Transcranial magnetic stimulation (TMS) applied over a particular cortical region can interfere with cognitive processing in that region, thereby creating a virtual disruption in the intact human cerebral cortex. Hence, this tool can be used to determine if a brain region is functionally involved in the performance of a given cognitive function. In the proposed study, we wish to use TMS to test the hypothesis that attentional filter mechanisms operating in extrastriate cortex are under "top-down" control by regions in the frontal and parietal cortex. Our approach will use a combination of TMS and functional magnetic resonance imaging (fMRI).