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Sensory stimulation triggers synchronized bioelectrical activity in the brain across various frequencies. This study delves into network-level activities, specifically focusing on local field potentials as a neural signature of visual category representation. Specifically, we studied the role of different local field potential frequency oscillation bands in visual stimulus category representation by presenting images of faces and objects to three monkeys while recording local field potential from inferior temporal cortex. We found category selective local field potential responses mainly for animate, but not inanimate, objects. Notably, face-selective local field potential responses were evident across all tested frequency bands, manifesting in both enhanced (above mean baseline activity) and suppressed (below mean baseline activity) local field potential powers. We observed four different local field potential response profiles based on frequency bands and face selective excitatory and suppressive responses. Low-frequency local field potential bands (1-30 Hz) were more prodominstaly suppressed by face stimulation than the high-frequency (30-170 Hz) local field potential bands. Furthermore, the low-frequency local field potentials conveyed less face category informtion than the high-frequency local field potential in both enhansive and suppressive conditions. Furthermore, we observed a negative correlation between face/object d-prime values in all the tested local field potential frequency bands and the anterior-posterior position of the recording sites. In addition, the power of low-frequency local field potential systematically declined across inferior temporal anterior-posterior positions, whereas high-frequency local field potential did not exhibit such a pattern. In general, for most of the above-mentioned findings somewhat similar results were observed for body, but not, other stimulus categories. The observed findings suggest that a balance of face selective excitation and inhibition across time and cortical space shape face category selectivity in inferior temporal cortex., (© The Author(s) 2023. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Our perceptual system processes only a selected subset of an incoming stream of stimuli due to sensory biases and limitations in spatial and temporal attention and working memory capacity. In this study, we investigated perceptual access to sensory information that was temporally predictable or unpredictable and spread across the visual field. In a visual recognition task, participants were presented with an array of different number of alphabetical stimuli that were followed by a probe with a delay. They had to indicate whether the probe was included in the stimulus-set or not. To test the impact of temporal attention, coloured cues that were displayed before the visual stimuli indicated the presentation onset of the stimulus-set. We found that temporal predictability of stimulus onset yields higher performance. In addition, recognition performance was biased across the visual field with higher performance for stimuli that were presented on the upper and right visual quadrants. Our findings demonstrate that recognition accuracy is enhanced by temporal cues and has an inherently asymmetric shape across the visual field., (© 2022. The Psychonomic Society, Inc.)

Rapid categorization of visual objects is critical for comprehending our complex visual world. The role of individual cortical neurons and neural populations in categorizing visual objects during passive vision has previously been studied. However, it is unclear whether and how perceptually guided behaviors affect the encoding of stimulus categories by neural population activity in the higher visual cortex. Here we studied the activity of the inferior temporal (IT) cortical neurons in macaque monkeys during both passive viewing and categorization of ambiguous body and object images. We found enhanced category information in the IT neural population activity during the correct, but not wrong, trials of the categorization task compared to the passive task. This encoding enhancement was task difficulty dependent with progressively larger values in trials with more ambiguous stimuli. Enhancement of IT neural population information for behaviorally relevant stimulus features suggests IT neural networks' involvement in perceptual decision-making behavior., (© 2022. The Author(s).)

Face-selective neurons in the inferior temporal (IT) cortex respond to faces by either increasing (ENH) or decreasing (SUP) their spiking activities compared with their baseline. Although nearly half of IT face neurons are selectively suppressed by face stimulation, their role in face representation is not clear. To address this issue, we recorded the spiking activities and local field potential (LFP) from IT cortex of three monkeys while they viewed a large set of visual stimuli. LFP high-gamma (HG-LFP) power indicated the presence of both ENH and SUP face-selective neural clusters in IT cortex. The magnitude of HG-LFP power of the recording sites was correlated with the magnitude of change in the evoked spiking activities of its constituent neurons for both ENH and SUP face clusters. Spatial distribution of the ENH and SUP face clusters suggests the presence of a complex and heterogeneous face hypercluster organization in IT cortex. Importantly, ENH neurons conveyed more face category and SUP neurons conveyed more face identity information at both the single-unit and neuronal population levels. Onset and peak of suppressive single-unit, neuronal population, and HG-LFP power activities lagged those of the ENH ones. These results demonstrate that IT neuronal code for face representation is optimized by increasing sparseness through selective suppression of a subset of face neurons. We suggest that IT cortex contains spatial clusters of both ENH and SUP face neurons with distinct specialized functional role in face representation. NEW & NOTEWORTHY Electrophysiological and imaging studies have suggested that face information is encoded by a network of clusters of enhancive face-selective neurons in the visual cortex of man and monkey. We show that nearly half of face-selective neurons are suppressed by face stimulation. The suppressive neurons form spatial clusters and convey more face identity information than the enhancive face neurons. Our results suggest the presence of two neuronal subsystems for coarse and fine face information processing.

Attention greatly influences sensory neural processing by enhancing firing rates of neurons that represent the attended stimuli and by modulating their tuning properties. The cholinergic system is believed to partly mediate the attention contingent improvement of cortical processing by influencing neuronal excitability, synaptic transmission and neural network characteristics. Here, we used a biophysically based model to investigate the mechanisms by which cholinergic system influences sensory information processing in the primary visual cortex (V1) layer 4C. The physiological properties and architectures of our model were inspired by experimental data and include feed-forward input from dorsal lateral geniculate nucleus that sets up orientation preference in V1 neural responses. When including a cholinergic drive, we found significant sharpening in orientation selectivity, desynchronization of LFP gamma power and spike-field coherence, decreased response variability and correlation reduction mostly by influencing intracortical interactions and by increasing inhibitory drive. Our results indicated that these effects emerged due to changes specific to the behavior of the inhibitory neurons. The behavior of our model closely resembles the effects of attention on neural activities in monkey V1. Our model suggests precise mechanisms through which cholinergic modulation may mediate the effects of attention in the visual cortex.

We investigated the neural correlates of pleasure induced by listening to highly pleasant and neutral musical excerpts using electroencephalography (EEG). Power spectrum analysis of EEG data showed a distinct gradual change in the power of low-frequency oscillations in response to highly pleasant, but not neutral, musical excerpts. Specifically, listening to highly pleasant music was associated with (i) relatively higher oscillatory activity in the theta band over the frontocentral (FC) area and in the alpha band over the parieto-occipital area, and (ii) a gradual increase in the oscillatory power over time. Correlation analysis between behavioral and electrophysiological data revealed that theta power over the FC electrodes was correlated with subjective assessment of pleasantness while listening to music. To study the link between attention and positive valence in our experiments, volunteers performed a delayed match-to-sample memory task while listening to the musical excerpts. The subjects' performances were significantly lower under highly pleasant conditions compared to neutral conditions. Listening to pleasant music requires higher degrees of attention, leading to the observed decline in memory performance. Gradual development of low-frequency oscillations in the frontal and posterior areas may be at least partly due to gradual recruitment of higher levels of attention over time in response to pleasurable music., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Time is an inseparable component of every physical event that we perceive, yet it is not clear how the brain processes time or how the neuronal representation of time affects our perception of events. Here we asked subjects to perform a visual discrimination task while we changed the temporal context in which the stimuli were presented. We collected electroencephalography (EEG) signals in two temporal contexts. In predictable blocks stimuli were presented after a constant delay relative to a visual cue, and in unpredictable blocks stimuli were presented after variable delays relative to the visual cue. Four subsecond delays of 83, 150, 400, and 800 ms were used in the predictable and unpredictable blocks. We observed that predictability modulated the power of prestimulus alpha oscillations in the parieto-occipital sites: alpha power increased in the 300-ms window before stimulus onset in the predictable blocks compared with the unpredictable blocks. This modulation only occurred in the longest delay period, 800 ms, in which predictability also improved the behavioral performance of the subjects. Moreover, learning the temporal context shaped the prestimulus alpha power: modulation of prestimulus alpha power grew during the predictable block and correlated with performance enhancement. These results suggest that the brain is able to learn the subsecond temporal context of stimuli and use this to enhance sensory processing. Furthermore, the neural correlate of this temporal prediction is reflected in the alpha oscillations. NEW & NOTEWORTHY It is not well understood how the uncertainty in the timing of an external event affects its processing, particularly at subsecond scales. Here we demonstrate how a predictable timing scheme improves visual processing. We found that learning the predictable scheme gradually shaped the prestimulus alpha power. These findings indicate that the human brain is able to extract implicit subsecond patterns in the temporal context of events., (Copyright © 2017 the American Physiological Society.)

Neuronal networks of the brain adapt their information processing according to the history of stimuli. Whereas most studies have linked adaptation to repetition suppression, recurrent connections within a network and disinhibition due to adaptation predict more complex response patterns. The main questions of this study are as follows: what is the effect of the selectivity of neurons on suppression/enhancement of neural responses? What are the consequences of adaptation on information representation in neural population and the temporal structure of response patterns? We studied rapid face adaptation using spiking activities of neurons in the inferior-temporal (IT) cortex. Investigating the responses of neurons, within a wide range from negative to positive face selectivity, showed that despite the peak amplitude suppression in highly positive selective neurons, responses were enhanced in most other neurons. This enhancement can be attributed to disinhibition due to adaptation. Delayed and distributed responses were observed for positive selective neurons. Principal component analysis of the IT population responses over time revealed that repetition of face stimuli resulted in temporal decorrelation of the network activity. The contributions of the main and higher neuronal dimensions were changed under an adaptation condition, where more neuronal dimensions were used to encode repeated face stimuli.

The representation of visual objects in primate brain is distributed and multiple neurons are involved in encoding each object. One way to understand the neural basis of object representation is to estimate the number of neural dimensions that are needed for veridical representation of object categories. In this study, the characteristics of the match between physical-shape and neural representational spaces in monkey inferior temporal (IT) cortex were evaluated. Specifically, we examined how the number of neural dimensions, stimulus behavioral saliency and stimulus category selectivity of neurons affected the correlation between shape and neural representational spaces in IT cortex. Single-unit recordings from monkey IT cortex revealed that there was a significant match between face space and its neural representation at lower neural dimensions, whereas the optimal match for the non-face objects was observed at higher neural dimensions. There was a statistically significant match between the face and neural spaces only in the face-selective neurons, whereas a significant match was observed for non-face objects in all neurons regardless of their category selectivity. Interestingly, the face neurons showed a higher match for the non-face objects than for the faces at higher neural dimensions. The optimal representation of face space in the responses of the face neurons was a low dimensional map that emerged early (~150 ms post-stimulus onset) and was followed by a high dimensional and relatively late (~300 ms) map for the non-face stimuli. These results support a multiplexing function for the face neurons in the representation of very similar shape spaces, but with different dimensionality and timing scales., (© 2017 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)

One goal of our nervous system is to form predictions about the world around us to facilitate our responses to upcoming events. One basis for such predictions could be the recently encountered visual stimuli, or the recent statistics of the visual environment. We examined the effect of recently experienced stimulus statistics on the visual representation of face stimuli by recording the responses of face-responsive neurons in the final stage of visual object recognition, the inferotemporal (IT) cortex, during blocks in which the probability of seeing a particular face was either 100% or only 12%. During the block with only face images, ∼30% of IT neurons exhibit enhanced anticipatory activity before the evoked visual response. This anticipatory modulation is followed by greater activity, broader view tuning, more distributed processing, and more reliable responses of IT neurons to the face stimuli. These changes in the visual response were sufficient to improve the ability of IT neurons to represent a variable property of the predictable face images (viewing angle), as measured by the performance of a simple linear classifier. These results demonstrate that the recent statistics of the visual environment can facilitate processing of stimulus information in the population neuronal representation., New & Noteworthy: Neurons in inferotemporal (IT) cortex anticipate the arrival of a predictable stimulus, and visual responses to an expected stimulus are more distributed throughout the population of IT neurons, providing an enhanced representation of second-order stimulus information (in this case, viewing angle). The findings reveal a potential neural basis for the behavioral benefits of contextual expectation., (Copyright © 2017 the American Physiological Society.)

Object categories are recognized at multiple levels of hierarchical abstractions. Psychophysical studies have shown a more rapid perceptual access to the mid-level category information (e.g., human faces) than the higher (superordinate; e.g., animal) or the lower (subordinate; e.g., face identity) level. Mid-level category members share many features, whereas few features are shared among members of different mid-level categories. To understand better the neural basis of expedited access to mid-level category information, we examined neural responses of the inferior temporal (IT) cortex of macaque monkeys viewing a large number of object images. We found an earlier representation of mid-level categories in the IT population and single-unit responses compared with superordinate- and subordinate-level categories. The short-latency representation of mid-level category information shows that visual cortex first divides the category shape space at its sharpest boundaries, defined by high/low within/between-group similarity. This short-latency, mid-level category boundary map may be a prerequisite for representation of other categories at more global and finer scales., (Copyright © 2016 the American Physiological Society.)

One of the characteristics of autism spectrum disorder (ASD) is atypical sensory processing and perceptual integration. Here, we used an object naming task to test the significance of deletion of vertices versus extended contours (edges) in naming fragmented line drawings of natural objects in typically developing and ASD children. The basic components of a fragmented image in perceptual closure need to be integrated to make a coherent visual perception. When vertices were missing and only edges were visible, typically developing and ASD subjects performed similarly. But typically developing children performed significantly better than ASD children when only vertex information was visible. These results indicate impairment of binding vertices but not edges to form a holistic representation of an object in children with ASD.

Visual object categorization is a critical task in our daily life. Many studies have explored category representation in the inferior temporal (IT) cortex at the level of single neurons and population. However, it is not clear how behavioral demands modulate this category representation. Here, we recorded from the IT single neurons in monkeys performing two different tasks with identical visual stimuli: passive fixation and body/object categorization. We found that category selectivity of the IT neurons was improved in the categorization compared with the passive task where reward was not contingent on image category. The category improvement was the result of larger rate enhancement for the preferred category and smaller response variability for both preferred and nonpreferred categories. These specific modulations in the responses of IT category neurons enhanced signal-to-noise ratio of the neural responses to discriminate better between the preferred and nonpreferred categories. Our results provide new insight into the adaptable category representation in the IT cortex, which depends on behavioral demands., (Copyright © 2014 the American Physiological Society.)

Spontaneous firing is a ubiquitous property of neural activity in the brain. Recent literature suggests that this baseline activity plays a key role in perception. However, it is not known how the baseline activity contributes to neural coding and behavior. Here, by recording from the single neurons in the inferior temporal cortex of monkeys performing a visual categorization task, we thoroughly explored the relationship between baseline activity, the evoked response, and behavior. Specifically we found that a low-frequency (<8 Hz) oscillation in the spike train, prior and phase-locked to the stimulus onset, was correlated with increased gamma power and neuronal baseline activity. This enhancement of the baseline activity was then followed by an increase in the neural selectivity and the response reliability and eventually a higher behavioral performance.

We have calculated the intrinsic dimensionality of visual object representations in anterior inferotemporal (AIT) cortex, based on responses of a large sample of cells stimulated with photographs of diverse objects. Because dimensionality was dependent on data set size, we determined asymptotic dimensionality as both the number of neurons and number of stimulus image approached infinity. Our final dimensionality estimate was 93 (SD: ± 11), indicating that there is basis set of approximately 100 independent features that characterize the dimensions of neural object space. We believe this is the first estimate of the dimensionality of neural visual representations based on single-cell neurophysiological data. The dimensionality of AIT object representations was much lower than the dimensionality of the stimuli. We suggest that there may be a gradual reduction in the dimensionality of object representations in neural populations going from retina to inferotemporal cortex as receptive fields become increasingly complex.

Inferior temporal (IT) cortex as the final stage of the ventral visual pathway is involved in visual object recognition. In our everyday life we need to recognize visual objects that are degraded by noise. Psychophysical studies have shown that the accuracy and speed of the object recognition decreases as the amount of visual noise increases. However, the neural representation of ambiguous visual objects and the underlying neural mechanisms of such changes in the behavior are not known. Here, by recording the neuronal spiking activity of macaque monkeys' IT we explored the relationship between stimulus ambiguity and the IT neural activity. We found smaller amplitude, later onset, earlier offset and shorter duration of the response as visual ambiguity increased. All of these modulations were gradual and correlated with the level of stimulus ambiguity. We found that while category selectivity of IT neurons decreased with noise, it was preserved for a large extent of visual ambiguity. This noise tolerance for category selectivity in IT was lost at 60% noise level. Interestingly, while the response of the IT neurons to visual stimuli at 60% noise level was significantly larger than their baseline activity and full (100%) noise, it was not category selective anymore. The latter finding shows a neural representation that signals the presence of visual stimulus without signaling what it is. In general these findings, in the context of a drift diffusion model, explain the neural mechanisms of perceptual accuracy and speed changes in the process of recognizing ambiguous objects.

Objective: Incapability in face perception and recognition is one of the main issues in autism spectrum disorders (ASD). Event related potential (ERP) studies have revealed controversial insights on autistic brain responses to faces and objects. The current investigation examined the ERP components of young children with ASD compared to a typically developing (TD) group when looking at the upright and inverted images of faces and cars., Methods: Fourteen children and adolescents aged between 9 and 17 diagnosed as having ASD were compared with 18 age- gender matched normally developing individuals. All participants' ERPs were recorded while they were seeing the images of human faces and objects in both upright and inverted positions. The ERP components including N170 (latency and amplitude) were compared between the two groups in two conditions of upright and inverted using the repeated measure analysis method., Results: The processing speed for upright faces was faster than the inverted faces in the TD group; however, the difference was not significant. A significant difference was observed in terms of N170 latency between the two groups for different stimulus categories such as objects and faces(p<0.05). Moreover, inverted vs. upright stimuli in both groups elicited a greater response in terms of N170 amplitude in both groups, and this effect was significantly prominent in the right hemisphere (p<0.05). The N170 amplitude turned out to be greater for the inverted vs. upright stimuli irrespective of the stimuli type and group., Conclusion: These data suggest youths with ASD have difficulty processing information, particularly in face perception regardless of the stimuli orientation.

Recognition of face-views is a crucial component of our social behavior. However, little is known about the neuronal basis of face-view recognition. Recent psychophysical studies have shown that adaptation to a particular face-view changes perception of other views in humans. To understand the neuronal basis of face-view representation and the mechanisms involved in face-view adaptation, we studied the responses of inferotemporal (IT) cortex neurons before and after face-view adaptation. We first used face-view adaptation to show that monkeys, as well as humans, exhibit face-view aftereffect, suggesting the presence of a view-sensitive representation in primates. Then, we recorded from IT neurons of monkeys and found that changes in responses of these neurons can account for changes in perception of face views attributable to adaptation, indicating IT as a neural correlate of face-view perception. More importantly, we provide evidence that IT neurons with wide-view tuning not only conveyed face-view information but also exhibited neural signatures of view aftereffect before neurons with narrower tuning. Our findings show that view-dependent representation of objects does not necessarily depend on narrow-tuned neurons. The finding that wide-view tuned neurons account for view-specific perceptual changes implies the importance of accumulating sensitivity and tolerance at the level of IT neurons, as the final stage of visual object recognition.

Impairments in attentional behaviors, including over-selectivity, under-selectivity, distractibility and difficulty in shift of attention, are widely reported in several developmental disorders, including autism. Uncharacteristic inhibitory to excitatory neuronal number ratio (IER) and abnormal synaptic strength levels in the brain are two broadly accepted neurobiological disorders observed in autistic individuals. These neurobiological findings are contrasting and their relation to the atypical attentional behaviors is not clear yet. In this paper, we take a computational approach to investigate the relation of imbalanced IER and abnormal synaptic strength to some well-documented spectrum of attentional impairments. The computational model is based on a modified version of a biologically plausible neural model of two competing minicolumns in IT cortex augmented with a simple model of top-down attention. Top-down attention is assumed to amplify (attenuates) attended (unattended) stimulus. The inhibitory synaptic strength parameter in the model is set such that typical attentional behavior is emerged. Then, according to related findings, the parameter is changed and the model's attentional behavior is considered. The simulation results show that, without any change in top-down attention, the abnormal inhibitory synaptic strength values--and IER imbalance- result in over-selectivity, under-selectivity, distractibility and difficulty in shift of attention in the model. It suggests that the modeled neurobiological abnormalities can be accounted for the attentional deficits. In addition, the atypical attentional behaviors do not necessarily point to impairments in top-down attention. Our simulations suggest that limited changes in the inhibitory synaptic strength and variations in top-down attention signal affect the model's attentional behaviors in the same way. So, limited deficits in the inhibitory strength may be alleviated by appropriate change in top-down attention biasing. Nevertheless, our model proposes that this compensation is not possible for very high and very low values of the inhibitory strength.

We have characterized selectivity and sparseness in anterior inferotemporal cortex, using a large data set. Responses were collected from 674 monkey inferotemporal cells, each stimulated by 806 object photographs. This 806 × 674 matrix was examined in two ways: columnwise, looking at responses of a single neuron to all images (single-neuron selectivity), and rowwise, looking at the responses of all neurons caused by a single image (population sparseness). Selectivity and sparseness were measured as kurtosis of probability distributions. Population sparseness exceeded single-neuron selectivity, with specific values dependent on the size of the data sample. This difference was principally caused by inclusion, within the population, of neurons with a variety of dynamic ranges (standard deviations of responses over all images). Statistics of large responses were examined by quantifying how quickly the upper tail of the probability distribution decreased (tail heaviness). This analysis demonstrated that population responses had heavier tails than single-neuron responses, consistent with the difference between sparseness and selectivity measurements. Population responses with spontaneous activity subtracted had the heaviest tails, following a power law. The very light tails of single-neuron responses indicate that the critical feature for each neuron is simple enough to have a high probability of occurring within a limited stimulus set. Heavy tails of population responses indicate that there are a large number of different critical features to which different neurons are tuned. These results are inconsistent with some structural models of object recognition that posit that objects are decomposed into a small number of standard features.

During natural vision, stimuli are viewed for different durations as the state of brain activity changes over time. Here we studied the effects of stimulus presentation duration on cell responses (n=259) in three subdivisions of the inferotemporal (IT) cortex of fixating macaque monkeys as neural baseline firing rates varied over the course of recording. First, cell responses to the presentation of 120 images were tested, and four images that elicited significant responses with various degrees of effectiveness were selected for further study. Then the four selected images were presented to the monkeys for five different presentation durations (18, 70, 140, 210, and 350 ms). We found that depending on the magnitude of neural baseline activity, stimulus presentation duration affected the response properties and efficiency of neural information processing in the IT cortex. Short stimulus presentation durations elicited phasic responses consisting of rhythmic activation and inactivation, which conveyed a lower amount of stimulus information, particularly following higher baseline firing rates. Longer presentation durations elicited a sustained pattern of response and carried a greater amount of information, particularly at lower baseline firing rates. Finally, a significantly higher proportion of cells in the posterior IT compared with the anterior IT had a tendency to have high baseline activity, recruit stronger phasic responses and convey less information. It is plausible that during natural vision, as stimuli with various exposure durations affect the visual system, top-down influence or competition within local neural networks differentially influences the function of IT cells by changing their baseline activity.

Inferior temporal (IT) object representations have been intensively studied in monkeys and humans, but representations of the same particular objects have never been compared between the species. Moreover, IT's role in categorization is not well understood. Here, we presented monkeys and humans with the same images of real-world objects and measured the IT response pattern elicited by each image. In order to relate the representations between the species and to computational models, we compare response-pattern dissimilarity matrices. IT response patterns form category clusters, which match between man and monkey. The clusters correspond to animate and inanimate objects; within the animate objects, faces and bodies form subclusters. Within each category, IT distinguishes individual exemplars, and the within-category exemplar similarities also match between the species. Our findings suggest that primate IT across species may host a common code, which combines a categorical and a continuous representation of objects.

In a dynamic environment stimulus task relevancy could be altered through time and it is not always possible to dissociate relevant and irrelevant objects from the very first moment they come to our sight. In such conditions, subjects need to retain maximum possible information in their WM until it is clear which items should be eliminated from WM to free attention and memory resources. Here, we examined the neural basis of irrelevant information filtering from WM by recording human ERP during a visual change detection task in which the stimulus irrelevancy was revealed in a later stage of the task forcing the subjects to keep all of the information in WM until test object set was presented. Assessing subjects' behaviour we found that subjects' RT was highly correlated with the number of irrelevant objects and not the relevant one, pointing to the notion that filtering, and not selection, process was used to handle the distracting effect of irrelevant objects. In addition we found that frontal N150 and parietal N200 peak latencies increased systematically as the amount of irrelevancy load increased. Interestingly, the peak latency of parietal N200, and not frontal N150, better correlated with subjects' RT. The difference between frontal N150 and parietal N200 peak latencies varied with the amount of irrelevancy load suggesting that functional connectivity between modules underlying fronto-parietal potentials vary concomitant with the irrelevancy load. These findings suggest the existence of two neural modules, responsible for irrelevant objects elimination, whose activity latency and functional connectivity depend on the number of irrelevant object.

Our mental representation of object categories is hierarchically organized, and our rapid and seemingly effortless categorization ability is crucial for our daily behavior. Here, we examine responses of a large number (>600) of neurons in monkey inferior temporal (IT) cortex with a large number (>1,000) of natural and artificial object images. During the recordings, the monkeys performed a passive fixation task. We found that the categorical structure of objects is represented by the pattern of activity distributed over the cell population. Animate and inanimate objects created distinguishable clusters in the population code. The global category of animate objects was divided into bodies, hands, and faces. Faces were divided into primate and nonprimate faces, and the primate-face group was divided into human and monkey faces. Bodies of human, birds, and four-limb animals clustered together, whereas lower animals such as fish, reptile, and insects made another cluster. Thus the cluster analysis showed that IT population responses reconstruct a large part of our intuitive category structure, including the global division into animate and inanimate objects, and further hierarchical subdivisions of animate objects. The representation of categories was distributed in several respects, e.g., the similarity of response patterns to stimuli within a category was maintained by both the cells that maximally responded to the category and the cells that responded weakly to the category. These results advance our understanding of the nature of the IT neural code, suggesting an inherently categorical representation that comprises a range of categories including the amply investigated face category.

The inferior temporal cortex (IT) of primates is thought to be the final visual area in the ventral stream of cortical areas responsible for object recognition. Consistent with this hypothesis, single IT neurons respond selectively to highly complex visual stimuli such as faces. However, a direct causal link between the activity of face-selective neurons and face perception has not been demonstrated. In the present study of macaque monkeys, we artificially activated small clusters of IT neurons by means of electrical microstimulation while the monkeys performed a categorization task, judging whether noisy visual images belonged to 'face' or 'non-face' categories. Here we show that microstimulation of face-selective sites, but not other sites, strongly biased the monkeys' decisions towards the face category. The magnitude of the effect depended upon the degree of face selectivity of the stimulation site, the size of the stimulated cluster of face-selective neurons, and the exact timing of microstimulation. Our results establish a causal relationship between the activity of face-selective neurons and face perception.

Several lines of evidence have confirmed that visual integrity among the two hemifields is severely impaired following transection of posterior part of the corpus callosum (splenium). But the extent of this impairment differs for various visual functions. Here, in a posterior callosectomized patient (MD), interhemispheric visual spatial integrity is investigated in three experiments. In the first experiment, MD was asked to make decision about horizontal alignment of two balls presented simultaneously in one of three conditions: both in right or left hemifield, or each in one hemifield. We have previously shown that she is not able to perform a shape-matching task for stimuli presented to different hemifields. But in this task, MD was able to compare location of the two bilaterally presented stimuli significantly above chance level. Then we investigated whether attentive visual object tracking across vertical meridian of the visual field is possible in the absence of splenium. MD had to attentively track one bouncing ball among three identical balls, while it crossed the vertical midline in half of trials. Her performance in crossed conditions was significantly above chance level, but it was lower than uncrossed conditions. Finally, we investigated the contribution of simple interhemispheric temporal signals in performing the attentive tracking task. Results suggest that the patient was not using such temporal signals. Our results suggest that interhemispheric connections other than splenium can contribute in making an integral visual map across hemifields. Such an integrated map can be used for bilateral visual spatial comparisons and visual spatial attention.

Capsaicin (Cap)-induced depletion of C-fiber afferents results in plasticity of somatosensory system which is manifested as a functional alteration at different levels of the somatosensory pathway. In the present study we examined the effect of Cap-induced neonatal depletion of C-fibers on the induction of long-term potentiation (LTP) and long-term depression (LTD) in the neocortex of freely moving rats. A stimulating electrode was implanted into corpus callosum and a recording electrode was implanted in the somatosensory cortex of control (Con: normal, without electrical stimulation), trained (normal, with electrical stimulation) and Cap-treated (C-fiber depleted, with electrical stimulation) adult rats. Two weeks after the surgery, evoked field potential responses were recorded before, during (12 days) and after (1 month) the induction period of LTP and LTD. The LTP and LTD response characteristics during the time course of recording were compared between different groups. In the train group, LTP and LTD appeared after 3 days of stimulation. LTP magnitude peaked after about 6 days while LTD magnitude peaked in about 12 days. C-fiber depletion postponed the development of LTP and LTD making the highest differential levels of LTP about 6 days after the initiation of LTP induction. The impact of C-fiber depletion on slowing the time course of LTD induction was more prolonged and lasted until day 12 of the initiation of LTD induction. These results suggest that intact C-fibers are necessary for normal plasticity and long-term synaptic modification of the somatosensory system.

Configural processing could develop for non-face visual objects as one becomes familiar with those objects through repeated exposure. To explore the role of familiarity in object recognition, we studied the effect of adaptation to a visual object (adapting stimulus) on the identification performance of other objects (test stimulus) while adapting and test stimuli were exactly the same, shared parts or were completely different. We used a subset of English alphabets (p, q, d and b) as familiar objects and an unfamiliar set of symbols constructed from same parts but with different configurations. Adaptation to a member of each set led to a lower identification performance for that object in a crowding paradigm. Adaptation to each member of the unfamiliar set resulted in decreased identification performance for the same object and those members of the set that shared parts with the adapting stimulus. But no such transfer of adaptation was observed for the familiar set. Our results support the notion that processing of object parts plays an important role in the recognition of unfamiliar objects while recognition of familiar objects is mainly based on configural processing mechanisms.

Neurons in the visual system respond to different visual stimuli with different onset latencies. However, it has remained unknown which stimulus features, aside from stimulus contrast, determine the onset latencies of responses. To examine the possibility that response onset latencies carry information about complex object images, we recorded single-cell responses in the inferior temporal cortex of alert monkeys, while they viewed >1,000 object stimuli. Many cells responded to human and non-primate animal faces with comparable magnitudes but responded significantly more quickly to human faces than to non-primate animal faces. Differences in onset latency may be used to increase the coding capacity or enhance or suppress information about particular object groups by time-dependent modulation.

C-fiber depletion results in expansion of low threshold somatosensory mechanoreceptive fields. In this study, we investigated the role of intact C-fibers in GABAA-mediated inhibition in barrel cortical neurons. We used electronically controlled mechanical stimulation of whiskers to quantitatively examine the responses of barrel cells to whisker displacements. After systemic injection of picrotoxin neuronal responses were recorded at 5 min intervals for 20 min and then at 10 min intervals for 100 min. Picrotoxin injection caused a 3-fold increase in response magnitude of adjacent whisker stimulation and 1.4-fold increase in response magnitude of principal whisker stimulation with a maximum enhancement 50 min after the injection. There was no significant change in spontaneous activity following picrotoxin injection. The response enhancement and receptive field expansion observed in normal rats were completely absent in the C-fiber-depleted rats. These results suggest that the GABAA-mediated inhibition that modulates the receptive field functional organization of the barrel cortex depends on intact C-fibers.

Controlled mechanical displacement was used to stimulate single whiskers in normal and C-fiber depleted rats to quantitatively examine the role of C-fibers in the response properties of barrel cortical cells. C-fiber depletion using neonatal capsaicin treatment increased the barrel single-unit response magnitude to deflection of both principal and adjacent whiskers while there was not any significant difference in the barrel cells' spontaneous activity. Capsaicin treatment increased the neural response duration of adjacent whisker stimulation but did not change that to the principal whisker deflection. There was no difference in response latencies of principal or adjacent whisker displacement between the normal and C-fiber-depleted groups. The efficiency of neural code for differentiation of principal and adjacent whiskers was measured by ROC analysis, which reflects the performance of an ideal observer in this discrimination using cells' firing rate. No significant difference was found in the performance of neurons in capsaicin-treated and control groups in distinguishing principal and adjacent whisker deflections from each other. These results suggest that neonatal C-fiber depletion causes an expansion of barrel cells receptive field but it does not affect the discrimination of individual whisker stimulation by the barrel cells.

Visual adaptation has been successfully used for studying the neural activity of different cortical areas in response to visual stimuli when observers do not have explicit conscious access to those stimuli. We compared the orientation selective adaptation to apparent motion and its effect on the perception of stimuli with bistable apparent motion in crowded and non-crowded conditions. In the crowding paradigm conscious access to a visual stimulus is severely impaired when it is flanked by other similar stimuli in the peripheral visual field. As expected, adaptation to the target stimulus occurred in the non-crowded condition in all of the individual subjects (n=4; P<0.001). Although in the crowded condition subjects were not able to discriminate the target stimulus, adaptation to that stimulus was still preserved (P<0.001). There was no significant difference between the adaptations in the two conditions of the apparent motion (P>0.05). Imaging studies have shown that V5 cortex is the earliest visual area that specifically responds to apparent motion. Our results suggest that in certain conditions V5 may be activated while there is no explicit conscious access to the apparent motion.

When a global moving pattern is superimposed on high-contrast stationary or slowly moving stimuli, the latter occasionally disappear for periods of several seconds (motion-induced blindness, MIB). Here, an adaptation paradigm was used to determine if orientation-selective adaptation still occurs for the stimulus that is no longer visible. Two slowly drifting high-contrast Gabor patches were presented to observers. As soon as both patches disappeared, one was eliminated from the screen. After 2 s, two low-contrast Gabor patches were presented as tests at the same locations and observers were asked to report their orientations. The observers' performance was significantly higher when the orientation of the low-contrast test patch was orthogonal to the orientation of the high-contrast adapting patch (p < 0.0001) for the location where the patch was present during MIB, even though it was perceptually invisible. The observers' performance was not significantly different at the adjacent control location where the stimulus was absent during the MIB. Although no stimulus was visible at either location, orientation-selective adaptation was preserved only for the location at which the patch remained present. Since orientation information is processed in low-level visual areas such as the primary visual cortex (V1), we conclude that MIB originates in an area higher than V1.

Subjects were asked to report the number of items in a display as the items moved along a circular path around the fixation point. As the rotation speed increased, the apparent number of items also increased. This motion-induced overestimation (MIO) effect was investigated in three experiments. In the first experiment, the effect of rotation speed and set size was explored with an enumeration task. The overestimation error increased with an increase in speed or number of items in the display. In the second experiment, we used an adjustment paradigm to measure the speed threshold of MIO effect onset. Temporal rate of the display, which was defined as product of rotation speed and the number of rotating items, was the determining factor of MIO onset. In the third experiment, moving items were marked with different colours. Surprisingly, the number of perceived items was still overestimated even though the number of perceived colours was not.

Effects of visual deprivation on the induction of epileptiform activity were studied in layer II/III of 29-39-day-old rat primary visual cortex. Field potentials were evoked by stimulation of layer IV in slices from control (CON) and dark-reared (DR) rats. Picrotoxin (PTX)-induced epileptiform activity was characterized by spontaneous and evoked epileptic field potentials (EFPs). The results showed that DR slices demonstrate greater susceptibility for induction of spontaneous EFP. PTX-induced changes in the characteristics of evoked field potentials also showed higher tendency of DR animals to generate epileptiform activity. In both groups, field potentials consisted of pEPSP(1) (population excitatory postsynaptic potential 1, i.e., first negativity) and pEPSP(2) (second negativity), respectively. There was no significant difference between the characteristics of field potentials in CON and DR slices. PTX significantly increased amplitude and duration of pEPSP(2), but it had no significant effect on pEPSP(1). Effects of PTX on pEPSP(2) were significantly higher in DR slices. It is concluded that visual deprivation results in a heightened potential in layer II/III of the rat visual cortex to generate PTX-induced epileptiform activity.