| Summary: | A complex visual scene will typically contain many different objects, few of which are currently relevant to behavior. Thus, attentional mechanisms are needed to select the relevant objects from the scene and to reject the irrelevant ones. Neurophysiological studies in monkeys have identified some of the neural mechanisms of attentional selection within the ventral, ldquoobject recognitionrdquo, stream of the cortex, which begins with area V1 and continues through areas V2, V4, and IT cortex. At each stage along this stream, attended, or behaviorally relevant, stimuli are processed preferentially compared to irrelevant distracters. The source of the attentional feedback to visual cortex seems to originate in parietal and prefrontal cortex. We proposed some years ago that this attentional feedback biased competitive interactions among neurons in visual cortex, in favor of neuronal responses to the most behaviorally relevant stimulus. More recent work indicates that these competitive interactions are one aspect of a more general visual mechanism for contrast normalization, which is present in most or all visual areas. By providing the appropriate input to this normalization mechanism, feedback from parietal and prefrontal cortex appears to shift the balance of visual cortical responses towards the attended stimulus.
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