Physiological studies have shown that neural activity in area V4 can be modulated by changes in the behavioral relevance of a stimulus. These studies suggest that extrastriate cortex may represent a stage in visual processing where 'top-down' attentional inputs can influence the representation of bottom-up stimulus information. To examine the role of cortical feedback in the mediation of attentional effects in extrastriate cortex, we removed prefrontal cortex unilaterally in combination with a transection of the corpus callosum and anterior commissure in two adult macaques. As a result, visual processing in only one hemisphere could be modulated by prefrontal feedback. Monkeys were trained to discriminate the orientation of a colored target presented among colored distractors. The color of the fixation spot cued the target for discrimination. The stimuli were presented in either the control or the affected hemifield. We found that orientation thresholds in the affected hemifield, but not the control hemifield, increased with increasing frequency of cue change. To determine if the performance deficit was the result of a disruption of top-down processing, we trained one of the monkeys to perform a 'bottom-up' variation of the task in which the target for discrimination was defined by color pop-out. For this task, performance in the affected hemifield was not differentially affected by the frequency of target change, relative to performance in the control hemifield. We attribute the difference in performance between the two tasks to an increase in the "attentional load" or top-down component in the cueing task. When the target identity was defined by color pop-out (bottom-up task), target selection was presumably accomplished by local cortical mechanisms. These findings support the notion that prefrontal cortex is involved in the attentive selection of behaviorally relevant stimuli.

Supported by NIMH IRP.