2 MODULATORY EFFECTS OF ATTENTION THAT MEDIATE OBJECT CATEGORIZATION IN HUMAN CORTEX

Attentional modulation of cortical activity was examined by varying th e rate of visual stimuli in object categorization tasks according to s ingle and conjoined features. Activation of dorsolateral frontal corte x was independent of the stimulus presentation rate and elicited by th e participant's attention to conjoined compared with single features. Several cortical regions showed attentionally modulated activity. In i nferior temporal cortex, modulation was due to an additional bias sign al underlying normal rate-correlated activity, In two other regions (p remotor cortex and cerebellum), attention modified the correlation of activity and the stimulus presentation rate, Attentional effects in th e human cortex are expressed by at least two physiologically distinct mechanisms acting on spatially distributed areas.