Our senses provide us with a vast amount of information at any given moment in time. For example, visual scenes contain many different objects that cannot all be processed simultaneously because of the limited processing capacity of the visual system. Therefore, attentional mechanisms are required to select relevant objects and filter out irrelevant ones. There is converging evidence from single-cell recording and lesion studies in monkeys and functional brain imaging studies in humans that irrelevant information from cluttered visual scenes is filtered out in extrastriate visual cortex. Functional brain imaging studies also suggest that these attentional filtering mechanisms are mediated by top-down feedback signals arising in higher-order areas in frontal and parietal cortex. However, it is not clear from these studies whether these frontal and parietal brain regions are functionally significant in attentional control of behavior.