 What is the source of this attentional modulation? Because neuroimaging studies have demonstrated that multiple cortical regions are recruited during tasks involving selective attention, it has proven difficult thus far to determine the differential contributions of each region. A central goal of the proposed research is to characterize the contributions of prefrontal cortex and parietal cortex and test the hypothesis that these regions exert top-down modulatory influences over visual processing areas. Specifically, we wish to investigate the interaction between areas involved in attentional control and visual areas modulated by attention. 