Mechanisms of visual attention in the human cortex

A typical scene contains many different objects that, because of the limite d processing capacity of the visual system, compete for neural representati on. The competition among multiple objects in visual cortex can be biased b y both bottom-up sensory-driven mechanisms and top-down influences, such as selective attention. Functional brain imaging studies reveal that, both in the absence and in the presence of visual stimulation, biasing signals due to selective attention can modulate neural activity in visual cortex in se veral ways. Although the competition among stimuli for representation is ul timately resolved within visual cortex, the source of top-down biasing sign als derives from a network of areas in frontal and parietal cortex.