000+04:30

Blais B, Frenkel M, Kuindersma S, Muhammad R, Shouval HZ, Cooper LN, Bear MF.
J Neurophysiol. 2008 Jul 23.

Ocular dominance (OD) plasticity is a robust paradigm for examining the functional consequences of synaptic plasticity. Previous experimental and theoretical results have shown that OD plasticity can be accounted for by known synaptic plasticity mechanisms, using the assumption that deprivation by lid suture eliminates spatial structure in the deprived channel. Here we show that in the mouse, recovery from monocular lid suture can be obtained by subsequent binocular lid suture but not by dark rearing. This poses a significant challenge to previous theoretical results. We therefore performed simulations with a natural input environment appropriate for mouse visual cortex. In contrast to previous work we assume that lid suture causes degradation but not elimination of spatial structure, whereas dark rearing produces elimination of spatial structure. We present experimental evidence that supports this assumption, measuring responses through sutured lids in the mouse. The change in assumptions about the input environment is sufficient to account for new experimental observations, while still accounting for previous experimental results.

PMID: 1865031

Full text: http://jn.physiology.org/cgi/reprint/90411.2008v1

Students and Professors

2008-07-27T17:00:00.003+04:30

Spatio-temporal correlations and visual signalling in a complete neuronal population

2008-07-26T13:21:00.001+04:30

Pillow JW, Shlens J, Paninski L, Sher A, Litke AM, Chichilnisky EJ, Simoncelli EP.
Nature. 2008 Jul 23.

Statistical dependencies in the responses of sensory neurons govern both the amount of stimulus information conveyed and the means by which downstream neurons can extract it. Although a variety of measurements indicate the existence of such dependencies, their origin and importance for neural coding are poorly understood. Here we analyse the functional significance of correlated firing in a complete population of macaque parasol retinal ganglion cells using a model of multi-neuron spike responses. The model, with parameters fit directly to physiological data, simultaneously captures both the stimulus dependence and detailed spatio-temporal correlations in population responses, and provides two insights into the structure of the neural code. First, neural encoding at the population level is less noisy than one would expect from the variability of individual neurons: spike times are more precise, and can be predicted more accurately when the spiking of neighbouring neurons is taken into account. Second, correlations provide additional sensory information: optimal, model-based decoding that exploits the response correlation structure extracts 20% more information about the visual scene than decoding under the assumption of independence, and preserves 40% more visual information than optimal linear decoding. This model-based approach reveals the role of correlated activity in the retinal coding of visual stimuli, and provides a general framework for understanding the importance of correlated activity in populations of neurons.

PMID: 18650810

Fulltext: http://www.nature.com/nature/journal/vaop/ncurrent/pdf/nature07140.pdf

Spatio-temporal correlations and visual signalling in a complete neuronal population

2008-07-26T13:21:00.000+04:30

Highly Selective Receptive Fields in Mouse Visual Cortex

2008-07-24T10:54:00.000+04:30

Cristopher M. Niell and Michael P. Stryker
The Journal of Neuroscience, July 23, 2008 • 28(30):7520 –7536

Genetic methods available in mice are likely to be powerful tools in dissecting cortical circuits. However, the visual cortex, in which
sensory coding has been most thoroughly studied in other species, has essentially been neglected in mice perhaps because of their poor
spatial acuity and the lack of columnar organization such as orientation maps.Wehave now applied quantitative methods to characterize
visual receptive fields in mouse primary visual cortex V1 by making extracellular recordings with silicon electrode arrays in anesthetized
mice.Weused current source density analysis to determine laminar location and spike waveforms to discriminate putative excitatory and
inhibitory units.Wefind that, although the spatial scale of mouse receptive fields is up to one or two orders of magnitude larger, neurons
show selectivity for stimulus parameters such as orientation and spatial frequency that is near to that found in other species. Furthermore,
typical response properties such as linear versus nonlinear spatial summation (i.e., simple and complex cells) and contrastinvariant
tuning are also present in mouse V1 and correlate with laminar position and cell type. Interestingly, we find that putative
inhibitory neurons generally have less selective, and nonlinear, responses. This quantitative description of receptive field properties
should facilitate the use of mouse visual cortex as a system to address longstanding questions of visual neuroscience and cortical
processing.

Free Fulltext: http://www.jneurosci.org/cgi/reprint/28/30/7520

Brain Magic

2008-07-24T00:11:00.000+04:30

Patches of face-selective cortex in the macaque frontal lobe.

2008-07-20T16:43:00.000+04:30

Tsao DY, Schweers N, Moeller S, Freiwald WA.
Nat Neurosci. 2008 Jul 11.

In primates, specialized occipital-temporal face areas support the visual analysis of faces, but it is unclear whether similarly specialized areas exist in the frontal lobe. Using functional magnetic resonance imaging in alert macaques, we identified three discrete regions of highly face-selective cortex in ventral prefrontal cortex, one of which was strongly lateralized to the right hemisphere. These prefrontal face patches may constitute dedicated modules for retrieving and responding to facial information.

PMID: 18622399

Fulltext: http://www.nature.com/neuro/journal/vaop/ncurrent/abs/nn.2158.html

Acetylcholine contributes through muscarinic receptors to attentional modulation in V1.

2008-07-20T16:19:00.000+04:30

Herrero JL, Roberts MJ, Delicato LS, Gieselmann MA, Dayan P, Thiele A.
Nature. 2008 Jul 16.

Attention exerts a strong influence over neuronal processing in cortical areas. It selectively increases firing rates and affects tuning properties, including changing receptive field locations and sizes. Although these effects are well studied, their cellular mechanisms are poorly understood. To study the cellular mechanisms, we combined iontophoretic pharmacological analysis of cholinergic receptors with single cell recordings in V1 while rhesus macaque monkeys (Macaca mulatta) performed a task that demanded top-down spatial attention. Attending to the receptive field of the V1 neuron under study caused an increase in firing rates. Here we show that this attentional modulation was enhanced by low doses of acetylcholine. Furthermore, applying the muscarinic antagonist scopolamine reduced attentional modulation, whereas the nicotinic antagonist mecamylamine had no systematic effect. These results demonstrate that muscarinic cholinergic mechanisms play a central part in mediating the effects of attention in V1.

PMID: 18633352

Fulltext: http://www.nature.com/nature/journal/vaop/ncurrent/pdf/nature07141.pdf

Functional Differentiation of Macaque Visual Temporal Cortical Neurons Using a Parametric Action Space.

2008-07-20T16:07:00.000+04:30

Vangeneugden J, Pollick F, Vogels R.
Cereb Cortex. 2008 Jul 16.

Neurons in the rostral superior temporal sulcus (STS) are responsive to displays of body movements. We employed a parametric action space to determine how similarities among actions are represented by visual temporal neurons and how form and motion information contributes to their responses. The stimulus space consisted of a stick-plus-point-light figure performing arm actions and their blends. Multidimensional scaling showed that the responses of temporal neurons represented the ordinal similarity between these actions. Further tests distinguished neurons responding equally strongly to static presentations and to actions ("snapshot" neurons), from those responding much less strongly to static presentations, but responding well when motion was present ("motion" neurons). The "motion" neurons were predominantly found in the upper bank/fundus of the STS, and "snapshot" neurons in the lower bank of the STS and inferior temporal convexity. Most "motion" neurons showed strong response modulation during the course of an action, thus responding to action kinematics. "Motion" neurons displayed a greater average selectivity for these simple arm actions than did "snapshot" neurons. We suggest that the "motion" neurons code for visual kinematics, whereas the "snapshot" neurons code for form/posture, and that both can contribute to action recognition, in agreement with computation models of action recognition.

PMID: 18632741

Free Fulltext: http://cercor.oxfordjournals.org/cgi/content/full/bhn109v1

Distinct Face-Processing Strategies in Parents of Autistic Children

2008-07-20T16:00:00.000+04:30

Adolphs R, Spezio ML, Parlier M, Piven J.
Curr Biol. 2008 Jul 15.

In his original description of autism, Kanner [1] noted that the parents of autistic children often exhibited unusual social behavior themselves, consistent with what we now know about the high heritability of autism [2]. We investigated this so-called Broad Autism Phenotype in the parents of children with autism, who themselves did not receive a diagnosis of any psychiatric illness. Building on recent quantifications of social cognition in autism [3], we investigated face processing by using the "bubbles" method [4] to measure how viewers make use of information from specific facial features in order to judge emotions. Parents of autistic children who were assessed as socially aloof (N = 15), a key component of the phenotype [5], showed a remarkable reduction in processing the eye region in faces, together with enhanced processing of the mouth, compared to a control group of parents of neurotypical children (N = 20), as well as to nonaloof parents of autistic children (N = 27, whose pattern of face processing was intermediate). The pattern of face processing seen in the Broad Autism Phenotype showed striking similarities to that previously reported to occur in autism [3] and for the first time provides a window into the endophenotype that may result from a subset of the genes that contribute to social cognition.

PMID: 18635351

Fulltext: Science Direct

Modulation of neural responses in inferotemporal cortex during the interpretation of ambiguous photographs.

2008-07-04T08:23:00.000+04:30

Liu Y, Jagadeesh B.
Eur J Neurosci. 2008 Jun;27(11):3059-73.

Ambiguous images are interpreted in the context of biases about what they might be; these biases and the behavioral consequences induced by them may influence the processing of images. In this report, we examine neural responses in inferotemporal cortex (IT) during the interpretation of ambiguous photographs created by morphing between two photographs. Monkeys classified different images as being one of two choices and learned to classify most of the samples correctly. For one image (the ambiguous sample) reward was administered randomly for either possible choice, and the monkeys were free to classify that image based on their own interpretation, with no learning possible. The ambiguous samples were not classified randomly: the monkey interpreted the samples differently during different sessions. The interpretation of the ambiguous sample was, in turn, highly correlated with the normalized response of individual neurons in IT to the ambiguous sample. If an ambiguous sample was interpreted as a particular choice during a session, the response to that ambiguous sample more closely resembled the response to that choice. Identical ambiguous images were interpreted differently during different sessions, and neural responses reflected the differing interpretations of the image during that session. The relationship between the interpretation of the image and neural responses strengthened over the course of a session because neural responses shifted to more closely resemble the response to the initial interpretation of the image. The data support a flexible representation of visual stimuli in higher visual areas.

PMID: 18588544

Population imaging of ongoing neuronal activity in the visual cortex of awake rats

2008-06-29T08:10:00.000+04:30

David S Greenberg, Arthur R Houweling, Jason N D Kerr
Nature Neuroscience 11, 749 - 751 (2008), doi:10.1038/nn.2140

It is unclear how the complex spatiotemporal organization of ongoing cortical neuronal activity recorded in anesthetized animals relates to the awake animal. We therefore used two-photon population calcium imaging in awake and subsequently anesthetized rats to follow action potential firing in populations of neurons across brain states, and examined how single neurons contributed to population activity. Firing rates and spike bursting in awake rats were higher, and pair-wise correlations were lower, compared with anesthetized rats. Anesthesia modulated population-wide synchronization and the relationship between firing rate and correlation. Overall, brain activity during wakefulness cannot be inferred using anesthesia.

Fulltext: http://www.nature.com/neuro/journal/v11/n7/full/nn.2140.html

Expressing fear enhances sensory acquisition

2008-06-29T08:06:00.001+04:30

Joshua M Susskind, Daniel H Lee, Andrée Cusi, Roman Feiman, Wojtek Grabski, Adam K Anderson
Nature Neuroscience 11, 843 - 850 (2008), doi:10.1038/nn.2138

It has been proposed that facial expression production originates in sensory regulation. Here we demonstrate that facial expressions of fear are configured to enhance sensory acquisition. A statistical model of expression appearance revealed that fear and disgust expressions have opposite shape and surface reflectance features. We hypothesized that this reflects a fundamental antagonism serving to augment versus diminish sensory exposure. In keeping with this hypothesis, when subjects posed expressions of fear, they had a subjectively larger visual field, faster eye movements during target localization and an increase in nasal volume and air velocity during inspiration. The opposite pattern was found for disgust. Fear may therefore work to enhance perception, whereas disgust dampens it. These convergent results provide support for the Darwinian hypothesis that facial expressions are not arbitrary configurations for social communication, but rather, expressions may have originated in altering the sensory interface with the physical world.

Fulltext: http://www.nature.com/neuro/journal/v11/n7/full/nn.2138.html

Top 10 TED Talks

2008-06-29T07:35:00.001+04:30

Download the Top 10 TED Talks highlights video:
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Privileged Coding of Convex Shapes in Human Object-Selective Cortex.

2008-06-28T07:27:00.001+04:30

Haushofer J, Baker CI, Livingstone MS, Kanwisher N.
J Neurophysiol. 2008 Jun 25.

What is the neural code for object shape? Despite intensive research, the precise nature of object representations in high-level visual cortex remains elusive. Here we use functional magnetic resonance imaging (fMRI) to show that convex shapes are encoded in a privileged fashion by human lateral occipital complex (LOC), a region which has been implicated in object recognition. On each trial, two convex or two concave shapes that were either identical or different were presented sequentially. Critically, the convex and concave stimuli were the same except for a binocular disparity change that reversed the figure-ground assignment. The fMRI response in LOC for convex stimuli was higher for different than for identical shape pairs, indicating sensitivity to differences in convex shape. However, when the same stimuli were seen as concave, the response for different and identical pairs was the same, indicating lower sensitivity to changes in concave shape than convex shape. This pattern was more pronounced in the anterior than in the posterior portion of LOC. These results suggest that convex contours could be basic building blocks of cortical object representations.

PMID: 18579661

Fulltext: http://jn.physiology.org/cgi/reprint/90310.2008v1

Dynamic Population Coding of Category Information in ITC and PFC.

2008-06-21T09:17:00.000+04:30

Meyers EM, Freedman DJ, Kreiman G, Miller EK, Poggio TA.
J Neurophysiol. 2008 Jun 18.

Most electrophysiology studies analyze the activity of each neuron separately. While such studies have given much insight into properties of the visual system, they have also potentially overlooked important aspects of information coded in changing patterns of activity that are distributed over larger populations of neurons. In this work, we apply a population decoding method, to better estimate what information is available in neuronal ensembles, and how this information is coded in dynamic patterns of neural activity in data recorded from inferior temporal cortex (ITC) and prefrontal cortex (PFC) as macaque monkeys engaged in a delayed match-to-category task (Freedman et al. 2003). Analyses of activity patterns in ITC and PFC revealed that both areas contain 'abstract' category information (i.e., category information that is not directly correlated with properties of the stimuli); however, in general, PFC has more task-relevant information, and ITC has more detailed visual information. Analyses examining how information coded in these areas show that almost all category information is available in a small fraction of the neurons in the population. Most remarkably, our results also show that category information is coded by a non-stationary pattern of activity that changes over the course of a trial, with individual neurons containing information on much shorter time scales than the population as a whole.

PMID: 18562555

Fulltext: http://jn.physiology.org/cgi/reprint/90248.2008v1

A framework for studying the neurobiology of value-based decision making

2008-06-21T07:34:00.001+04:30

Antonio Rangel, Colin Camerer, P. Read Montague
Nature Reviews Neuroscience 9, 545-556 (July 2008) | doi:10.1038/nrn2357

Neuroeconomics is the study of the neurobiological and computational basis of value-based decision making. Its goal is to provide a biologically based account of human behaviour that can be applied in both the natural and the social sciences. This Review proposes a framework to investigate different aspects of the neurobiology of decision making. The framework allows us to bring together recent findings in the field, highlight some of the most important outstanding problems, define a common lexicon that bridges the different disciplines that inform neuroeconomics, and point the way to future applications.

Fulltext: http://www.nature.com/nrn/journal/v9/n7/pdf/nrn2357.pdf

Mechanisms of face perception.

2008-06-20T10:50:00.001+04:30

Tsao DY, Livingstone MS.
Annu Rev Neurosci. 2008;31:411-37.

Faces are among the most informative stimuli we ever perceive: Even a split-second glimpse of a person's face tells us his identity, sex, mood, age, race, and direction of attention. The specialness of face processing is acknowledged in the artificial vision community, where contests for face-recognition algorithms abound. Neurological evidence strongly implicates a dedicated machinery for face processing in the human brain to explain the double dissociability of face- and object-recognition deficits. Furthermore, recent evidence shows that macaques too have specialized neural machinery for processing faces. Here we propose a unifying hypothesis, deduced from computational, neurological, fMRI, and single-unit experiments: that what makes face processing special is that it is gated by an obligatory detection process. We clarify this idea in concrete algorithmic terms and show how it can explain a variety of phenomena associated with face processing.

PMID: 18558862
Fulltext: http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.neuro.30.051606.094238

Psychology of Security

2008-06-19T12:27:00.004+04:30

Bruce Schneier on Psychology of Security
Black Hat USA 2007

Fascinating presentation about how our many brain and mind biases affect the way we make security decisions.

What do you do?

2008-06-17T12:07:00.003+04:30

A Primer on Python for Life Science Researchers

2008-06-15T07:49:00.000+04:30

Sebastian Bassi
PLoS Comput Biol. 2007 Nov;3(11):e199.Click here to read

PMID: 18052533

Fulltext: http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.0030199

What we can do and what we cannot do with fMRI.

2008-06-14T07:30:00.000+04:30

Logothetis NK.
Nature. 2008 Jun 12;453(7197):869-78

Functional magnetic resonance imaging (fMRI) is currently the mainstay of neuroimaging in cognitive neuroscience. Advances in scanner technology, image acquisition protocols, experimental design, and analysis methods promise to push forward fMRI from mere cartography to the true study of brain organization. However, fundamental questions concerning the interpretation of fMRI data abound, as the conclusions drawn often ignore the actual limitations of the methodology. Here I give an overview of the current state of fMRI, and draw on neuroimaging and physiological data to present the current understanding of the haemodynamic signals and the constraints they impose on neuroimaging data interpretation.

PMID: 18548064
Fulltext: http://www.nature.com/nature/journal/v453/n7197/full/nature06976.html

Patches with links: a unified system for processing faces in the macaque temporal lobe.

2008-06-10T09:10:00.001+04:30

Moeller S, Freiwald WA, Tsao DY.
Science. 2008 Jun 6;320(5881):1355-9.

The brain processes objects through a series of regions along the ventral visual pathway, but the circuitry subserving the analysis of specific complex forms remains unknown. One complex form category, faces, selectively activates six patches of cortex in the macaque ventral pathway. To identify the connectivity of these face patches, we used electrical microstimulation combined with simultaneous functional magnetic resonance imaging. Stimulation of each of four targeted face patches produced strong activation, specifically within a subset of the other face patches. Stimulation outside the face patches produced an activation pattern that spared the face patches. These results suggest that the face patches form a strongly and specifically interconnected hierarchical network.

PMID: 18535247

Fulltext: http://www.sciencemag.org/cgi/reprint/320/5881/1355.pdf

In vivo two-photon voltage-sensitive dye imaging reveals top-down control of cortical layers 1 and 2 during wakefulness

2008-06-01T11:23:00.000+04:30

B. Kuhn, W. Denk, R. M. Bruno
PNAS | May 27, 2008 | vol. 105 | no. 21 | 7588-7593

Conventional methods of imaging membrane potential changes have limited spatial resolution, particularly along the axis perpendicular to the cortical surface. The laminar organization of the cortex suggests, however, that the distribution of activity in depth is not uniform. We developed a technique to resolve network activity of different cortical layers in vivo using two-photon microscopy of the voltage-sensitive dye (VSD) ANNINE-6. We imaged spontaneous voltage changes in the barrel field of the somatosensory cortex of head-restrained mice and analyzed their spatiotemporal correlations during anesthesia and wakefulness. EEG recordings always correlated more strongly with VSD signals in layer (L) 2 than in L1. Nearby (<200 µm) cortical areas were correlated with one another during anesthesia. Waking the mouse strongly desynchronized neighboring cortical areas in L1 in the 4- to 10-Hz frequency band. Wakefulness also slightly increased synchrony of neighboring territories in L2 in the 0.5- to 4.0-Hz range. Our observations are consistent with the idea that, in the awake animal, long-range inputs to L1 of the sensory cortex from various cortical and thalamic areas exert top-down control on sensory processing.

Fulltext: http://www.pnas.org/cgi/reprint/105/21/7588

Low-frequency local field potentials and spikes in primary visual cortex convey independent visual information.

2008-05-31T07:13:00.001+04:30

Belitski A, Gretton A, Magri C, Murayama Y, Montemurro MA, Logothetis NK, Panzeri S.
J Neurosci. 2008 May 28;28(22):5696-709.

Local field potentials (LFPs) reflect subthreshold integrative processes that complement spike train measures. However, little is yet known about the differences between how LFPs and spikes encode rich naturalistic sensory stimuli. We addressed this question by recording LFPs and spikes from the primary visual cortex of anesthetized macaques while presenting a color movie. We then determined how the power of LFPs and spikes at different frequencies represents the visual features in the movie. We found that the most informative LFP frequency ranges were 1-8 and 60-100 Hz. LFPs in the range of 12-40 Hz carried little information about the stimulus, and may primarily reflect neuromodulatory inputs. Spike power was informative only at frequencies <12 Hz. We further quantified "signal correlations" (correlations in the trial-averaged power response to different stimuli) and "noise correlations" (trial-by-trial correlations in the fluctuations around the average) of LFPs and spikes recorded from the same electrode. We found positive signal correlation between high-gamma LFPs (60-100 Hz) and spikes, as well as strong positive signal correlation within high-gamma LFPs, suggesting that high-gamma LFPs and spikes are generated within the same network. LFPs <24 Hz shared strong positive noise correlations, indicating that they are influenced by a common source, such as a diffuse neuromodulatory input. LFPs <40 Hz showed very little signal and noise correlations with LFPs >40 Hz and with spikes, suggesting that low-frequency LFPs reflect neural processes that in natural conditions are fully decoupled from those giving rise to spikes and to high-gamma LFPs.

PMID: 18509031

Fulltext: http://www.jneurosci.org/cgi/reprint/28/22/5696

Nature NeuroTechniques

2008-05-28T21:29:00.002+04:30

http://www.nature.com/focus/neurotechniques/index.html

Imagine Jane and Identify John: Face Identity Aftereffects Induced by Imagined Faces

2008-05-24T15:49:00.003+04:30

Jae-Jin Ryu, Karen Borrmann, Avi Chaudhuri
PlosOne

It is not known whether prolonged exposure to perceived and imagined complex visual images produces similar shifts in subsequent perception through selective adaptation. This question is important because a positive finding would suggest that perception and imagery of visual stimuli are mediated by shared neural networks. In this study, we used a selective adaptation procedure designed to induce high-level face-identity aftereffects—a phenomenon in which extended exposure to a particular face facilitates recognition of subsequent faces with opposite features while impairing recognition of all other faces. We report here that adaptation to either real or imagined faces produces a similar shift in perception and that identity boundaries represented in real and imagined faces are equivalent. Together, our results show that identity information contained in imagined and real faces produce similar behavioral outcomes. Our findings of high-level visual aftereffects induced by imagined stimuli can be taken as evidence for the involvement of shared neural networks that mediate perception and imagery of complex visual stimuli.

Free Fulltext: PlosOne

A Map for Horizontal Disparity in Monkey V2

2008-05-24T10:27:00.001+04:30

Gang Chen, Haidong D. Lu, Anna W. Roe
Volume 58, Issue 3, 8 May 2008, Pages 442-450

The perception of visual depth is determined by integration of spatial disparities of inputs from the two eyes. Single cells in visual cortex of monkeys are known to respond to specific binocular disparities; however, little is known about their functional organization. We now show, using intrinsic signal optical imaging and single-unit physiology, that, in the thick stripe compartments of the second visual area (V2), there is a clustered organization of Near cells and Far cells, and moreover, there are topographic maps for Near to Far disparities within V2. Our findings suggest that maps for visual disparity are calculated in V2, and demonstrate parallels in functional organization between the thin, pale, and thick stripes of V2.

Fulltext: ScienceDirect

Saccadic latency during electrical stimulation of the human subthalamic nucleus

2008-05-24T10:12:00.001+04:30

Yasin Temel, Veerle Visser-Vandewalle, R.H.S. Carpenter
Current Biology, Vol 18, R412-R414, 20 May 2008

High-frequency electrical stimulation of the subthalamic nucleus (‘deep brain stimulation’) has rapidly become a popular method for treating patients with Parkinson's disease [1], and is now widely recognised as one of the most effective long-term treatments. So far, the neural mechanisms underlying its effectiveness have been elusive. However, measuring saccadic latency — the time taken to look at a sudden visual stimulus — seems a promising approach. Latency varies randomly from trial to trial, and analysis of the resultant statistical distributions provides information about the parameters of the underlying decision-making mechanisms of the brain. Measurement of these parameters can then provide a sensitive and non-invasive way of quantifying the effects of clinical interventions, and providing information about the underlying neural mechanisms. In a group of Parkinson patients with electrodes previously implanted in the subthalamic nuclear complex, we found that bilateral electrical stimulation dramatically reduces the time taken to initiate a saccade. The effect on the distribution of latency corresponds to an increase in the rate of accumulation of the underlying decision signal, suggesting that stimulating this region specifically enhances the gain of descending pathways through the basal ganglia that contribute to saccadic initiation.

Fulltext: http://download.current-biology.com/pdfs/0960-9822/PIIS0960982208003059.pdf

Prefrontal-inferotemporal interaction is not always necessary for reversal learning.

2008-05-24T08:16:00.000+04:30

Wilson CR, Gaffan D.
J Neurosci. 2008 May 21;28(21):5529-38.

Prefrontal cortex (PFC) is thought to have a wide-ranging role in cognition, often described as executive function or behavioral inhibition. A specific example of such a role is the inhibition of representations in more posterior regions of cortex in a "top-down" manner, a function thought to be tested by reversal learning tasks. The direct action of PFC on posterior regions can be directly tested by disconnecting PFC from the region in question. We tested whether PFC directly inhibits visual object representations in inferotemporal cortex (IT) during reversal learning by studying the effect, in macaque monkeys, of disconnecting PFC from IT by crossed unilateral ablations. We tested two visual object reversal learning tasks, namely serial and concurrent reversal learning. We found that the disconnection severely impairs serial reversal learning but leaves concurrent reversal learning completely intact. Thus, PFC cannot be said to always have direct inhibitory control over visual object representations in reversal learning. Furthermore, our results cannot be explained by generalized theories of PFC function such as executive function and behavioral inhibition, because those theories do not make predictions that differentiate different forms of reversal learning. The results do, however, support our proposal, based on other experimental evidence from macaque monkeys, that PFC has a highly specific role in the representation of temporally complex events.

PMID: 18495887

Fulltext: http://www.jneurosci.org/cgi/reprint/28/21/5529

Decision-making with multiple alternatives.

2008-05-21T13:58:00.002+04:30

Churchland AK, Kiani R, Shadlen MN.
Nat Neurosci. 2008 May 18.

Simple perceptual tasks have laid the groundwork for understanding the neurobiology of decision-making. Here, we examined this foundation to explain how decision-making circuitry adjusts in the face of a more difficult task. We measured behavioral and physiological responses of monkeys on a two- and four-choice direction-discrimination decision task. For both tasks, firing rates in the lateral intraparietal area appeared to reflect the accumulation of evidence for or against each choice. Evidence accumulation began at a lower firing rate for the four-choice task, but reached a common level by the end of the decision process. The larger excursion suggests that the subjects required more evidence before making a choice. Furthermore, on both tasks, we observed a time-dependent rise in firing rates that may impose a deadline for deciding. These physiological observations constitute an effective strategy for handling increased task difficulty. The differences appear to explain subjects' accuracy and reaction times.

PMID: 18488024

The neural systems that mediate human perceptual decision making

2008-05-21T08:09:00.001+04:30

Hauke R. Heekeren, Sean Marrett & Leslie G. Ungerleider
Nature Reviews Neuroscience 9, 467-479 (June 2008) | doi:10.1038/nrn2374

Heekeren and colleagues review neurophysiological and neuroimaging studies of monkeys and humans making perceptual decisions, highlighting both the similarities and the differences in their decision-making processes and providing a new model for the neural architecture that underlies perceptual decision making in humans.

Fulltext: http://www.nature.com/nrn/journal/v9/n6/pdf/nrn2374.pdf

Fragment-Based Learning of Visual Object Categories

2008-05-20T12:17:00.000+04:30

Jay Hegdé, Evgeniy Bart, Daniel Kersten
Current Biology, Vol 18, 597-601, 22 April 2008

When we perceive a visual object, we implicitly or explicitly associate it with a category we know [1, 2, 3]. It is known that the visual system can use local, informative image fragments of a given object, rather than the whole object, to classify it into a familiar category [4, 5, 6, 7, 8]. How we acquire informative fragments has remained unclear. Here, we show that human observers acquire informative fragments during the initial learning of categories. We created new, but naturalistic, classes of visual objects by using a novel “virtual phylogenesis” (VP) algorithm that simulates key aspects of how biological categories evolve. Subjects were trained to distinguish two of these classes by using whole exemplar objects, not fragments. We hypothesized that if the visual system learns informative object fragments during category learning, then subjects must be able to perform the newly learned categorization by using only the fragments as opposed to whole objects. We found that subjects were able to successfully perform the classification task by using each of the informative fragments by itself, but not by using any of the comparable, but uninformative, fragments. Our results not only reveal that novel categories can be learned by discovering informative fragments but also introduce and illustrate the use of VP as a versatile tool for category-learning research.

Fulltext: http://download.current-biology.com/pdfs/0960-9822/PIIS096098220800448X.pdf

Electrical microstimulation thresholds for behavioral detection and saccades in monkey frontal eye fields.

2008-05-19T07:20:00.001+04:30

Murphey DK, Maunsell JH
Proc Natl Acad Sci U S A. 2008 May 13.

The frontal eye field (FEF) is involved in the transformation of visual signals into saccadic eye movements. Although it is often considered an oculomotor structure, several lines of evidence suggest that the FEF also contributes to visual perception and attention. To better understand the range of behaviors to which the FEF can contribute, we tested whether monkeys could detect activation of their FEF by electrical microstimulation with currents below those that cause eye movements. We found that stimulation of FEF neurons could almost always be detected at levels below those needed to generate saccades and that the electrical current needed for detection was highly correlated with that needed to generate a saccade. This relationship between detection and saccade thresholds can be explained if FEF neurons represent preparation to make particular saccades and subjects can be aware of such preparations without acting on them when the representation is not strong.

PMID: 18477698

Free Fulltext: http://www.pnas.org/cgi/reprint/0710820105v1

Electric stimulation fMRI of the perforant pathway to the rat hippocampus

2008-05-19T07:14:00.003+04:30

Canals S, Beyerlein M, Murayama Y, Logothetis NK.
Magn Reson Imaging. 2008 May 12.

The hippocampal formation is a brain system that is implicated in learning and memory. The major input to the hippocampus arrives from the entorhinal cortex (EC) to the dentate gyrus (DG) through the perforant path. In the present work, we have investigated the functional properties of this connection by concomitantly applying electrophysiological techniques, deep-brain electric microstimulation and functional magnetic resonance imaging in anesthetized rats. We systematically delivered different current intensities at diverse stimulation frequencies to the perforant path while recording electrophysiological and blood-oxygenation-level-dependent (BOLD) signals. We observed a linear relationship between the current intensity used to stimulate the hippocampal formation and the amplitude and extension of the induced BOLD response. In addition, we found a frequency-dependent spatial pattern of activation. With stimulation protocols and train frequencies used for kindling, the activity strongly spreads ipsilaterally through the hippocampus, DG, subiculum and EC.

PMID: 18479870

Fulltext: ScienceDirect

PhD Comics

2008-05-13T09:26:00.001+04:30

Transient Induced Gamma-Band Response in EEG as a Manifestation of Miniature Saccades

2008-05-11T07:28:00.002+04:30

Shlomit Yuval-Greenberg, Orr Tomer, Alon S. Keren, Israel Nelken, Leon Y. Deouell
Neuron, Vol 58, 429-441, 08 May 2008

The induced gamma-band EEG response (iGBR) recorded on the scalp is widely assumed to reflect synchronous neural oscillation associated with object representation, attention, memory, and consciousness. The most commonly reported EEG iGBR is a broadband transient increase in power at the gamma range ∼200–300 ms following stimulus onset. A conspicuous feature of this iGBR is the trial-to-trial poststimulus latency variability, which has been insufficiently addressed. Here, we show, using single-trial analysis of concomitant EEG and eye tracking, that this iGBR is tightly time locked to the onset of involuntary miniature eye movements and reflects a saccadic “spike potential.” The time course of the iGBR is related to an increase in the rate of saccades following a period of poststimulus saccadic inhibition. Thus, whereas neuronal gamma-band oscillations were shown conclusively with other methods, the broadband transient iGBR recorded by scalp EEG reflects properties of miniature saccade dynamics rather than neuronal oscillations.

Fulltext: http://download.neuron.org/pdfs/0896-6273/PIIS0896627308003012.pdf

The neural systems that mediate human perceptual decision making

2008-05-11T07:24:00.002+04:30

Heekeren HR, Marrett S, Ungerleider LG.
Nat Rev Neurosci. 2008 May 9

Perceptual decision making is the act of choosing one option or course of action from a set of alternatives on the basis of available sensory evidence. Thus, when we make such decisions, sensory information must be interpreted and translated into behaviour. Neurophysiological work in monkeys performing sensory discriminations, combined with computational modelling, has paved the way for neuroimaging studies that are aimed at understanding decision-related processes in the human brain. Here we review findings from human neuroimaging studies in conjunction with data analysis methods that can directly link decisions and signals in the human brain on a trial-by-trial basis. This leads to a new view about the neural basis of human perceptual decision-making processes.

PMID: 18464792

Short-term memory trace in rapidly adapting synapses of inferior temporal cortex

2008-05-11T07:15:00.001+04:30

Sugase-Miyamoto Y, Liu Z, Wiener MC, Optican LM, Richmond BJ.
PLoS Comput Biol. 2008 May 9;4(5):e1000073.

Visual short-term memory tasks depend upon both the inferior temporal cortex (ITC) and the prefrontal cortex (PFC). Activity in some neurons persists after the first (sample) stimulus is shown. This delay-period activity has been proposed as an important mechanism for working memory. In ITC neurons, intervening (nonmatching) stimuli wipe out the delay-period activity; hence, the role of ITC in memory must depend upon a different mechanism. Here, we look for a possible mechanism by contrasting memory effects in two architectonically different parts of ITC: area TE and the perirhinal cortex. We found that a large proportion (80%) of stimulus-selective neurons in area TE of macaque ITCs exhibit a memory effect during the stimulus interval. During a sequential delayed matching-to-sample task (DMS), the noise in the neuronal response to the test image was correlated with the noise in the neuronal response to the sample image. Neurons in perirhinal cortex did not show this correlation. These results led us to hypothesize that area TE contributes to short-term memory by acting as a matched filter. When the sample image appears, each TE neuron captures a static copy of its inputs by rapidly adjusting its synaptic weights to match the strength of their individual inputs. Input signals from subsequent images are multiplied by those synaptic weights, thereby computing a measure of the correlation between the past and present inputs. The total activity in area TE is sufficient to quantify the similarity between the two images. This matched filter theory provides an explanation of what is remembered, where the trace is stored, and how comparison is done across time, all without requiring delay period activity. Simulations of a matched filter model match the experimental results, suggesting that area TE neurons store a synaptic memory trace during short-term visual memory.

PMID: 18464917

Free Fulltext: http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1000073

Value Representations in the Primate Striatum during Matching Behavior.

2008-05-11T07:12:00.001+04:30

Lau B, Glimcher PW.
Neuron. 2008 May 8;58(3):451-63.

Choosing the most valuable course of action requires knowing the outcomes associated with the available alternatives. The striatum may be important for representing the values of actions. We examined this in monkeys performing an oculomotor choice task. The activity of phasically active neurons (PANs) in the striatum covaried with two classes of information: action-values and chosen-values. Action-value PANs were correlated with value estimates for one of the available actions, and these signals were frequently observed before movement execution. Chosen-value PANs were correlated with the value of the action that had been chosen, and these signals were primarily observed later in the task, immediately before or persistently after movement execution. These populations may serve distinct functions mediated by the striatum: some PANs may participate in choice by encoding the values of the available actions, while other PANs may participate in evaluative updating by encoding the reward value of chosen actions.

PMID: 18466754

Fulltext: http://download.neuron.org/pdfs/0896-6273/PIIS089662730800175X.pdf

Learning to recognize visual objects with microstimulation in inferior temporal cortex

2008-05-10T09:49:00.001+04:30

Kawasaki K, Sheinberg DL.
J Neurophysiol. 2008 May 7

The malleability of object representations by experience is essential for adaptive behavior. It has been hypothesized that neurons in inferior temporal cortex (IT) in monkeys are pivotal in visual association learning, evidenced by experiments revealing changes in neural selectivity following visual learning, as well as by lesion studies, wherein functional inactivation of IT impairs learning. A critical question remaining to be answered is whether IT neuronal activity is sufficient for learning. To address this question directly, we conducted experiments combining visual classification learning with microstimulation in IT. We assessed the effects of IT microstimulation during learning in cases where the stimulation was exclusively informative, conditionally informative, and informative but not necessary for the classification task. The results show that localized microstimulation in IT can be used to establish visual classification learning and the same stimulation applied during learning can predictably bias judgments on subsequent recognition. The effect of induced activity can neither be explained by direct stimulation-motor association nor by simple detection of cortical stimulation. We also found that the learning effects are specific to IT stimulation, as they are not observed by microstimulation in an adjacent auditory area. Our results add the evidence that the differential activity in IT during visual association learning is sufficient for establishing new associations. The results suggest that experimentally manipulated activity patterns within IT can be effectively combined with ongoing visually induced activity during the formation of new associations.

PMID: 18463185

Fulltext: http://jn.physiology.org/cgi/reprint/90247.2008v1

Spatial summation can explain the attentional modulation of neuronal responses to multiple stimuli in area V4.

2008-05-10T07:38:00.000+04:30

Ghose GM, Maunsell JH.
J Neurosci. 2008 May 7;28(19):5115-26

Although many studies have shown that the activity of individual neurons in a variety of visual areas is modulated by attention, a fundamental question remains unresolved: can attention alter the visual representations of individual neurons? One set of studies, primarily relying on the attentional modulations observed when a single stimulus is presented within the receptive field of a neuron, suggests that neuronal selectivities, such as orientation or direction tuning, are not fundamentally altered by attention (Salinas and Abbott, 1997; McAdams and Maunsell, 1999; Treue and Martinez Trujillo, 1999). Another set of studies, relying on modulations observed when multiple stimuli are presented within a receptive field, suggests that attention can alter the weighting of sensory inputs (Moran and Desimone, 1985; Luck et al., 1997; Reynolds et al., 1999; Chelazzi et al., 2001). In these studies, when preferred and nonpreferred stimuli are simultaneously presented, responses are much stronger when attention is directed to the preferred stimulus than when it is directed to the nonpreferred stimulus. In this study, we recorded neuronal responses from individual neurons in visual cortical area V4 to both single and paired stimuli with a variety of attentional allocations and stimulus combinations. For each neuron studied, we constructed a quantitative model of input summation and then tested various models of attention. In many neurons, we are able to explain neuronal responses across the entire range of stimuli and attentional allocations tested. Specifically, we are able to reconcile seemingly inconsistent observations of single and paired stimuli attentional modulation with a new model in which attention can facilitate or suppress specific inputs to a neuron but does not fundamentally alter the integration of these inputs.

PMID: 18463265

Interactions between the superior temporal sulcus and auditory cortex mediate dynamic face/voice integration in rhesus monkeys

2008-05-06T07:30:00.001+04:30

Ghazanfar AA, Chandrasekaran C, Logothetis NK.
J Neurosci. 2008 Apr 23;28(17):4457-69

The existence of multiple nodes in the cortical network that integrate faces and voices suggests that they may be interacting and influencing each other during communication. To test the hypothesis that multisensory responses in auditory cortex are influenced by visual inputs from the superior temporal sulcus (STS), an association area, we recorded local field potentials and single neurons from both structures concurrently in monkeys. The functional interactions between the auditory cortex and the STS, as measured by spectral analyses, increased in strength during presentations of dynamic faces and voices relative to either communication signal alone. These interactions were not solely modulations of response strength, because the phase relationships were significantly less variable in the multisensory condition as well. A similar analysis of functional interactions within the auditory cortex revealed no similar interactions as a function of stimulus condition, nor did a control condition in which the dynamic face was replaced with a dynamic disk mimicking mouth movements. Single neuron data revealed that these intercortical interactions were reflected in the spiking output of auditory cortex and that such spiking output was coordinated with oscillations in the STS. The vast majority of single neurons that were responsive to voices showed integrative responses when faces, but not control stimuli, were presented in conjunction. Our data suggest that the integration of faces and voices is mediated at least in part by neuronal cooperation between auditory cortex and the STS and that interactions between these structures are a fast and efficient way of dealing with the multisensory communication signals.

PMID: 1843452

Fulltext: http://www.jneurosci.org/cgi/reprint/28/17/4457

The effects of visual stimulation and selective visual attention on rhythmic neuronal synchronization in macaque area V4.

2008-05-03T09:04:00.000+04:30

Fries P, Womelsdorf T, Oostenveld R, Desimone R.
J Neurosci. 2008 Apr 30;28(18):4823-35

Selective attention lends relevant sensory input priority access to higher-level brain areas and ultimately to behavior. Recent studies have suggested that those neurons in visual areas that are activated by an attended stimulus engage in enhanced gamma-band (30-70 Hz) synchronization compared with neurons activated by a distracter. Such precise synchronization could enhance the postsynaptic impact of cells carrying behaviorally relevant information. Previous studies have used the local field potential (LFP) power spectrum or spike-LFP coherence (SFC) to indirectly estimate spike synchronization. Here, we directly demonstrate zero-phase gamma-band coherence among spike trains of V4 neurons. This synchronization was particularly evident during visual stimulation and enhanced by selective attention, thus confirming the pattern inferred from LFP power and SFC. We therefore investigated the time course of LFP gamma-band power and found rapid dynamics consistent with interactions of top-down spatial and feature attention with bottom-up saliency. In addition to the modulation of synchronization during visual stimulation, selective attention significantly changed the prestimulus pattern of synchronization. Attention inside the receptive field of the recorded neuronal population enhanced gamma-band synchronization and strongly reduced alpha-band (9-11 Hz) synchronization in the prestimulus period. These results lend further support for a functional role of rhythmic neuronal synchronization in attentional stimulus selection.

PMID: 18448659
Fulltext: http://www.jneurosci.org/cgi/reprint/28/18/4823

Neurophysiology of the BOLD fMRI Signal in Awake Monkeys

2008-05-02T09:11:00.001+04:30

Goense JB, Logothetis NK.
Curr Biol. 2008 Apr 23

BACKGROUND: Simultaneous intracortical recordings of neural activity and blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) in primary visual cortex of anesthetized monkeys demonstrated varying degrees of correlation between fMRI signals and the different types of neural activity, such as local field potentials (LFPs), multiple-unit activity (MUA), and single-unit activity (SUA). One important question raised by the aforementioned investigation is whether the reported correlations also apply to alert subjects. RESULTS: Monkeys were trained to perform a fixation task while stimuli within the receptive field of each recording site were used to elicit neural responses followed by a BOLD response. We show - also in alert behaving monkeys - that although both LFP and MUA make significant contributions to the BOLD response, LFPs are better and more reliable predictors of the BOLD signal. Moreover, when MUA responses adapt but LFP remains unaffected, the BOLD signal remains unaltered. CONCLUSIONS: The persistent coupling of the BOLD signal to the field potential when LFP and MUA have different time evolutions suggests that BOLD is primarily determined by the local processing of inputs in a given cortical area. In the alert animal the largest portion of the BOLD signal's variance is explained by an LFP range (20-60 Hz) that is most likely related to neuromodulation. Finally, the similarity of the results in alert and anesthetized subjects indicates that at least in V1 anesthesia is not a confounding factor. This enables the comparison of human fMRI results with a plethora of electrophysiological results obtained in alert or anesthetized animals.

PMID: 18439825

Fulltext: Science Direct

Theta phase–specific codes for two-dimensional position, trajectory and heading in the hippocampus

2008-04-28T21:07:00.002+04:30

John R Huxter, Timothy J Senior, Kevin Allen, Jozsef Csicsvari
Nature Neuroscience 11, 587 - 594 (2008)

Temporal coding is a means of representing information by the time, as opposed to the rate, at which neurons fire. Evidence of temporal coding in the hippocampus comes from place cells, whose spike times relative to theta oscillations reflect a rat's position while running along stereotyped trajectories. This arises from the backwards shift in cell firing relative to local theta oscillations (phase precession). Here we demonstrate phase precession during place-field crossings in an open-field foraging task. This produced spike sequences in each theta cycle that disambiguate the rat's trajectory through two-dimensional space and can be used to predict movement direction. Furthermore, position and movement direction were maximally predicted from firing in the early and late portions of the theta cycle, respectively. This represents the first direct evidence of a combined representation of position, trajectory and heading in the hippocampus, organized on a fine temporal scale by theta oscillations.

Fulltext: http://www.nature.com/neuro/journal/v11/n5/pdf/nn.2106.pdf

Unconscious determinants of free decisions in the human brain

2008-04-28T21:04:00.001+04:30

Chun Siong Soon, Marcel Brass, Hans-Jochen Heinze, John-Dylan Haynes
Nature Neuroscience 11, 543 - 545 (2008)

There has been a long controversy as to whether subjectively 'free' decisions are determined by brain activity ahead of time. We found that the outcome of a decision can be encoded in brain activity of prefrontal and parietal cortex up to 10 s before it enters awareness. This delay presumably reflects the operation of a network of high-level control areas that begin to prepare an upcoming decision long before it enters awareness.

Fulltext: http://www.nature.com/neuro/journal/v11/n5/pdf/nn.2112.pdf