Vision is an active and dynamic process. The brain's analysis of scenes requires an interaction between context, perceptual learning and top-down influences. Contextual influences enable neurons to integrate information over relatively large parts of the visual field. They confer selectivity for complex stimulus configurations. Top-down influences are mediated by re-entrant or feedback pathways between cortical areas. They carry rich and varied information about behavioral context, including attention, expectation, perceptual tasks, working memory and motor commands. As a consequence neurons function as adaptive processors that are able to assume different functional states according to the task being executed. Each cortical area represents an association field, linking different pieces of information, which is mediated by long range horizontal connections. We propose an interaction between feedback connections and horizontal connections that changes effective connectivity within this network and that allows neurons to select subsets of inputs, express different components of the association field. As a consequence they take on different functional roles. Perceptual learning requires establishing the appropriate linkage between feedback and intrinsic connections, whereby top-down influences are engaged in the encoding and recall of learned information. The receptive field is therefore a dynamic entity, allowing neurons to carry information that is relevant to current perceptual demands. Moreover, all cortical levels of sensory processing are capable of undergoing long term experience dependent functional changes, resulting from the normal process of perceptual learning and from functional recovery following CNS lesions. The circuitry underlying these changes includes the plexus of long range horizontal connections formed by cortical pyramidal cells and the axons of inhibitory interneurons. Alteration of sensory experience leads to a rapid and massive modification of these connections, involving exuberant outgrowth of new axon collaterals and a parallel process of pruning of existing and newly grown axon collaterals. The circuitry of the adult cortex therefore is under a continual long term process of modification as we assimilate new experiences, and short term dynamics as we analyze the constituents of visual scenes.
Charles Gilbert is the Arthur and Janet Ross professor at The Rockefeller University. He obtained his M.D. and Ph.D. from Harvard Medical School. Charles Gilbert's work focuses on the brain mechanisms of visual perception and learning at the level of cortical circuitry. He studies the way in which the brain analyzes visual images, how this analysis is shaped by experience and by higher order cognitive influences. He investigates the mechanisms of information processing by the brain, at the molecular, circuit and perceptual levels. A major focus of his work is on plasticity of the visual cortex: the way in which cortical circuitry and function change during normal perceptual learning and during functional recovery following lesions of the central nervous system. He has revealed its dynamic nature, showing the influence of attention, expectation and perceptual task on cortical function.
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