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

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

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

PMID: 18922950

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

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

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

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

PMID: 18923039

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

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

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

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

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

Improved visual sensitivity during smooth pursuit eye movements

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

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

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

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

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

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

PMID: 18787171

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

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

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

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

PMID: 18772261

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

Emergence of binocular functional properties in a monocular neural circuit

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

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

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

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

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

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

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

Believing is seeing: expectations alter visual awareness

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

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

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

Neural basis for unique hues

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

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

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

The Orientation Selectivity of Color-Responsive Neurons in Macaque V1

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

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

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

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

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

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

PMID: 18669590

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

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

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

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

PMID: 18676033

Neural repetition suppression reflects fulfilled perceptual expectations.

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

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

PMID: 18677308

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

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

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

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

PMID: 18666833

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

Influence of Reward Delays on Responses of Dopamine Neurons

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

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

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

Complementary Contributions of Prefrontal Neuron Classes in Abstract Numerical Categorization

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

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

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

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

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

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

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

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

The temporal precision of reward prediction in dopamine neurons.

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

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

PMID: 18660807

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

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

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

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

PMID: 1865031

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

Students and Professors

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

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

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

PMID: 18650810

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

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

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

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

PMID: 18650810

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

Highly Selective Receptive Fields in Mouse Visual Cortex

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

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

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

Brain Magic

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

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

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

PMID: 18622399

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

Acetylcholine contributes through muscarinic receptors to attentional modulation in V1.

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

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

PMID: 18633352

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

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

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

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

PMID: 18632741

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

Distinct Face-Processing Strategies in Parents of Autistic Children

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

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

PMID: 18635351

Fulltext: Science Direct

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

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

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

PMID: 18588544

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

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

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

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

Expressing fear enhances sensory acquisition

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

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

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

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Privileged Coding of Convex Shapes in Human Object-Selective Cortex.

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

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

PMID: 18579661

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

Dynamic Population Coding of Category Information in ITC and PFC.

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

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

PMID: 18562555

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

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

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

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

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

Mechanisms of face perception.

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

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

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

Psychology of Security

Bruce Schneier on Psychology of Security
Black Hat USA 2007

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

What do you do?

A Primer on Python for Life Science Researchers

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


PMID: 18052533

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

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

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

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

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

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

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

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

PMID: 18535247

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

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

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

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

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

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

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

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

PMID: 18509031

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

Nature NeuroTechniques

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

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

Jae-Jin Ryu, Karen Borrmann, Avi Chaudhuri
PlosOne

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

Free Fulltext: PlosOne

A Map for Horizontal Disparity in Monkey V2

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

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

Fulltext: ScienceDirect

Saccadic latency during electrical stimulation of the human subthalamic nucleus

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

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

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

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

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

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

PMID: 18495887

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

Decision-making with multiple alternatives.

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

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

PMID: 18488024

The neural systems that mediate human perceptual decision making

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

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

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

Fragment-Based Learning of Visual Object Categories

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

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

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

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

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

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

PMID: 18477698

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

Electric stimulation fMRI of the perforant pathway to the rat hippocampus

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

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

PMID: 18479870

Fulltext: ScienceDirect

PhD Comics

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

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

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

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

The neural systems that mediate human perceptual decision making

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

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

PMID: 18464792

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

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

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

PMID: 18464917

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

Value Representations in the Primate Striatum during Matching Behavior.

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

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

PMID: 18466754

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

Learning to recognize visual objects with microstimulation in inferior temporal cortex

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

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

PMID: 18463185

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

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

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

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

PMID: 18463265

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

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

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

PMID: 1843452

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

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

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

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

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

Neurophysiology of the BOLD fMRI Signal in Awake Monkeys

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

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

PMID: 18439825

Fulltext: Science Direct

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

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

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

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

Unconscious determinants of free decisions in the human brain

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

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

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

Reduction of stimulus visibility compresses apparent time intervals

Masahiko Terao, Junji Watanabe, Akihiro Yagi, Shin'ya Nishida
Nature Neuroscience 11, 541 - 542 (2008)

The neural mechanisms underlying visual estimation of subsecond durations remain unknown, but perisaccadic underestimation of interflash intervals may provide a clue as to the nature of these mechanisms. Here we found that simply reducing the flash visibility, particularly the visibility of transient signals, induced similar time underestimation by human observers. Our results suggest that weak transient responses fail to trigger the proper detection of temporal asynchrony, leading to increased perception of simultaneity and apparent time compression.

Fulltext: http://www.nature.com/neuro/journal/v11/n5/pdf/nn.2111.pdf

Instantaneous correlation of excitation and inhibition during ongoing and sensory-evoked activities

Michael Okun, Ilan Lampl
Nature Neuroscience 11, 535 - 537 (2008)

Temporal and quantitative relations between excitatory and inhibitory inputs in the cortex are central to its activity, yet they remain poorly understood. In particular, a controversy exists regarding the extent of correlation between cortical excitation and inhibition. Using simultaneous intracellular recordings in pairs of nearby neurons in vivo, we found that excitatory and inhibitory inputs are continuously synchronized and correlated in strength during spontaneous and sensory-evoked activities in the rat somatosensory cortex.

Fulltext: http://www.nature.com/neuro/journal/v11/n5/pdf/nn.2105.pdf

Choices in neuroscience careers

Tamas Bartfai, Tom Insel, Gord Fishell & Nancy Rothwell
Nature Reviews Neuroscience 9, 401-405 (May 2008) | doi:10.1038/nrn2386

How do I choose a mentor?
How do I decide what field of neuroscience to work in?
Should I consider doing research in industry?
Most students and postdoctoral researchers aiming for a successful career in neuroscience ask themselves these questions.
In this article, Nature Reviews Neuroscience asks four successful neuroscientists for their thoughts on the factors one should consider when making these decisions.
We hope that this Viewpoint will serve as a useful resource for junior neuroscientists who have to make important and sometimes difficult decisions that might have long-lasting consequences for their careers.

Fulltext: http://www.nature.com/nrn/journal/v9/n5/pdf/nrn2386.pdf

Adaptation across the Cortical Hierarchy: Low-Level Curve Adaptation Affects High-Level Facial-Expression Judgments

Hong Xu,1 Peter Dayan,2 Richard M. Lipkin,1 and Ning Qian
The Journal of Neuroscience, March 26, 2008, 28(13):3374-3383; doi:10.1523/JNEUROSCI.0182-08.2008

Adaptation is ubiquitous in sensory processing. Although sensory processing is hierarchical, with neurons at higher levels exhibiting greater degrees of tuning complexity and invariance than those at lower levels, few experimental or theoretical studies address how adaptation at one hierarchical level affects processing at others. Nevertheless, this issue is critical for understanding cortical coding and computation. Therefore, we examined whether perception of high-level facial expressions can be affected by adaptation to low-level curves (i.e., the shape of a mouth). After adapting to a concave curve, subjects more frequently perceived faces as happy, and after adapting to a convex curve, subjects more frequently perceived faces as sad. We observed this multilevel aftereffect with both cartoon and real test faces when the adapting curve and the mouths of the test faces had the same location. However, when we placed the adapting curve 0.2° below the test faces, the effect disappeared. Surprisingly, this positional specificity held even when real faces, instead of curves, were the adapting stimuli, suggesting that it is a general property for facial-expression aftereffects. We also studied the converse question of whether face adaptation affects curvature judgments, and found such effects after adapting to a cartoon face, but not a real face. Our results suggest that there is a local component in facial-expression representation, in addition to holistic representations emphasized in previous studies. By showing that adaptation can propagate up the cortical hierarchy, our findings also challenge existing functional accounts of adaptation.

Fulltext: http://www.jneurosci.org/cgi/reprint/28/13/3374

One-dimensional dynamics of attention and decision making in LIP

Ganguli S, Bisley JW, Roitman JD, Shadlen MN, Goldberg ME, Miller KD.
Neuron. 2008 Apr 10;58(1):15-25

Where we allocate our visual spatial attention depends upon a continual competition between internally generated goals and external distractions. Recently it was shown that single neurons in the macaque lateral intraparietal area (LIP) can predict the amount of time a distractor can shift the locus of spatial attention away from a goal. We propose that this remarkable dynamical correspondence between single neurons and attention can be explained by a network model in which generically high-dimensional firing-rate vectors rapidly decay to a single mode. We find direct experimental evidence for this model, not only in the original attentional task, but also in a very different task involving perceptual decision making. These results confirm a theoretical prediction that slowly varying activity patterns are proportional to spontaneous activity, pose constraints on models of persistent activity, and suggest a network mechanism for the emergence of robust behavioral timing from heterogeneous neuronal populations.

PMID: 18400159

Fulltext: http://download.neuron.org/pdfs/0896-6273/PIIS0896627308001682.pdf

Prefrontal cortex function in the representation of temporally complex events

Browning PG, Gaffan D.
J Neurosci. 2008 Apr 9;28(15):3934-40

The frontal cortex and inferior temporal cortex are strongly functionally interconnected. Previous experiments on prefrontal function in monkeys have shown that a disconnection of prefrontal cortex from inferior temporal cortex impairs a variety of complex visual learning tasks but leaves simple concurrent object-reward association learning intact. We investigated the possibility that temporal components of visual learning tasks determine the sensitivity of those tasks to prefrontal-temporal disconnection by adding specific temporal components to the concurrent object-reward association learning task. Monkeys with crossed unilateral lesions of prefrontal cortex and inferior temporal cortex were impaired compared with unoperated controls at associating two-item sequences of visual objects with reward. The impairment was specific to the learning of visual sequences, because disconnection was without effect on object-reward association learning for an equivalent delayed reward. This result was replicated in monkeys with transection of the uncinate fascicle, thus determining the anatomical specificity of the dissociation. Previous behavioral results suggest that monkeys represent the two-item serial compound stimuli in a configural manner, similar to the way monkeys represent simultaneously presented compound stimuli. The representation of simultaneously presented configural stimuli depends on the perirhinal cortex. The present experiments show that the representation of serially presented compound stimuli depends on the interaction of prefrontal cortex and inferior temporal cortex. We suggest that prefrontal-temporal disconnection impairs a wide variety of learning tasks because in those tasks monkeys lay down similar temporally complex representations.

PMID: 18400892

Fulltext: http://www.jneurosci.org/cgi/reprint/28/15/3934

Maps of visual space in human occipital cortex are retinotopic, not spatiotopic

Gardner JL, Merriam EP, Movshon JA, Heeger DJ.
J Neurosci. 2008 Apr 9;28(15):3988-99

We experience the visual world as phenomenally invariant to eye position, but almost all cortical maps of visual space in monkeys use a retinotopic reference frame, that is, the cortical representation of a point in the visual world is different across eye positions. It was recently reported that human cortical area MT (unlike monkey MT) represents stimuli in a reference frame linked to the position of stimuli in space, a "spatiotopic" reference frame. We used visuotopic mapping with blood oxygen level-dependent functional magnetic resonance imaging signals to define 12 human visual cortical areas, and then determined whether the reference frame in each area was spatiotopic or retinotopic. We found that all 12 areas, including MT, represented stimuli in a retinotopic reference frame. Although there were patches of cortex in and around these visual areas that were ostensibly spatiotopic, none of these patches exhibited reliable stimulus-evoked responses. We conclude that the early, visuotopically organized visual cortical areas in the human brain (like their counterparts in the monkey brain) represent stimuli in a retinotopic reference frame.

PMID: 18400898

Fulltext: http://www.jneurosci.org/cgi/reprint/28/15/3988