Inferring spike trains from local field potentials

Rasch MJ, Gretton A, Murayama Y, Maass W, Logothetis NK
J Neurophysiol. 2007 Dec 26

We investigated whether it is possible to infer spike trains solely on the basis of the underling local field potentials (LFPs). Employing support vector machines and linear regression models, we found that in the primary visual cortex (V1) of monkeys, spikes can indeed be inferred from LFPs, at least with moderate success. Although there is a considerable degree of variation across electrodes, the low-frequency structure in spike trains (in the 100 ms range) can be inferred with reasonable accuracy, whereas exact spike positions are not reliably predicted. Two kinds of features of the LFP are exploited for prediction: the frequency power of bands in the high gamma-range (40-90 Hz), and information contained in low-frequency oscillations (<10 Hz), where both phase and power modulations are informative. Information analysis revealed that both features code (mainly) independent aspects of the spike-to-LFP relationship, with the low-frequency LFP phase coding for temporally clustered spiking activity. Although both features and prediction quality are similar during semi-natural movie stimuli and spontaneous activity, prediction performance during spontaneous activity degrades much more slowly with increasing electrode distance. The general trend of data obtained with anesthetized animals is qualitatively mirrored in that of a more limited data set recorded in V1 of awake monkeys. In contrast to the cortical field potentials, thalamic LFPs (e.g. LFPs derived from recordings in dLGN) hold no useful information for predicting spiking activity.

PMID: 18160425

Single neuron studies of inferior temporal cortex

Gross CG.
Neuropsychologia. 2007 Nov 21

This paper reviews our experiments on the response properties of single neurons in inferior temporal (IT) cortex in the monkey that were carried out starting in 1965. It describes situational factors that led us to find neurons sensitive to images of faces and hands and summarizes the basic sensory properties of IT neurons. Subsequent developments on the cognitive properties of IT neurons and on imaging the responses of human temporal cortex to facial images are outlined. Finally, this paper summarizes recent results on fMRI imaging of the responses of temporal cortex to facial images.

PMID: 18155735

Fulltext: Science Direct

Individual faces elicit distinct response patterns in human anterior temporal cortex

Kriegeskorte N, Formisano E, Sorger B, Goebel R.
Proc Natl Acad Sci U S A. 2007 Dec 11

Visual face identification requires distinguishing between thousands of faces we know. This computational feat involves a network of brain regions including the fusiform face area (FFA) and anterior inferotemporal cortex (aIT), whose roles in the process are not well understood. Here, we provide the first demonstration that it is possible to discriminate cortical response patterns elicited by individual face images with high-resolution functional magnetic resonance imaging (fMRI). Response patterns elicited by the face images were distinct in aIT but not in the FFA. Individual-level face information is likely to be present in both regions, but our data suggest that it is more pronounced in aIT. One interpretation is that the FFA detects faces and engages aIT for identification.

PMID: 18077383

Retinotopy of the face aftereffect.

Afraz SR, Cavanagh P.
Vision Res. 2007 Dec 11

Physiological results for the size of face-specific units in inferotemporal cortex (IT) support an extraordinarily large range of possible sizes-from 2.5 degrees to 30 degrees or more. We use a behavioral test of face-specific aftereffects to measure the face analysis regions and find a coarse retinotopy consistent with receptive fields of intermediate size (10 degrees -12 degrees at 3 degrees eccentricity). In the first experiment, observers were adapted to a single face at 3 degrees from fixation. A test (a morph of the face and its anti-face) was then presented at different locations around fixation and subjects classified it as face or anti-face. The face aftereffect (FAE) was not constant at all test locations-it dropped to half its maximum value for tests 5 degrees from the adapting location. Simultaneous adaptation to both a face and its anti-face, placed at opposite locations across fixation, produced two separate regions of opposite aftereffects. However, with four stimuli, faces alternating with anti-faces equally spaced around fixation, the FAE was greatly reduced at all locations, implying a fairly coarse localization of the aftereffect. In the second experiment, observers adapted to a face and its anti-face presented either simultaneously or in alternation. Results showed that the simultaneous presentation of a face and its anti-face leads to stronger FAEs than sequential presentation, suggesting that face processing has a dynamic nature and its region of analysis is sharpened when there is more than one face in the scene. In the final experiment, a face and two anti-face flankers with different spatial offsets were presented during adaptation and the FAE was measured at the face location. Results showed that FAE at the face location was inhibited more as the distance of anti-face flankers to the face stimulus was reduced. This confirms the spatial extent of face analysis regions in a test with a fixed number of stimuli where only distance varied.

PMID: 18078975

Complementary circuits connecting the orbital and medial prefrontal networks with the temporal, insular, and opercular cortex in the macaque monkey

Saleem KS, Kondo H, Price JL
J Comp Neurol. 2008 Feb 1;506(4):659-93.

The origin and termination of axonal connections between the orbital and medial prefrontal cortex (OMPFC) and the temporal, insular, and opercular cortex have been analyzed with anterograde and retrograde axonal tracers, injected in the OMPFC or temporal cortex. The results show that there are two distinct, complementary, and reciprocal neural systems, related to the previously defined "orbital" and "medial" prefrontal networks. The orbital prefrontal network, which includes areas in the central and lateral part of the orbital cortex, is connected with vision-related areas in the inferior temporal cortex (especially area TEav) and the fundus and ventral bank of the superior temporal sulcus (STSf/v), and with somatic sensory-related areas in the frontal operculum (OPf) and dysgranular insular area (Id). No connections were found between the orbital network and auditory areas. The orbital network is also connected with taste and olfactory cortical areas and the perirhinal cortex and appears to be involved in assessment of sensory objects, especially food. The medial prefrontal network includes areas on the medial surface of the frontal lobe, medial orbital areas, and two caudolateral orbital areas. It is connected with the rostral superior temporal gyrus (STGr) and the dorsal bank of the superior temporal sulcus (STSd). This region is rostral to the auditory parabelt areas, and there are only relatively light connections between the auditory areas and the medial network. This system, which is also connected with the entorhinal, parahippocampal, and cingulate/retrosplenial cortex, may be involved in emotion and other self-referential processes. J. Comp. Neurol. 506:659-693, 2008. (c) 2007 Wiley-Liss, Inc.

PMID: 18067141

Neural mechanisms of visual categorization: Insights from neurophysiology

Freedman DJ, Miller EK.
Neurosci Biobehav Rev. 2007 Aug 15

How does the brain recognize the meaning of sensory stimuli? Through experience, we easily learn to group stimuli into meaningful categories such as "chair", "table" and "vehicle". Although much is known about how the brain processes and encodes basic visual features (e.g. color, orientation, and motion direction), much less is known about how the brain learns and represents the behavioral relevance, or category, of stimuli. This article will review a number of recent experiments which suggest that neuronal activity in primate prefrontal, temporal and parietal cortical areas likely plays significant, though complementary, roles in visual categorization and category learning.

PMID: 17950874

Fulltext: sciencedirect

Surface-based and probabilistic atlases of primate cerebral cortex

Van Essen DC, Dierker DL.
Neuron. 2007 Oct 25;56(2):209-25

Brain atlases play an increasingly important role in neuroimaging, as they are invaluable for analysis, visualization, and comparison of results across studies. For both humans and macaque monkeys, digital brain atlases of many varieties are in widespread use, each having its own strengths and limitations. For studies of cerebral cortex there is particular utility in hybrid atlases that capitalize on the complementary nature of surface and volume representations, are based on a population average rather than an individual brain, and include measures of variation as well as averages. Linking different brain atlases to one another and to online databases containing a growing body of neuroimaging data will enable powerful forms of data mining that accelerate discovery and improve research efficiency.

PMID: 17964241

Fulltext: http://www.neuron.org/content/article/abstract?uid=PIIS089662730700760X

Trade-off between object selectivity and tolerance in monkey inferotemporal cortex

Zoccolan D, Kouh M, Poggio T, DiCarlo JJ
J Neurosci. 2007 Nov 7;27(45):12292-307

Object recognition requires both selectivity among different objects and tolerance to vastly different retinal images of the same object, resulting from natural variation in (e.g.) position, size, illumination, and clutter. Thus, discovering neuronal responses that have object selectivity and tolerance to identity-preserving transformations is fundamental to understanding object recognition. Although selectivity and tolerance are found at the highest level of the primate ventral visual stream [the inferotemporal cortex (IT)], both properties are highly varied and poorly understood. If an IT neuron has very sharp selectivity for a unique combination of object features ("diagnostic features"), this might automatically endow it with high tolerance. However, this relationship cannot be taken as given; although some IT neurons are highly object selective and some are highly tolerant, the empirical connection of these key properties is unknown. In this study, we systematically measured both object selectivity and tolerance to different identity-preserving image transformations in the spiking responses of a population of monkey IT neurons. We found that IT neurons with high object selectivity typically have low tolerance (and vice versa), regardless of how object selectivity was quantified and the type of tolerance examined. The discovery of this trade-off illuminates object selectivity and tolerance in IT and unifies a range of previous, seemingly disparate results. This finding also argues against the idea that diagnostic conjunctions of features guarantee tolerance. Instead, it is naturally explained by object recognition models in which object selectivity is built through AND-like tuning mechanisms.

PMID: 17989294

Fulltext: http://www.jneurosci.org/cgi/content/full/27/45/12292

The representation of multiple objects in prefrontal neuronal delay activity

Warden MR, Miller EK
Cereb Cortex. 2007 Sep;17 Suppl 1:i41-50.

The ability to retain multiple items in short-term memory is fundamental for intelligent behavior, yet little is known about its neural basis. To explore the mechanisms underlying this ability, we trained 2 monkeys to remember a sequence of 2 objects across a short delay. We then recorded the activity of neurons from the lateral prefrontal cortex during task performance and found that most neurons had activity that depended on the identity of both objects while a minority reflected just one object. Further, the activity driven by a particular combination of objects was not a simple addition of the activity elicited by individual objects. Instead, the representation of the first object was altered by the addition of the second object to memory, and the form of this change was not systematically predictable. These results indicate that multiple objects are not stored in separate groups of prefrontal neurons. Rather, they are represented by a single population of neurons in a complex fashion. We also found that the strength of the memory trace associated with each object decayed over time, leading to a relatively stronger representation of more recently seen objects. This is a potential mechanism for representing the temporal order of objects.

PMID: 17726003

Fulltext: http://cercor.oxfordjournals.org/cgi/content/full/17/suppl_1/i41

Electrical Stimulation of the Midbrain for Hearing Restoration: Insight into the Functional Organization of the Human Central Auditory System

Hubert H. Lim, Thomas Lenarz, Gert Joseph, Rolf-Dieter Battmer, Amir Samii, Madjid Samii, James F. Patrick, and Minoo Lenarz

The cochlear implant can restore speech perception in patients with sensorineural hearing loss. However, it is ineffective for those without an implantable cochlea or a functional auditory nerve. These patients can be implanted with the auditory brainstem implant (ABI), which stimulates the surface of the cochlear nucleus. Unfortunately, the ABI has achieved limited success in its main patient group [i.e., those with neurofibromatosis type 2 (NF2)] and requires a difficult surgical procedure. These limitations have motivated us to develop a new hearing prosthesis that stimulates the midbrain with a penetrating electrode array. We recently implanted three patients with the auditory midbrain implant (AMI), and it has proven to be safe with minimal movement over time. The AMI provides loudness, pitch, temporal, and directional cues, features that have shown to be important for speech perception and more complex sound processing. Thus far, all three patients obtain enhancements in lip reading capabilities and environmental awareness and some improvements in speech perception comparable with that of NF2 ABI patients. Considering that our midbrain target is more surgically exposable than the cochlear nucleus, this argues for the use of the AMI as an alternative to the ABI. Fortunately, we were able to stimulate different midbrain regions in our patients and investigate the functional organization of the human central auditory system. These findings provide some insight into how we may need to stimulate the midbrain to improve hearing performance with the AMI.

Abstract: http://www.jneurosci.org/cgi/content/abstract/27/49/13541

Correlation between neural spike trains increases with firing rate

de la Rocha J, Doiron B, Shea-Brown E, Josić K, Reyes A.
Nature. 2007 Aug 16;448(7155):802-6

Populations of neurons in the retina, olfactory system, visual and somatosensory thalamus, and several cortical regions show temporal correlation between the discharge times of their action potentials (spike trains). Correlated firing has been linked to stimulus encoding, attention, stimulus discrimination, and motor behaviour. Nevertheless, the mechanisms underlying correlated spiking are poorly understood, and its coding implications are still debated. It is not clear, for instance, whether correlations between the discharges of two neurons are determined solely by the correlation between their afferent currents, or whether they also depend on the mean and variance of the input. We addressed this question by computing the spike train correlation coefficient of unconnected pairs of in vitro cortical neurons receiving correlated inputs. Notably, even when the input correlation remained fixed, the spike train output correlation increased with the firing rate, but was largely independent of spike train variability. With a combination of analytical techniques and numerical simulations using 'integrate-and-fire' neuron models we show that this relationship between output correlation and firing rate is robust to input heterogeneities. Finally, this overlooked relationship is replicated by a standard threshold-linear model, demonstrating the universality of the result. This connection between the rate and correlation of spiking activity links two fundamental features of the neural code.

PMID: 17700699

A temporal frequency-dependent functional architecture in human V1 revealed by high-resolution fMRI

Sun P, Ueno K, Waggoner RA, Gardner JL, Tanaka K, Cheng K.
Nat Neurosci. 2007 Oct 14;

Although cortical neurons with similar functional properties often cluster together in a columnar organization, only ocular dominance columns, the columnar structure representing segregated anatomical input (from one of the two eyes), have been found in human primary visual cortex (V1). It has yet to be shown whether other columnar organizations that arise only from differential responses to stimulus properties also exist in human V1. Using high-resolution functional magnetic resonance imaging, we have found such a functional architecture containing domains that respond preferentially to either low or high temporal frequency.

PMID: 17934459

Shape Selectivity for Camouflage-Breaking Dynamic Stimuli in Dorsal V4 Neurons

Mysore SG, Vogels R, Raiguel SE, Orban GA.
Cereb Cortex. 2007 Oct 12;

Motion is a potent cue for breaking camouflage in the natural world. To understand the neural basis of this phenomenon, one must utilize moving shapes defined by coherent motion of random texture elements against a similar, but stationary texture. To investigate how well neurons in area V4 process this novel, ecologically relevant stimulus and to compare shape selectivity for these shapes with static and other moving shapes, we tested V4 neurons with 5 static or moving shapes defined either by luminance or kinetic cues. The kinetic cues included a temporal frequency cue due to the difference in temporal frequencies of the moving dots inside the shape boundary and stationary dots outside the boundary. Therefore, static opponent motion-defined shapes without this cue were tested as an additional control. Approximately 44% (95/216) of V4 neurons showed shape selectivity. Analyses of these selective neurons both at single-neuron and population levels revealed that the shape-selective V4 neurons responded selectively to the moving kinetic shapes and that these neurons demonstrated robust invariance for shape preference across different shape conditions. Cue-invariant shape selectivity was more pronounced when kinetic shapes included the temporal frequency cue. This invariance may be rooted in nonlinearities occurring early in the visual pathway.

PMID: 17934186

TMS evidence for the involvement of the right occipital face area in early face processing

Pitcher D, Walsh V, Yovel G, Duchaine B.
Curr Biol. 2007 Sep 18;17(18):1568-73. Epub 2007 Aug 30.

Extensive research has demonstrated that several specialized cortical regions respond preferentially to faces [1-6]. One such region, located in the inferior occipital gyrus, has been dubbed the occipital face area (OFA) [7]. The OFA is the first stage in two influential face-processing models [8, 9], both of which suggest that it constructs an initial representation of a face, but how and when it does so remains unclear. The present study revealed that repetitive transcranial magnetic stimulation (rTMS) targeted at the right OFA (rOFA) disrupted accurate discrimination of face parts but had no effect on the discrimination of spacing between these parts. rTMS to left OFA had no effect. A matched part and spacing discrimination task that used house stimuli showed no impairment. In a second experiment, rTMS to rOFA replicated the face-part impairment but did not produce the same effect in an adjacent area, the lateral occipital cortex. A third experiment delivered double pulses of TMS separated by 40 ms at six periods after stimulus presentation during face-part discrimination. Accuracy dropped when pulses were delivered at 60 and 100 ms only. These findings indicate that the rOFA processes face-part information at an early stage in the face-processing stream.

PMID: 17764942

The timecourse of higher-level face aftereffects.

Rhodes G, Jeffery L, Clifford CW, Leopold DA.
Vision Res. 2007 Aug;47(17):2291-6. Epub 2007 Jul 9

Perceptual aftereffects for simple visual attributes processed early in the cortical hierarchy increase logarithmically with adapting duration and decay exponentially with test duration. This classic timecourse has been reported recently for a face identity aftereffect [Leopold, D. A., Rhodes, G., Müller, K.-M., & Jeffery, L. (2005). The dynamics of visual adaptation to faces. Proceedings of the Royal Society of London, Series B, 272, 897-904], suggesting that the dynamics of visual adaptation may be similar throughout the visual system. An alternative interpretation, however, is that the classic timecourse is a flow-on effect of adaptation of a low-level, retinotopic component of the face identity aftereffect. Here, we examined the timecourse of the higher-level (size-invariant) components of two face aftereffects, the face identity aftereffect and the figural face aftereffect. Both showed the classic pattern of logarithmic build-up and exponential decay. These results indicate that the classic timecourse of face aftereffects is not a flow-on effect of low-level retinotopic adaptation, and support the hypothesis that dynamics of visual adaptation are similar at higher and lower levels of the cortical visual hierarchy. They also reinforce the perceptual nature of face aftereffects, ruling out demand characteristics and other post-perceptual factors as plausible accounts.

PMID: 17619045

Fall Break

Recent History of Stimulus Speeds Affects the Speed Tuning of Neurons in Area MT

Anja Schlack, Bart Krekelberg, Thomas D. Albright
The Journal of Neuroscience, October 10, 2007, 27(41):11009-11018

Visual motion processing plays a key role in enabling primates' successful interaction with their dynamic environments. Although in natural environments the speed of visual stimuli continuously varies, speed tuning of neurons in the prototypical motion area MT has traditionally been assessed with stimuli that moved at constant speeds. We investigated whether the representation of speed in a continuously varying stimulus context differs from the representation of constant speeds. We recorded from individual MT neurons of fixating macaques while stimuli moved either at a constant speed or in a linearly accelerating or decelerating manner. We found clear speed tuning even when the stimulus consisted of visual motion with gradual speed changes. There were, however, important differences with the speed tuning as measured with constant stimuli: the stimulus context affected neuronal preferred speed as well as the associated tuning width of the speed tuning curves. These acceleration-dependent changes in response lead to an accurate representation of the acceleration of these stimuli in the MT cells. To elucidate the mechanistic basis of this signal, we constructed a stochastic firing rate model based on the constant speed response profiles. This model incorporated each cell's speed tuning and response adaptation dynamics and accurately predicted the response to constant speeds as well as accelerating and decelerating stimuli. Because the response of the model neurons had no explicit acceleration dependence, we conclude that speed-dependent adaptation creates a strong influence of temporal context on the MT response and thereby results in the representation of acceleration signals.

Perseverance pays off

Chris Rowan
POSTDOC JOURNAL, NATURE|Vol 449|11 October 2007

At long last, I've published the final part of my PhD research. Now what?

I've just heard that the final bit of research from my PhD has been accepted for publication. I'm obviously pleased, but I feel more like sighing in relief than punching the air. It's taken me two-and-a-half years, and two major rewrites, to get to this point — and there's only so many times I can read and recraft the same sentences before I start to get jaded....

Free Fulltext: http://www.nature.com/naturejobs/2007/071011/full/nj7163-752c.html

Mutating Pictures Gallery

A population of 1,000 random pictures each, created in October 2007. You allow the fittest pictures to survive. The higher your rating for a pic the more mutated offspring it produces.

http://mutatingpictures.com/
Read more: http://blogoscoped.com/archive/2007-10-09-n18.html
Results: http://mutatingpictures.com/progress

What would you do if you could publish only 20 papers throughout your career?

Editorial
NATURE MEDICINE VOLUME 13 | NUMBER 10 | OCTOBER 2007

There are researchers who think that something is seriously
wrong with the way science is published.
Some complain about power: “Editors of ‘high-profile’
journals have too much power over scientists, because their
decisions profoundly affect one’s chances to get a grant/job/
tenure.” However, with a number of excellent journals to
choose from, an important paper should find a good home
in one of them, despite any poor editorial decisions from the
others. Besides, high-profile status is ultimately conferred to
a journal by the community, and publications that don’t do
a good job of filtering scientific information (an important
raison d’etre for professional editors) do pay the price.
Others complain about money: “Publishing firms make a
fortune selling back to universities the papers that scientists
produce.” The open-access movement stems at least in part
from this complaint. So much ink has been spilled over this
argument that it is hard to say anything new. Instead, let’s
acknowledge that if there are so many journals on the market,
it’s because of the tendency of authors to submit very incremental
papers—what one could call ‘minimal publishable
units’. There are as many scientists who ‘slice the salami’ as
thin as possible as there are specialized journals willing to
publish their work.
It’s certainly possible to argue that the proliferation of papers
also results from an explosion of research disciplines, each of
which needs its own set of journals, and from the need the community
has for the independent replication of published results.
One could also say that trainees need papers to advance to the
next stage of their careers and that researchers need papers to
show funding agencies where their money has been going.
For the sake of argument, though, let’s agree that there are
problems about cost and power in scientific publishing that
need to be fixed. So, here’s a bold proposal to reduce both the
number of publications that your library has to pay for and
the influence of editors on what is granted visibility in highprofile
journals—let’s set a limit on the number of papers that
scientists can publish during their careers.
These are the basic rules: whenever you get your first academic
job (that is, the first lab of your own), you get 20 tickets.
Ticket scalpers
What would you do if you could publish only 20 papers throughout your career?
Every time you publish a paper, you hand over one of them.
Once you run out of tickets, your publishing days are over. As
simple as that.
If we adopted this model, many articles reporting incremental
advances would no longer be written, and many specialized
journals would disappear. And with far fewer papers to read,
each one reporting a much more complete piece of research,
search committees or funding bodies could directly evaluate
the work of a given scientist, instead of (as is often the case)
leaning on surrogate indicators such as a journal’s impact factor
or number of citations.
At the extreme, we might not even need journals (and editors)
anymore; everything would be published in preprint
servers like those used by physicists, and the community
would simply evaluate and rank the different contributions
as they become available. This way, the whole community could
act as reviewers, doing away with the existing peer-review process.
This is somewhat reminiscent of what some websites are
already trying to do, so far with limited success. But if everybody
agreed to publish just 20 papers to keep the size of the
literature manageable, then the journal of the future might
conceivably be a preprint server.
As this model is adopted, it will certainly need some tweaking.
Maybe a particularly seminal paper would be exempt from
ticket usage. Would review articles require a ticket? Maybe a
much coveted award could come with half an extra ticket,
and a very competitive postdoctoral fellowship could get you
one-fourth of a ticket. If you collaborate with another lab, you
could be sole senior author of a paper at the expense of one
ticket or, alternatively, ‘split the bill’ with your collaborator.
Authors of fraudulent papers could be penalized by taking away
from them, say, three tickets. Maybe we would even see the birth
of a new class of ticket scalper and the production of counterfeit
tickets. And if you are one of those extraordinarily persuasive
scientists, you can always try to convince another researcher to
give you one of their tickets so you can publish another influential
piece of work.
The key question is: if you are unhappy with scientific publishing,
would you agree to the 20-paper limit?

Sex-contingent face aftereffects depend on perceptual category rather than structural encoding

Bestelmeyer PE, Jones BC, Debruine LM, Little AC, Perrett DI, Schneider A, Welling LL, Conway CA.

Many studies have used visual adaptation to investigate how recent experience with faces influences perception. While faces similar to those seen during adaptation phases are typically perceived as more 'normal' after adaptation, it is possible to induce aftereffects in one direction for one category (e.g. female) and simultaneously induce aftereffects in the opposite direction for another category (e.g. male). Such aftereffects could reflect 'category-contingent' adaptation of neurons selective for perceptual category (e.g. male or female) or 'structure-contingent' adaptation of lower-level neurons coding the physical characteristics of different face patterns. We compared these explanations by testing for simultaneous opposite after effects following adaptation to (a) two groups of faces from distinct sex categories (male and female) or (b) two groups of faces from the same sex category (female and hyper-female) where the structural differences between the female and hyper-female groups were mathematically identical to those between male and female groups. We were able to induce opposite aftereffects following adaptation between sex categories but not after adaptation within a sex category. These findings indicate the involvement of neurons coding perceptual category in sex-contingent face aftereffects and cannot be explained by neurons coding only the physical aspects of face patterns.

PMID: 17870064

Dynamics of Visual Information Integration in the Brain for Categorizing Facial Expressions

Philippe G. Schyns, Lucy S. Petro, Marie L. Smith
Current Biology, Volume 17, Issue 18, 18 September 2007, Pages 1580-1585

A key to understanding visual cognition is to determine when, how, and with what information the human brain distinguishes between visual categories. So far, the dynamics of information processing for categorization of visual stimuli has not been elucidated. By using an ecologically important categorization task (seven expressions of emotion), we demonstrate, in three human observers, that an early brain event (the N170 Event Related Potential, occurring 170 ms after stimulus onset [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] and [16]) integrates visual information specific to each expression, according to a pattern. Specifically, starting 50 ms prior to the ERP peak, facial information tends to be integrated from the eyes downward in the face. This integration stops, and the ERP peaks, when the information diagnostic for judging a particular expression has been integrated (e.g., the eyes in fear, the corners of the nose in disgust, or the mouth in happiness). Consequently, the duration of information integration from the eyes down determines the latency of the N170 for each expression (e.g., with “fear” being faster than “disgust,” itself faster than “happy”). For the first time in visual categorization, we relate the dynamics of an important brain event to the dynamics of a precise information-processing function.

Technical breakthroughs in neuroscience

Journal of Visualized Experiments

http://www.myjove.com/ is a web site which is named YouTube for Science.

Specificity and randomness in the visual cortex

Ohki K, Reid RC
Curr Opin Neurobiol. 2007 Aug 24;

Research on the functional anatomy of visual cortical circuits has recently zoomed in from the macroscopic level to the microscopic. High-resolution functional imaging has revealed that the functional architecture of orientation maps in higher mammals is built with single-cell precision. By contrast, orientation selectivity in rodents is dispersed on visual cortex in a salt-and-pepper fashion, despite highly tuned visual responses. Recent studies of synaptic physiology indicate that there are disjoint subnetworks of interconnected cells in the rodent visual cortex. These intermingled subnetworks, described in vitro, may relate to the intermingled ensembles of cells tuned to different orientations, described in vivo. This hypothesis may soon be tested with new anatomic techniques that promise to reveal the detailed wiring diagram of cortical circuits.

PMID: 17720489

NeuroMorpho.org

NeuroMorpho.org is a centrally curated inventory of digitally reconstructed neurons. NeuroMorpho.Org contains contributions from over two-dozen labs and is continuously updated as new morphological reconstructions are collected, published, and shared, with the goal of densely covering all available data.

Iranian Affiliations in SFN

It is quite interesting that there are 12 abstracts for this year's society for neuroscience meeting that have Iranian affiliations.
Just go to SFN website ( http://www.sfn.org and go to the abstracts section) or simply follow this link and search for "Iran".
Following are the titles:

1	(24.8/MMM8) Concepts of caloric restriction in the therapy of epilepsy by Avicenna and Rhazas	M. RAZA1, N. PARINEJAD
2	(750.22/HHH2) Does classical music influence depression?	M. MOHAMMADIAN1, *P. MOHAMMADIAN, M. AMINI, K. HASHEMIAN
3	(29.7/PPP27-PPP28) Annual workshops: taking the excitement of neuroscience to high school pupils in Iran	A. LAK1,2, *M. OMRANI3,1,2, B. BABADI1,2, S. R. AFRAZ
4	(590.25/U14) Effect of Ethosuximide on parkinsonian tremor: insight to pathophysiology from clinical evidence	M. OMRANI1,2,5, N. ALAVI TABARI2,3, M. A. KHOSHNOODI2,5, M. MOTAMEDI
5	(554.7) The effect of stimulus presentation duration on response properties of inferior temporal cortex of macaque monkeys	K. MIRPOUR1, *H. ESTEKY
6	(375.7/V9) Behavioral effects of caloric restriction in a model of temporal lobe epilepsy in rats	N. PARINEJAD1, S. KESHAVARZI2, *M. RAZA
7	(96.11/VV10) Decreased locomotor and ingestive behaviors accompany increased neuroinflammation in aged BALB/cBy mice	S. J. BONASERA1, V. MOJTAHEDZADEH5, E. H. GOULDING2, K. A. SCHENK3, W. XU4, R. BARBEAU4, L. H. TECOTT
8	(189.1/TT3) Muscle cocontraction following motor learning	M. DARAINY1,2, D. OSTRY
9	(253.4/L10) Network Eigen Kinetic Analysis (NEKA): A novel data-driven approach to modeling the uptake of dopamine in the brain	A. H. ASSADI1, M. E. KLOC2, H. TORABI-DASHTI3, Q. LIU4, C. DAVIDSON4, T. H. LEE
10	(674.15/E6) Purification of neurons from neural stem cells	H. AZARI1, G. OSBORNE2, B. SCHEFFLER3, D. STEINDLER3, *B. A. REYNOLDS
11	(362.12/M5) Type-5 metabotropic glutamate receptor-dependent LTP of excitatory synapses on fast-spiking GABAergic neurons in mouse visual cortex	A. SARIHI1, B. JIANG1,3, A.-R. KOMAKI4, K. SOHYA1, K. OBATA1, Y. YANAGAWA5, *T. TSUMOTO2
12	(683.13/H1) Na,K-ATPase alpha2 isoform heterozygous knockout mice show increased susceptibility to cortical spreading depression and altered neuronal excitability	C. REIFFURTH1, M. ZAHEDI KHORASANI2, M. ALAM1, A. E. MOSELEY3, J. B. LINGREL3, A. FRIEDMAN4, J. P. DREIER

Enhanced affective brain representations of chocolate in cravers vs. non-cravers

Rolls ET, McCabe C.
Eur J Neurosci. 2007 Aug;26(4):1067-1076

To examine the neural circuitry involved in food craving, in making food particularly appetitive and thus in driving wanting and eating, we used fMRI to measure the response to the flavour of chocolate, the sight of chocolate and their combination in cravers vs. non-cravers. Statistical parametric mapping (SPM) analyses showed that the sight of chocolate produced more activation in chocolate cravers than non-cravers in the medial orbitofrontal cortex and ventral striatum. For cravers vs. non-cravers, a combination of a picture of chocolate with chocolate in the mouth produced a greater effect than the sum of the components (i.e. supralinearity) in the medial orbitofrontal cortex and pregenual cingulate cortex. Furthermore, the pleasantness ratings of the chocolate and chocolate-related stimuli had higher positive correlations with the fMRI blood oxygenation level-dependent signals in the pregenual cingulate cortex and medial orbitofrontal cortex in the cravers than in the non-cravers. To our knowledge, this is the first study to show that there are differences between cravers and non-cravers in their responses to the sensory components of a craved food in the orbitofrontal cortex, ventral striatum and pregenual cingulate cortex, and that in some of these regions the differences are related to the subjective pleasantness of the craved foods. Understanding individual differences in brain responses to very pleasant foods helps in the understanding of the mechanisms that drive the liking for specific foods and thus intake of those foods.

PMID: 17714197

Masking disrupts reentrant processing in human visual cortex

Fahrenfort JJ, Scholte HS, Lamme VA
J Cogn Neurosci. 2007 Sep;19(9):1488-97

In masking, a stimulus is rendered invisible through the presentation of a second stimulus shortly after the first. Over the years, authors have typically explained masking by postulating some early disruption process. In these feedforward-type explanations, the mask somehow "catches up" with the target stimulus, disrupting its processing either through lateral or interchannel inhibition. However, studies from recent years indicate that visual perception-and most notably visual awareness itself-may depend strongly on cortico-cortical feedback connections from higher to lower visual areas. This has led some researchers to propose that masking derives its effectiveness from selectively interrupting these reentrant processes. In this experiment, we used electroencephalogram measurements to determine what happens in the human visual cortex during detection of a texture-defined square under nonmasked (seen) and masked (unseen) conditions. Electro-encephalogram derivatives that are typically associated with reentrant processing turn out to be absent in the masked condition. Moreover, extrastriate visual areas are still activated early on by both seen and unseen stimuli, as shown by scalp surface Laplacian current source-density maps. This conclusively shows that feedforward processing is preserved, even when subject performance is at chance as determined by objective measures. From these results, we conclude that masking derives its effectiveness, at least partly, from disrupting reentrant processing, thereby interfering with the neural mechanisms of figure-ground segmentation and visual awareness itself.

PMID: 17714010

Implicit trustworthiness decisions: automatic coding of face properties in the human amygdala

Engell AD, Haxby JV, Todorov A
J Cogn Neurosci. 2007 Sep;19(9):1508-19

Deciding whether an unfamiliar person is trustworthy is one of the most important decisions in social environments. We used functional magnetic resonance imaging to show that the amygdala is involved in implicit evaluations of trustworthiness of faces, consistent with prior findings. The amygdala response increased as perceived trustworthiness decreased in a task that did not demand person evaluation. More importantly, we tested whether this response is due to an individual's idiosyncratic perception or to face properties that are perceived as untrustworthy across individuals. The amygdala response was better predicted by consensus ratings of trustworthiness than by an individual's own judgments. Individual judgments accounted for little residual variance in the amygdala after controlling for the shared variance with consensus ratings. These findings suggest that the amygdala automatically categorizes faces according to face properties commonly perceived to signal untrustworthiness.

PMID: 17714012

Spatio-temporal prediction and inference by V1 neurons

Guo K, Robertson RG, Pulgarin M, Nevado A, Panzeri S, Thiele A, Young MP
Eur J Neurosci. 2007 Aug;26(4):1045-1054

In normal vision, visual scenes are predictable, as they are both spatially and temporally redundant. Evidence suggests that the visual system may use the spatio-temporal regularities of the external world, available in the retinal signal, to extract information from the visual environment and better reconstruct current and future stimuli. We studied this by recording neuronal responses of primary visual cortex (area V1) in anaesthetized and paralysed macaques during the presentation of dynamic sequences of bars, in which spatio-temporal regularities and local information were independently manipulated. Most V1 neurons were significantly modulated by events prior to and distant from stimulation of their classical receptive fields (CRFs); many were more strongly tuned to prior and distant events than they were to CRFs bars; and several showed tuning to prior information without any CRF stimulation. Hence, V1 neurons do not simply analyse local contours, but impute local features to the visual world, on the basis of prior knowledge of a visual world in which useful information can be distributed widely in space and time.

PMID: 17714195

Experience-dependent coding of facial expression in superior temporal sulcus

Nicholas Furl, Nicola J. van Rijsbergen, Alessandro Treves, Karl J. Friston, Raymond J. Dolan
PNAS | August 14, 2007 | vol. 104 | no. 33 | 13485-13489
Sensory information from the external world is inherently ambiguous, necessitating prior experience as a constraint on perception. Prolonged experience (adaptation) induces perception of ambiguous morph faces as a category different from the adapted category, suggesting sensitivity in underlying neural codes to differences between input and recent experience. Using magnetoencephalography, we investigated the neural dynamics of such experience-dependent visual coding by focusing on the timing of responses to morphs after facial expression adaptation. We show that evoked fields arising from the superior temporal sulcus (STS) reflect the degree to which a morph and adapted expression deviate. Furthermore, adaptation effects within STS predict the magnitude of behavioral aftereffects. These findings show that the STS codes expressions relative to recent experience rather than absolutely and may bias perception of expressions. One potential neural mechanism for the late timing of both effects appeals to hierarchical models that ascribe a central role to backward connections in mediating predictive codes.

Human cortical representation of oral temperature

Guest S, Grabenhorst F, Essick G, Chen Y, Young M, McGlone F, de Araujo I, Rolls ET.
Physiol Behav. 2007 Jul 13;

The temperature of foods and fluids is a major factor that determines their pleasantness and acceptability. Studies of nonhuman primates have shown that many neurons in cortical taste areas receive and process not only chemosensory inputs, but oral thermosensory (temperature) inputs as well. We investigated whether changes in oral temperature activate these areas in humans, or middle or posterior insular cortex, the areas most frequently identified for the encoding of temperature information from the human hand. In the fMRI study we identified areas of activation in response to innocuous, temperature-controlled (cooled and warmed, 5, 20 and 50 degrees C) liquid introduced into the mouth. The oral temperature stimuli activated the insular taste cortex (identified by glucose taste stimuli), a part of the somatosensory cortex, the orbitofrontal cortex, the anterior cingulate cortex, and the ventral striatum. Brain regions where activations correlated with the pleasantness ratings of the oral temperature stimuli included the orbitofrontal cortex and pregenual cingulate cortex. We conclude that a network of taste- and reward-responsive regions of the human brain is also activated by intra-oral thermal stimulation, and that the pleasant subjective states elicited by oral thermal stimuli are correlated with the activations in the orbitofrontal cortex and pregenual cingulate cortex. Thus the pleasantness of oral temperature is represented in brain regions shown in previous studies to represent the pleasantness of the taste and flavour of food. Bringing together these different oral representations in the same brain regions may enable particular combinations to influence the pleasantness of foods.

PMID: 17689575
Free Fulltext: Science-Direct

MRIcro free viewing software

http://www.sph.sc.edu/comd/rorden/mricro.html
This software could be used for localizing and comparing lesions across patients and in a patient across the course of disease or treatment. It supports MRI along with other imaging modalities (?).

Action-Related Properties Shape Object Representations in the Ventral Stream

Bradford Z. Mahon, Shawn C. Milleville, Gioia A.L. Negri, Raffaella I. Rumiati, Alfonso Caramazza, Alex Martin
Neuron, Vol 55, 507-520, 02 August 2007

The principles driving the organization of the ventral object-processing stream remain unknown. Here, we show that stimulus-specific repetition suppression (RS) in one region of the ventral stream is biased according to motor-relevant properties of objects. Quantitative analysis confirmed that this result was not confounded with similarity in visual shape. A similar pattern of biases in RS according to motor-relevant properties of objects was observed in dorsal stream regions in the left hemisphere. These findings suggest that neural specificity for “tools” in the ventral stream is driven by similarity metrics computed over motor-relevant information represented in dorsal structures. Support for this view is provided by converging results from functional connectivity analyses of the fMRI data and a separate neuropsychological study. More generally, these data suggest that a basic organizing principle giving rise to “category specificity” in the ventral stream may involve similarity metrics computed over information represented elsewhere in the brain.

Fulltext: http://download.neuron.org/pdfs/0896-6273/PIIS0896627307005387.pdf

Vision and the Brain: Unseen Complexities – Part 2

Why do we need vision? As it turns out, there are two answers to this question. On the one hand, we need vision to give us detailed knowledge of the world beyond ourselves, knowledge that allows us to recognize things from minute to minute and day to day. On the other hand, we also need vision to guide our actions in that world at the very moment they occur. These are two quite different job descriptions, and nature seems to have given us two different visual systems to carry them out. Dr. Goodale discusses how separate but interacting visual systems have evolved for the perception of objects on the one hand and the control of actions directed at those objects on the other, examining how both systems process information but each using the information in different ways.
By: Melvyn Goodale, Ph.D., C.Psych., F.R.S.C., research professor in visual neuroscience, University of Western Ontario
http://www.researchchannel.org/prog/displayevent.aspx?rID=16250&fID=345

Context familiarity enhances target processing by inferior temporal cortex neurons.

Mruczek RE, Sheinberg DL
J Neurosci. 2007 Aug 8;27(32):8533-45

Experience-dependent changes in the response properties of ventral visual stream neurons are thought to underlie our ability to rapidly and efficiently recognize visual objects. How these neural changes are related to efficient visual processing during natural vision remains unclear. Here, we demonstrate a neurophysiological correlate of efficient visual search through highly familiar object arrays. Humans and monkeys are faster at locating the same target when it is surrounded by familiar compared with unfamiliar distractors. We show that this behavioral enhancement is driven by an increased sensitivity of target-selective neurons in inferior temporal cortex. This results from an increased "signal" for target representations and decreased "noise" from neighboring familiar distractors. These data highlight the dynamic properties of the inferior temporal cortex neurons and add to a growing body of evidence demonstrating how experience shapes neural processing in the ventral visual stream.

PMID: 17687031

Fulltext: http://www.jneurosci.org/cgi/content/full/27/32/8533

The unsolved mystery of vision

Richard H. Masland, Paul R. Martin
Current Biology Vol 17 No 15

Vision looms large in neuroscience — it is the subject of a gigantic literature and four Nobel prizes — but there is a growing realization that there are problems with the textbook explanation of how mammalian vision works. Here we will summarize the evidence behind this disquiet. In effect, we shall present a portrait of a field that is ‘stuck’. Our initial focus, because it is our area of expertise, is on evidence that the early steps of mammalian vision are more diverse and more interesting than is usually imagined, so that our understanding of the later stages is in trouble right from the start. But we will also summarize problems, raised by others, with the later stages themselves.

Hardware/Software Hacking: Joining the Real and the Virtual

For the experimental neuroscientists who need to connect their computers to the external world!

Research Channel

http://www.researchchannel.org For Scientific lecture fans! Who download the entire Internet! ;-)
Specifically see Science section, with some lectures on Neurosciences.

Is That My Brother? Perceptual and Neurobiological Factors in Face Blindness

Face blindness (technically known as prosopagnosia) is a condition in which people with otherwise normal vision cannot discriminate one ... all » face from another. They may not be able to pick out their own husband or children in a crowded room or even themselves in a mirror. One woman reported she once had to crinkle her face in a crowded rest room to discriminate herself from others in the mirror. This problem can occur through injury to particular areas within the brain (either through head trauma, stoke or surgery), but it can also occur developmentally. In the latter case, the brain appears completely normal, yet developmental prosopagnosics (DP) have never learned to accurately discriminate faces.

There is a large scientific body of work on face perception published in the psychological, social and neurobiological literature, and I will highlight some of the more important findings. I will then discuss work from my own laboratory on perceptual processing of faces; emphasizing training methods we have developed to help individuals with DP identify faces, sometimes for the first time in their lives. This discussion will be complemented by inclusion of documented neurobiological and cognitive changes that accompany the emergence of face recognition abilities.

Technology Insight: future neuroprosthetic therapies for disorders of the nervous system

Richard A Normann
Nature Clinical Practice Neurology (2007) 3, 444-452 doi:10.1038/ncpneuro0556

Most disorders of the nervous system result from localized sensory or motor pathologies attributable to disease or trauma. The emerging field of neuroprosthetics is focused on the development of therapeutic interventions that will be able to restore some of this lost neural function by selective electrical stimulation of sensory or motor pathways, or by harnessing activity recorded from remnant neural pathways. A key element in this restoration of function has been the development of a new generation of penetrating microelectrode arrays that provide unprecedented selective access to the neurons of the CNS and PNS. The active tips of these microelectrode arrays penetrate the nervous tissues and abut against small populations of neurons or nerve fibers, thereby providing selective access to these cells. These electrode arrays are not only beginning to provide researchers with the ability to better study the spatiotemporal information processing performed by the nervous system, they can also form the basis for new therapies for disorders of the nervous system. In this Review, three examples of this new generation of microelectrode arrays are described, as are potential therapeutic applications in blindness and spinal cord injury, and for the control of prosthetic limbs.

Free full text: http://www.nature.com/ncpneuro/journal/v3/n8/pdf/ncpneuro0556.pdf

Behavioural improvements with thalamic stimulation

N. D. Schiff, J. T. Giacino, K. Kalmar, J. D. Victor, K. Baker, M. Gerber, B. Fritz, B. Eisenberg, J. O’Connor, E. J. Kobylarz, S. Farris, A. Machado, C. McCagg, F. Plum, J. J. Fins, A. R. Rezai
Nature 448, 600-603(2 August 2007) doi:10.1038/nature06041

Widespread loss of cerebral connectivity is assumed to underlie
the failure of brain mechanisms that support communication and
goal-directed behaviour following severe traumatic brain injury.
Disorders of consciousness that persist for longer than 12 months
after severe traumatic brain injury are generally considered to be
immutable; no treatment has been shown to accelerate recovery or
improve functional outcome in such cases1,2. Recent studies have
shown unexpected preservation of large-scale cerebral networks in
patients in the minimally conscious state (MCS)3,4, a condition
that is characterized by intermittent evidence of awareness of self
or the environment5. These findings indicate that there might be
residual functional capacity in some patients that could be supported
by therapeutic interventions. We hypothesize that further
recovery in some patients in the MCS is limited by chronic underactivation
of potentially recruitable large-scale networks. Here, in
a 6-month double-blind alternating crossover study, we show that
bilateral deep brain electrical stimulation (DBS) of the central
thalamus modulates behavioural responsiveness in a patient
who remained in MCS for 6 yr following traumatic brain injury
before the intervention. The frequency of specific cognitively
mediated behaviours (primary outcome measures) and functional
limb control and oral feeding (secondary outcome measures)
increased during periods in which DBS was on as compared with
periods in which it was off. Logistic regression modelling shows a
statistical linkage between the observed functional improvements
and recent stimulation history. We interpret the DBS effects as
compensating for a loss of arousal regulation that is normally
controlled by the frontal lobe in the intact brain. These findings
provide evidence that DBS can promote significant late functional
recovery from severe traumatic brain injury. Our observations,
years after the injury occurred, challenge the existing practice of
early treatment discontinuation for patients with only inconsistent
interactive behaviours and motivate further research to
develop therapeutic interventions.

PMID: 17671503
Fulltext: http://www.nature.com/nature/journal/v448/n7153/full/448539a.html

Abnormal FMRI adaptation to unfamiliar faces in a case of developmental prosopamnesia

Williams MA, Berberovic N, Mattingley JB
Curr Biol. 2007 Jul 17;17(14):1259-64

In rare cases, damage to the temporal lobe causes a selective impairment in the ability to learn new faces, a condition known as prosopamnesia [1]. Here we present the case of an individual with prosopamnesia in the absence of any acquired structural lesion. "C" shows intact processing of simple and complex nonface objects, but her ability to learn new faces is severely impaired. We used a neural marker of perceptual learning known as repetition suppression to examine functioning within C's fusiform face area (FFA), a region of cortex involved in face perception [2]. For comparison, we examined repetition suppression in the scene-selective parahippocampal place area (PPA) [3]. As expected, normal controls showed significant region-specific attenuation of neural activity across repetitions of each stimulus class. C also showed normal attenuation within the PPA to familiar and unfamiliar scenes, and within the FFA to familiar faces. Critically, however, she failed to show any adaptive change within the FFA for repeated unfamiliar faces, despite a face-specific blood-oxygen-dependent response (BOLD) response in her FFA during viewing of face stimuli. Our findings suggest that in developmental prosopamnesia, the FFA cannot maintain stable representations of new faces for subsequent recall or recognition.

PMID: 17614283

Fulltext: http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6VRT-4P48D06-8-B&_cdi=6243&_user=10&_orig=search&_coverDate=07%2F17%2F2007&_sk=999829985&view=c&wchp=dGLbVtz-zSkWb&md5=77db69cb4705be2249a6b7f4aeaca0a0&ie=/sdarticle.pdf

Statistics of Midbrain Dopamine Neuron Spike Trains in the Awake Primate

Bayer HM, Lau B, Glimcher PW
J Neurophysiol. 2007 Jul 5;

Work in behaving primates indicates that midbrain dopamine neurons encode a prediction error, the difference between an obtained reward and the reward expected. Studies of dopamine action potential timing in the alert and anaesthetized rat indicate that dopamine neurons respond in tonic and phasic modes, a distinction that has been less well characterized in the primates. We used spike train models to examine the relationship between the tonic and burst modes of activity in dopamine neurons while monkeys were performing a reinforced visuo-saccadic movement task. We studied spiking activity during four task-related intervals; two of these were intervals during which no task-related events occurred, while two were periods marked by task-related phasic activity. We found that dopamine neuron spike trains during the intervals when no events occurred were well described as tonic. Action potentials appeared to be independent, to occur at low frequency, and to be almost equally well described by Gaussian and Poisson-like (Gamma) processes. Unlike in the rat, interspike intervals as low as 20 ms were often observed during these presumptively tonic epochs. Having identified these periods of presumptively tonic activity we were able to quantitatively define phasic modulations (both increases and decreases in activity) during the intervals in which task-related events occurred. This analysis revealed that the phasic modulations of these neurons include both bursting, as has been described previously, and pausing. Together bursts and pauses seemed to provide a continuous, although non-linear, representation of the theoretically defined reward prediction error of reinforcement learning.

PMID: 17615124

Fulltext: http://jn.physiology.org/cgi/reprint/01140.2006v1

Individuation and holistic processing of faces in rhesus monkeys

Dahl CD, Logothetis NK, Hoffman KL
Proc Biol Sci. 2007 Sep 7;274(1622):2069-76

Despite considerable evidence that neural activity in monkeys reflects various aspects of face perception, relatively little is known about monkeys' face processing abilities. Two characteristics of face processing observed in humans are a subordinate-level entry point, here, the default recognition of faces at the subordinate, rather than basic, level of categorization, and holistic effects, i.e. perception of facial displays as an integrated whole. The present study used an adaptation paradigm to test whether untrained rhesus macaques (Macaca mulatta) display these hallmarks of face processing. In experiments 1 and 2, macaques showed greater rebound from adaptation to conspecific faces than to other animals at the individual or subordinate level. In experiment 3, exchanging only the bottom half of a monkey face produced greater rebound in aligned than in misaligned composites, indicating that for normal, aligned faces, the new bottom half may have influenced the perception of the whole face. Scan path analysis supported this assertion: during rebound, fixation to the unchanged eye region was renewed, but only for aligned stimuli. These experiments show that macaques naturally display the distinguishing characteristics of face processing seen in humans and provide the first clear demonstration that holistic information guides scan paths for conspecific faces.

PMID: 17609192

Free Fulltext: http://www.journals.royalsoc.ac.uk/content/9w71833h18243416/

The Neural Basis of Decision Making

Gold JI, Shadlen MN
Annu Rev Neurosci. 2007 Jul 21;30:535-574

The study of decision making spans such varied fields as neuroscience, psychology, economics, statistics, political science, and computer science. Despite this diversity of applications, most decisions share common elements including deliberation and commitment. Here we evaluate recent progress in understanding how these basic elements of decision formation are implemented in the brain. We focus on simple decisions that can be studied in the laboratory but emphasize general principles likely to extend to other settings.

PMID: 17600525

Fulltext: http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.neuro.29.051605.113038

Multiple dopamine functions at different time courses

Schultz W
Annu Rev Neurosci. 2007;30:259-88

Many lesion studies report an amazing variety of deficits in behavioral functions that cannot possibly be encoded in great detail by the relatively small number of midbrain dopamine neurons. Although hoping to unravel a single dopamine function underlying these phenomena, electrophysiological and neurochemical studies still give a confusing, mutually exclusive, and partly contradictory account of dopamine's role in behavior. However, the speed of observed phasic dopamine changes varies several thousand fold, which offers a means to differentiate the behavioral relationships according to their time courses. Thus dopamine is involved in mediating the reactivity of the organism to the environment at different time scales, from fast impulse responses related to reward via slower changes with uncertainty, punishment, and possibly movement to the tonic enabling of postsynaptic motor, cognitive, and motivational systems deficient in Parkinson's disease.

PMID: 17600522

Fulltext: http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.neuro.28.061604.135722

A Model of V4 Shape Selectivity and Invariance

Cadieu C, Kouh M, Pasupathy A, Connor C, Riesenhuber M, Poggio TA
J Neurophysiol. 2007 Jun 27;

Object recognition in primates is mediated by the ventral visual pathway and is classically described as a feedforward hierarchy of increasingly sophisticated representations. Neurons in macaque monkey area V4, an intermediate stage along the ventral pathway, have been shown to exhibit selectivity to complex boundary conformation and invariance to spatial translation. How could such a representation be derived from the signals in lower visual areas such as V1? We show that a quantitative model of hierarchical processing, which is part of a larger model of object recognition in the ventral pathway, provides a plausible mechanism for the translation-invariant shape representation observed in area V4. Simulated model neurons successfully reproduce V4 selectivity and invariance through a nonlinear, translation-invariant combination of locally selective subunits, suggesting that a similar transformation may occur or culminate in area V4. Specifically, this mechanism models the selectivity of individual V4 neurons to boundary conformation stimuli, exhibits the same degree of translation invariance observed in V4, and produces observed V4 population responses to bars and non-Cartesian gratings. This work provides a quantitative model of the widely described shape selectivity and invariance properties of area V4 and points toward a possible canonical mechanism operating throughout the ventral pathway.

PMID: 17596412

Fulltext: http://jn.physiology.org/cgi/reprint/01265.2006v1

Quantitative comparison between neural response in macaque inferotemporal cortex and behavioral discrimination of photographic images

Allred SR, Jagadeesh B
J Neurophysiol. 2007 Jun 27;

Inferotemporal (IT) cortex plays a critical role in the primate ability to perceive and discriminate between images, but the relationship between responses of single neurons and behavioral capacities is poorly understood. We studied this relationship by recording from IT neurons while monkeys performed a delayed-match-to-sample task with two images. On each day, two sample images were chosen to maximize the selectivity of the neuron, and task difficulty was manipulated by varying sample duration and by masking the sample. On each trial, monkeys reported which of the two sample images was presented. Neural performance was described using an ideal observer analysis. Across the population, neural and behavioral sensitivity to changes in sample duration were indistinguishable. Neural sensitivity was dependent on epoch used to analyze neural response; maximal neural sensitivity was achieved in the 128 ms epoch that began 85 ms after sample onset. At most sample durations, the epoch that yielded optimal neural performance was longer than the sample duration, suggesting that neural selectivity persisted after the presentation of the mask during performance of the task. A control experiment showed that neural and behavioral performance improved in the absence of the mask. These observations suggest that the responses of individual IT neurons contain sufficient information to allow behavioral discrimination of images in a demanding task.

PMID: 17596424

Modulation of neuronal interactions through neuronal synchronization

Womelsdorf T, Schoffelen JM, Oostenveld R, Singer W, Desimone R, Engel AK, Fries P
Science. 2007 Jun 15;316(5831):1609-12

Brain processing depends on the interactions between neuronal groups. Those interactions are governed by the pattern of anatomical connections and by yet unknown mechanisms that modulate the effective strength of a given connection. We found that the mutual influence among neuronal groups depends on the phase relation between rhythmic activities within the groups. Phase relations supporting interactions between the groups preceded those interactions by a few milliseconds, consistent with a mechanistic role. These effects were specific in time, frequency, and space, and we therefore propose that the pattern of synchronization flexibly determines the pattern of neuronal interactions.

PMID: 17569862
Fulltext: http://www.sciencemag.org/cgi/content/full/316/5831/1609

Temporal Activity Patterns in Thermosensory Neurons of Freely Moving Caenorhabditis elegans Encode Spatial Thermal Gradients

Damon A. Clark, Christopher V. Gabel, Harrison Gabel, and Aravinthan D. T. Samuel
The Journal of Neuroscience, June 6, 2007, 27(23):6083-6090;

Our understanding of the operation of neurons and neuronal circuits has come primarily from probing their activity in dissected, anesthetized, or restrained animals. However, the behaviorally relevant operation of neurons and neuronal circuits occurs within intact animals as they freely perform behavioral tasks. The small size and transparency of the nematode Caenorhabditis elegans make it an ideal system for noninvasive, optical measurements of neuronal activity. Here, we use a high signal-to-noise version of cameleon, a fluorescent calcium-binding protein, to quantify the activity of the AFD thermosensory neuron of individual worms freely navigating spatial thermal gradients. We find that AFD activity is directly coupled to the worm's exploratory movements in spatial thermal gradients. We show that the worm is able, in principle, to evaluate and guide its own thermotactic behaviors with respect to ambient spatial thermal gradients by monitoring the activity of this single thermosensory neuron.

Enhanced Category Tuning Revealed by Intracranial Electroencephalograms in High-Order Human Visual Areas

Eran Privman, Yuval Nir, Uri Kramer, Svetlana Kipervasser, Fani Andelman, Miri Y. Neufeld, Roy Mukamel, Yehezkel Yeshurun, Itzhak Fried, and Rafael Malach
The Journal of Neuroscience, June 6, 2007, 27(23):6234-6242;

The functional organization of human sensory cortex was studied by comparing intracranial EEG (iEEG) recordings of local field potentials in neurosurgical patients with functional magnetic resonance imaging (fMRI) obtained in healthy subjects. Using naturalistic movie stimuli, we found a tight correlation between these two measures throughout the human sensory cortex. Importantly, the correlation between the iEEG and fMRI signals was site-specific, exhibiting neuroanatomically specific coupling. In several cortical sites the iEEG activity was confined strictly to one object category. This site selectivity was not limited to faces but included other object categories such as houses and tools. The selectivity of the iEEG signals to images of different object categories was remarkably higher when compared with the selectivity of the corresponding fMRI signals. A plausible interpretation of the fMRI and iEEG results concerns cortical organization in which object categories are organized in a mosaic of narrowly tuned object-selective clusters.

Cortical Connections of Area V4 in the Macaque

Ungerleider LG, Galkin TW, Desimone R, Gattass R.
Cereb Cortex. 2007 Jun 4

To determine the locus, full extent, and topographic organization of cortical connections of area V4 (visual area 4), we injected anterograde and retrograde tracers under electrophysiological guidance into 21 sites in 9 macaques. Injection sites included representations ranging from central to far peripheral eccentricities in the upper and lower fields. Our results indicated that all parts of V4 are connected with occipital areas V2 (visual area 2), V3 (visual area 3), and V3A (visual complex V3, part A), superior temporal areas V4t (V4 transition zone), MT (medial temporal area), and FST (fundus of the superior temporal sulcus [STS] area), inferior temporal areas TEO (cytoarchitectonic area TEO in posterior inferior temporal cortex) and TE (cytoarchitectonic area TE in anterior temporal cortex), and the frontal eye field (FEF). By contrast, mainly peripheral field representations of V4 are connected with occipitoparietal areas DP (dorsal prelunate area), VIP (ventral intraparietal area), LIP (lateral intraparietal area), PIP (posterior intraparietal area), parieto-occipital area, and MST (medial STS area), and parahippocampal area TF (cytoarchitectonic area TF on the parahippocampal gyrus). Based on the distribution of labeled cells and terminals, projections from V4 to V2 and V3 are feedback, those to V3A, V4t, MT, DP, VIP, PIP, and FEF are the intermediate type, and those to FST, MST, LIP, TEO, TE, and TF are feedforward. Peripheral field projections from V4 to parietal areas could provide a direct route for rapid activation of circuits serving spatial vision and spatial attention. By contrast, the predominance of central field projections from V4 to inferior temporal areas is consistent with the need for detailed form analysis for object vision.

PMID: 17548798
Free fulltext: http://cercor.oxfordjournals.org/cgi/reprint/bhm061v1.pdf

Probabilistic reasoning by neurons

Yang T, Shadlen MN.
Nature. 2007 Jun 3;

Our brains allow us to reason about alternatives and to make choices that are likely to pay off. Often there is no one correct answer, but instead one that is favoured simply because it is more likely to lead to reward. A variety of probabilistic classification tasks probe the covert strategies that humans use to decide among alternatives based on evidence that bears only probabilistically on outcome. Here we show that rhesus monkeys can also achieve such reasoning. We have trained two monkeys to choose between a pair of coloured targets after viewing four shapes, shown sequentially, that governed the probability that one of the targets would furnish reward. Monkeys learned to combine probabilistic information from the shape combinations. Moreover, neurons in the parietal cortex reveal the addition and subtraction of probabilistic quantities that underlie decision-making on this task.

PMID: 17546027

Fulltest: http://www.nature.com/nature/journal/vaop/ncurrent/pdf/nature05852.pdf

Victim of the Brain

1988 docudrama about "the ideas of Douglas Hofstadter". It was created by Dutch director Piet Hoenderdos. Features interviews with Doug ... all » Hofstadter and Dan Dennett. Dennett also stars as himself.

Original acquired from the Center for Research in Concepts and Cognition at Indiana University. Uploaded with permission from Douglas Hofstadter.

A Multi-Feature Part-Based Object Detection System

Google Tech Talks May 25, 2007 - Speaker: Bernd Heisele, Honda

ABSTRACT
I will start with an overview on object recognition systems which use local features and ... all » analyze their strengths and weaknesses. I will then present a general purpose part-based object detection system which we evaluated on a benchmark pedestrian detection data set . In a first step, the system computes feature maps from the training images. It then randomly extracts a large number of rectangular parts from the feature maps and clusters the parts based on their feature similarity and their x-y-location in the feature maps. The cluster centers build an initial set of part templates from which the system selects a subset using the gentle-boost algorithm. The localization of the parts during classification is performed by normalized cross-correlation of the part templates with feature maps. Three different types of feature maps were used in our experiments: Original gray value images, the magnitudes of the gradient, and Gabor filtered images. In experiments on a benchmark pedestrian detection database, we investigate how the number of the components, the feature type and the training data affects the detection performance. The system is compared to state-of-the-art pedestrian detectors.

A Labeled-Line Code for Small and Large Numerosities in the Monkey Prefrontal Cortex

Andreas Nieder and Katharina Merten
The Journal of Neuroscience, May 30, 2007, 27(22):5986-5993

How single neurons represent information about the magnitude of a stimulus remains controversial. Neurons encoding purely sensory magnitude typically show monotonic response functions ("summation coding"), and summation units are usually implemented in models of numerosity representation. In contrast, cells representing numerical quantity exhibit nonmonotonic tuning functions that peak at their preferred numerosity ("labeled-line code"), but the restricted range of tested quantities in these studies did not permit a definite answer. Here, we analyzed both behavioral and neuronal representations of a broad range of numerosities from 1 to 30 in the prefrontal cortex of monkeys. Numerosity-selective neurons showed a clear and behaviorally relevant labeled-line code for all numerosities. Moreover, both the behavioral and neuronal tuning functions obeyed the Weber–Fechner Law and were best represented on a nonlinearly compressed scale. Our single-cell study is in good agreement with functional imaging data reporting peaked tuning functions in humans, demonstrating neuronal precursors for human number competence in a nonhuman primate. Our findings also emphasize that the manner in which neurons encode and maintain magnitude information may depend on the precise task at hand as well as the type of magnitude to represent and memorize.

Fulltext: http://www.jneurosci.org/cgi/reprint/27/22/5986

Nature focus on Cerebral Investigations: MRI & PET imaging

Free Fulltext Articles available at Nature web site:
http://www.nature.com/jcbfm/webfocus/mri/index.html

Interactions between higher and lower visual areas improve shape selectivity of higher level neurons-Explaining crowding phenomena

Jehee JF, Roelfsema PR, Deco G, Murre JM, Lamme VA
Brain Res. 2007 Apr 12;

Recent theories of visual perception propose that feedforward cortical processing enables rapid and automatic object categorizations, yet incorporates a limited amount of detail. Subsequent feedback processing highlights high-resolution representations in early visual areas and provides spatial detail. To verify this hypothesis, we separate the contributions of feedforward and feedback signals to the selectivity of cortical neurons in a neural network simulation that is modeled after the hierarchical feedforward-feedback organization of cortical areas. We find that in such a network the responses of high-level neurons can initially distinguish between low-resolution aspects of objects but are 'blind' to differences in detail. After several feedback-feedforward cycles of processing, however, they can also distinguish between objects that differ in detail. Moreover, we find that our model captures recent paradoxical results of crowding phenomena, showing that spatial detail that is lost in visual crowding is nevertheless able to evoke specific adaptation effects. Our results thus provide an existence proof of the feasibility of novel theoretical models and provide a mechanism to explain various psychophysical and physiological results.

PMID: 17540349

Experience-dependent rescaling of entorhinal grids

Caswell Barry, Robin Hayman, Neil Burgess and Kathryn J Jeffery
Nature Neuroscience - 10, 682 - 684 (2007)
Published online: 7 May 2007; | doi:10.1038/nn1905

The firing pattern of entorhinal 'grid cells' is thought to provide an intrinsic metric for space. We report a strong experience-dependent environmental influence: the spatial scales of the grids (which are aligned and have fixed relative sizes within each animal) vary parametrically with changes to a familiar environment's size and shape. Thus grid scale reflects an interaction between intrinsic, path-integrative calculation of location and learned associations to the external environment.

Fulltext: http://www.nature.com/neuro/journal/v10/n6/pdf/nn1905.pdf

Rapid learning in cortical coding of visual scenes

Haishan Yao, Lei Shi, Feng Han, Hongfeng Gao and Yang Dan
Nature Neuroscience - 10, 772 - 778 (2007)
Published online: 29 April 2007; | doi:10.1038/nn1895

Experience-dependent plasticity in adult visual cortex is believed to have important roles in visual coding and perceptual learning. Here we show that repeated stimulation with movies of natural scenes induces a rapid improvement in response reliability in cat visual cortex, whereas stimulation with white noise or flashed bar stimuli does not. The improved reliability can be accounted for by a selective increase in spiking evoked by preferred stimuli, and the magnitude of improvement depends on the sparseness of the response. The increase in reliability persists for at least several minutes in the absence of further movie stimulation. During this period, spontaneous spiking activity shows detectable reverberation of the movie-evoked responses. Thus, repeated exposure to natural stimuli not only induces a rapid improvement in cortical response reliability, but also leaves a 'memory trace' in subsequent spontaneous activity.


Fulltext: http://www.nature.com/neuro/journal/v10/n6/pdf/nn1895.pdf

Only some spatial patterns of fMRI response are read out in task performance

Mark A Williams1, Sabin Dang1 and Nancy G Kanwisher
Nature Neuroscience - 10, 685 - 686 (2007)
Published online: 7 May 2007; | doi:10.1038/nn1900

Classification methods show that the spatial pattern of a functional magnetic resonance imaging response across the cortex contains category information, but whether such patterns are used, or 'read out', in behavioral performance remains untested. We show that although the spatial pattern in both the retinotopic and lateral occipital cortex (LOC) in humans contains category information, only in the LOC is the pattern stronger for correct than for incorrect trials. Thus, some, but not all, spatial patterns are read out during task performance.

Fulltext: http://www.nature.com/neuro/journal/v10/n6/pdf/nn1900.pdf

Primate Reaching Cued by Multichannel Spatiotemporal Cortical Microstimulation

N. A. Fitzsimmons, W. Drake, T. L. Hanson, M. A. Lebedev, and M. A. L. Nicolelis
The Journal of Neuroscience, May 23, 2007, 27(21):5593-5602;

Both humans and animals can discriminate signals delivered to sensory areas of their brains using electrical microstimulation. This opens the possibility of creating an artificial sensory channel that could be implemented in neuroprosthetic devices. Although microstimulation delivered through multiple implanted electrodes could be beneficial for this purpose, appropriate microstimulation protocols have not been developed. Here, we report a series of experiments in which owl monkeys performed reaching movements guided by spatiotemporal patterns of cortical microstimulation delivered to primary somatosensory cortex through chronically implanted multielectrode arrays. The monkeys learned to discriminate microstimulation patterns, and their ability to learn new patterns and new behavioral rules improved during several months of testing. Significantly, information was conveyed to the brain through the interplay of microstimulation patterns delivered to multiple electrodes and the temporal order in which these electrodes were stimulated. This suggests multichannel microstimulation as a viable means of sensorizing neural prostheses.

Fulltext: http://www.jneurosci.org/cgi/reprint/27/21/5593

Motor Learning with Unstable Neural Representations

Uri Rokni, Andrew G. Richardson, Emilio Bizzi and H. Sebastian Seung
Neuron, Vol 54, 653-666, 24 May 2007

It is often assumed that learning takes place by changing an otherwise stable neural representation. To test this assumption, we studied changes in the directional tuning of primate motor cortical neurons during reaching movements performed in familiar and novel environments. During the familiar task, tuning curves exhibited slow random drift. During learning of the novel task, random drift was accompanied by systematic shifts of tuning curves. Our analysis suggests that motor learning is based on a surprisingly unstable neural representation. To explain these results, we propose that motor cortex is a redundant neural network, i.e., any single behavior can be realized by multiple configurations of synaptic strengths. We further hypothesize that synaptic modifications underlying learning contain a random component, which causes wandering among synaptic configurations with equivalent behaviors but different neural representations. We use a simple model to explore the implications of these assumptions.

Fulltext: http://download.neuron.org/pdfs/0896-6273/PIIS0896627307003339.pdf

A Critical Period for Enhanced Synaptic Plasticity in Newly Generated Neurons of the Adult Brain

Shaoyu Ge, Chih-hao Yang, Kuei-sen Hsu, Guo-li Ming and Hongjun Song
Neuron, Vol 54, 559-566, 24 May 2007

Active adult neurogenesis occurs in discrete brain regions of all mammals and is widely regarded as a neuronal replacement mechanism. Whether adult-born neurons make unique contributions to brain functions is largely unknown. Here we systematically characterized synaptic plasticity of retrovirally labeled adult-born dentate granule cells at different stages during their neuronal maturation. We identified a critical period between 1 and 1.5 months of the cell age when adult-born neurons exhibit enhanced long-term potentiation with increased potentiation amplitude and decreased induction threshold. Furthermore, such enhanced plasticity in adult-born neurons depends on developmentally regulated synaptic expression of NR2B-containing NMDA receptors. Our study demonstrates that adult-born neurons exhibit the same classic critical period plasticity as neurons in the developing nervous system. The transient nature of such enhanced plasticity may provide a fundamental mechanism allowing adult-born neurons within the critical period to serve as major mediators of experience-induced plasticity while maintaining stability of the mature circuitry.

Fulltext: http://download.neuron.org/pdfs/0896-6273/PIIS0896627307003340.pdf

Everything is Miscellaneous

Google Tech Talks May 10, 2007
By David Weinberger.

Modulation of Transient and Sustained Response Components of V4 Neurons by Temporal Crowding in Flashed Stimulus Sequences

Brad C. Motter
The Journal of Neuroscience, September 20, 2006 • 26(38):9683–9694

The responses of extrastriate area V4 neurons to flashed visual stimuli were examined to determine whether the responses to stimulus
sequences occurring at normal saccade and fixation timing intervals were degraded relative to longer timing intervals. Stimuli were
flashed in receptive fields in the near periphery while monkeys maintained steady fixation. Short interstimulus intervals (ISIs) resulted
in an overall habituation style response reduction. The transient component of responses to preferred stimuli was suppressed, often
completely, when the ISI was reduced below 100 ms into the range of saccadic durations. The sustained response component weakened
but remained intact. At short ISIs the trailing activity from the preceding stimulus blended with onset activity from the next stimulus,
making it difficult to detect individual stimulus onset events within the spike train. Habituation or conditioning effects were correlated
with the stimulus tuning sensitivity of the neuron but only loosely associated with the actual level of V4 activation elicited by preceding
stimuli. The results suggest that sharply tuned neurons, because of their probabilistic inactivity, are particularly sensitive to temporal
change, whereas the sustained components of broadly tuned neurons could provide a continuity of information flow across visual
transients, such as saccades, that do not alter significantly the view by the neuron of the scene.

A Fast, Reciprocal Pathway between the Lateral Geniculate Nucleus and Visual Cortex in the Macaque Monkey

Farran Briggs and W. Martin Usrey
The Journal of Neuroscience, May 16, 2007, 27(20):5431-5436;

Neurons in the lateral geniculate nucleus (LGN) not only provide feedforward input to primary visual cortex (V1), but also receive robust feedback from the cortex. Accordingly, visual processing in the LGN is continuously influenced by previous patterns of activity. This study examines the temporal properties of feedforward and feedback pathways between the LGN and V1 in the macaque monkey to provide a lower bound on how quickly the cortex can influence the LGN. In so doing, we identified a subclass of corticogeniculate neurons that receives direct, suprathreshold input from the LGN that is similar in latency to that directed to other recipient neurons (4.2 ± 0.4 vs 4.0 ± 0.2 ms). These neurons also provide feedback to the LGN that is significantly shorter in latency than that supplied by corticogeniculate neurons lacking LGN input (5.1 ± 1.3 vs 11.1 ± 2.3 ms, respectively). Across our sample of corticogeniculate neurons, the shortest combined visual response latency and feedback latency was 37 ms (mean, 52.5 ± 3.8 ms), indicating that visual signals can rapidly travel from the periphery to the cortex and back to the LGN.

Fulltext: http://www.jneurosci.org/cgi/reprint/27/20/5431

Visual word processing and experiential origins of functional selectivity in human extrastriate cortex

Baker CI, Liu J, Wald LL, Kwong KK, Benner T, Kanwisher N
Proc Natl Acad Sci U S A. 2007 May 14

How do category-selective regions arise in human extrastriate cortex? Visually presented words provide an ideal test of the role of experience: Although individuals have extensive experience with visual words, our species has only been reading for a few thousand years, a period not thought to be long enough for natural selection to produce a genetically specified mechanism dedicated to visual word recognition per se. Using relatively high-resolution functional magnetic resonance imaging (1.4 x 1.4 x 2-mm voxels), we identified a small region of extrastriate cortex in most participants that responds selectively to both visually presented words and consonant strings, compared with line drawings, digit strings, and Chinese characters. Critically, we show that this pattern of selectivity is dependent on experience with specific orthographies: The same region responds more strongly to Hebrew words in Hebrew readers than in nonreaders of Hebrew. These results indicate that extensive experience with a given visual category can produce strong selectivity for that category in discrete cortical regions.

PMID: 17502592

Return to the RNAi World: Rethinking Gene Expression and Evolution

Google Tech Talks April 9, 2007
The speaker, Craig Mello, was awarded the Nobel Prize in Physiology or Medicine in 2006.

Context-dependent perceptual modulation of single neurons in primate visual cortex

Maier A, Logothetis NK, Leopold DA
Proc Natl Acad Sci U S A. 2007 Mar 27;104(13):5620-5

Some neurons in the visual cortex alter their spiking rate according to the perceptual interpretation of an observed stimulus, rather than its physical structure alone. Experiments in monkeys have suggested that, although the proportion of neurons showing this effect differs greatly between cortical areas, this proportion remains similar across different stimuli. These findings have raised the intriguing questions of whether the same neurons always participate in the disambiguation of sensory patterns and whether such neurons might represent a special class of cortical cells that relay perceptual signals to higher cortical areas. Here we explore this question by measuring activity in the middle temporal cortex of monkeys and asking to what degree the percept-related responses of individual neurons depend upon the specific sensory input. In contrast to our expectations, we found that even small differences in the stimuli led to significant changes in the signaling of the perceptual state by single neurons. We conclude that nearly all feature-responsive neurons in this area, rather than a select subset, can contribute to the resolution of sensory conflict, and that the role of individual cells in signaling the perceptual outcome is tightly linked to the fine details of the stimuli involved.

PMID: 17369363

Free Fulltext: http://www.pnas.org/cgi/reprint/104/13/5620

Effects of Long-Term Object Familiarity on Event-Related Potentials in the Monkey

Jessie J. Peissig, Jedediah Singer, Keisuke Kawasaki, David L. Sheinberg
Cerebral Cortex June 2007;17:1323--1334 doi:10.1093/cercor/bhl043

Although some change in the neural representation of an object
must occur as it becomes familiar, the nature of this change is not
fully understood. In humans, it has been shown that the N170—an
evoked visual potential—is enhanced for classes of objects for
which people have visual expertise. In this study, we explored
whether monkeys show a similar modulation in event-related
potential (ERP) amplitude as a result of long-term familiarity by
recording ERPs with chronically implanted electrodes over extended
training periods spanning many sessions. In each of 3
experiments, we found larger amplitude visual evoked responses to
highly familiar images for the time period of 120--250 ms after
stimulus onset. This difference was found when the monkeys were
trained in an individual-level discrimination task, in a task that
required only color discrimination, and even following a viewingonly
task. We thus observed this familiarity effect across several
tasks and different object categories and further found that the
difference between ‘‘familiar’’ and ‘‘novel’’ became smaller as the
animals gained experience with the previously unfamiliar objects
across multiple test sessions. These data suggest that changes in
visual responses associated with familiarity are evident early in the
evoked visual response, are robust, and may be automatic, driven at
least in part by repeated object exposure.