The neural mechanisms for the visual recognition of objects extend beyond the primary visual cortex into multiple extrastriate cortical areas within the occipital and temporal lobes. To understand the neural mechanisms of perception, attention and memory in these areas, we are recording the activity of neurons both in anesthetized, immobilized monkeys and in awake monkeys engaged in a task requiring visual discrimination, selective attention, and memory. We have found that neurons in one extrastriate area, area V4, code many different stimulus features useful for object recognition, such as the length and width of contours, textures, and colors. Since neurons in this area are sensitive to form and color differences between a stimulus and its background, they may play a role in separating figure from ground. In both V4 and the inferior temporal cortex, we have found that selective attention gates visual processing by filtering unwanted information from receptive fields. Even the degree to which attended stimuli are processed in these areas depends on """"""""how much"""""""" attention, or effort, is devoted to them. Thus, the information-processing capacity of cortical neurons depends not only on hard-wired mechanisms but on cognitive state. Since we do not find neuronal effects of spatially directed attention in either the primary visual cortex or area V2, whatever structures gate extrastriate responses to attended stimuli must work at the level of V4 and beyond. We are currently attempting to identify these structures.
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