Bharathi Jagadeesh

Clustering of perirhinal neurons with similar properties following visual experience in adult monkeys

The functional organization of early visual areas seems to be largely determined during development. However, the organization of areas important for learning and memory, such as perirhinal cortex, may be modifiable in adults. To test this hypothesis, we recorded from pairs of neurons in perirhinal cortex of macaques while they viewed multiple complex stimuli. For novel stimuli, neuronal response preferences for pairs of nearby neurons and far-apart neurons were uncorrelated. However, after one day of experience with the stimuli, response preferences of nearby neurons became more similar. We conclude that specific visual experience induces development of clusters of perirhinal neurons with similar stimulus preferences.

Time Course and Stimulus Dependence of Repetition-Induced Response Suppression in Inferotemporal Cortex

Neural responses throughout the sensory system are affected by stimulus history. In the inferotemporal cortex (IT)--an area important for processing information about object shape--there is a substantially reduced response to the second presentation of an image. Understanding the mechanisms underlying repetition suppression may provide important insights into the circuitry that generates responses in IT. In addition, repetition suppression may have important perceptual consequences. The characteristics of repetition suppression in IT are poorly understood, and the details, including the interaction between the content of the first and second stimulus and the time course of suppression, are not clear. Here, we examined the time course of suppression in IT by varying both the duration and stimulus content of two stimuli presented in sequence. The data show that the degree of suppression does not depend directly on the response evoked by the first stimulus in the recorded neuron. Repetition suppression was also limited in duration, peaking at approximately 200 ms after the onset of the second (test) image and disappearing before the end of the response. Neural selectivity to a continuum of related images was enhanced if the first stimulus produced a weak response in the cell. The dynamics of the response suggests that different parts of the input and recurrent circuitry that gives rise to neural responses in IT are differentially modulated by repetition suppression. The selectivity of the sustained response was preserved in spite of substantial suppression of the early part of the response. The data suggest that suppression in IT is a property of the input and recurrent circuitry in IT and is not directly related to the degree of response in the recorded neuron itself.

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

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 ∼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.

Neural selectivity in anterior inferotemporal cortex for morphed photographic images during behavioral classification or fixation.

Anterior inferotemporal cortex (aIT) contributes to the ability to discriminate and classify complex images. To determine whether and what proportion of single neurons in aIT cortex can yield enough information to classify complex images, we recorded from aIT neurons during the presentation of morphed photographic images in sessions in which monkeys classified images in a two alternative forced-choice--delayed-match-to-sample (2AFC-DMS) task or in sessions in which they performed a fixation task. The sample stimuli were chosen from a sequence in which one image was gradually morphed into another in a pair, while the original pair of images served as choices. Responses of many individual neurons in aIT cortex during the behavioral classification of the images, decoded using an ideal observer analysis, were sufficiently selective to account for the observed behavioral classification of the images. The responses of a separate population of neurons in aIT cortex recorded in subsequent sessions while the monkeys viewed the same images, were less selective than neural responses measured during sessions in which the 2AFC-DMS task was performed. Our findings show that many neurons in aIT could provide sensory information sufficient for the classification of images when a 2AFC-DMS task was performed.

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

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.

Detection of spontaneous class-specific visual stimuli with high temporal accuracy in human electrocorticography

Most brain-computer interface classification experiments from electrical potential recordings have been focused on the identification of classes of stimuli or behavior where the timing of experimental parameters is known or pre-designated. Real world experience, however, is spontaneous, and to this end we describe an experiment predicting the occurrence, timing, and types of visual stimuli perceived by a human subject from electrocorticographic recordings. All 300 of 300 presented stimuli were correctly detected, with a temporal precision of order 20ms. The type of stimulus (face/house) was correctly identified in 95% of these cases. There were ~20 false alarm events, corresponding to a late 2nd neuronal response to a previously identified event.

Inhibition in inferotemporal cortex: generating selectivity for objectfeatures

Wang and colleagues demonstrate how local inhibition contributes to generating the remarkable specificity of responses of inferotemporal cortical neurons for complex stimulus features.

Algorithms for image database navigation and tuning of object selective neurons in the non-human primate

The problem of object perception has long plagued physiologists, psychologists and computer scientists. Many lines of evidence suggest that neurons in inferior temporal (IT) cortex are critical for object perception. However, despite years of physiological studies and computational models, little is known about the specific neural or mathematical dimensions appropriate for analysis of realistic images. In another field, and without directly referencing physiological mechanisms, computer scientists have successfully developed algorithms to navigate image databases. To the extent that algorithms for image similarity succeed in capturing human perceptual similarity, they should also explain the responses of neurons involved in perception. Here we take two such algorithms and compare algorithm-assessed similarity of images and neural response similarity to those same images.

Color-based estimates of stimulus similarity predict perceptual similarity of image pairs to monkeys

Introduction: Studies of object perception are hampered by the difficulty of mathematically describing the similarity of realistic images. Recently, color based metrics have been developed to search for similar images in large databases of realistic images. Such metrics have been shown to predict human subjects’ similarity rankings of realistic images, because the color of realistic images often depends on their content. Can color similarity metrics also predict perceptual similarity of realistic images to nonhuman primates? Methods: The color similarity metric used was the Earth Mover Distance (EMD, Y. Rubner, Stanford Vision Lab). We measured performance in a delayed-match-to-sample task using pairs of images. In each trial, the sample was presented for 16-512 ms, followed by a mask. After a delay period, two target images appeared, and the monkey was rewarded for making a saccade to the image that matched the sample. Image pairs were chosen so that the EMD between them spanned the range in our database. For each pair, we estimated the threshold viewing duration as the presentation time required to reach 75% accuracy. If the images are perceptually similar, a longer presentation time should be required to complete the task correctly. Results & Conclusions: Performance is correlated with EMD. Low EMD pairs have higher threshold duration (take longer to discriminate) than high EMD pairs. In addition, low EMD pairs incurred performance improvements during daily training sessions, while high EMD pairs did not. This finding is consistent with previous findings that experience improves performance in perceptually difficult tasks. A recent study has also shown that EMD can explain some of the response properties of single neurons in IT cortex, an area of the macaque brain critical for object perception (Allred et al, SFN, 2002). Taken together, these results indicate that a color similarity metric (EMD) may predict Journal of Vision Color-based estimates of stimulus similarity pred... http://www.journalofvision.org/3/9/511/ 2 of 2 7/2/08 1:14 PM perceptual similarity of images to nonhuman primates. History Received August 22, 2003; published October 22, 2003 Citation Allred, S. R., Thompson, J. Y. S., & Jagadeesh, B. (2003). Color-based estimates of stimulus similarity predict perceptual similarity of image pairs to monkeys [Abstract]. Journal of Vision, 3(9):511, 511a, http://journalofvision.org/3/9/511/, doi:10.1167/3.9.511.