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