Neural Coding of Visual Stimuli

National Institute Of Mental Health

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Abstract

Primates rapidly categorize visual stimuli by recognizing shared features across individual stimuli. Previous studies identified two subregions of the primate brain, inferior temporal cortex (IT), TEO and TE, that contribute to categorizing visual images. To identify a stimulus as a member of a category it is necessary to compare the viewed stimulus with a remembered template. To examine the neural plasticity underlying category learning, we recorded neuronal activity simultaneously from TEO and TE using chronically implanted electrode arrays. In two monkeys we monitored the neural activity during training on a visual categorization task. The stimuli were morphs of cats and dogs, that is, images that were mixtures of images of cats and dogs in different proportions. As the monkey learned, the activity of TE neurons increased in both category specificity (distinguishing between stimuli of different categories) and category generalization (generalizing across stimuli within the same category), whereas TEO neurons showed little change in either measure. Category representation in TE was enhanced during trials where the monkey correctly identified the stimulus versus error trials. Single-cell analyses revealed that category selectivity in TE strengthened with learning, primarily through enhanced encoding of one category. Together with our previous behavioral evidence, these results highlight a key role for experience-dependent plasticity in TE in supporting visual category learning. Thus, there is stronger learning in TE than TEO.