Visual search: Bottom-up or top-down?

The aim of the experiments in this paper was to explore the relationship be tween top-down and bottom-up processes in visual search. Employing behavior al techniques, we first consider the possible role of the magnocellular vis ual pathway in visual search, and find that visual search does not necessar ily depend on processing by this visual sub-system. We next use functional imaging (positron emission tomography) to explore the effect of varying top -down strategy during visual search. Our findings indicate that the neural processes underlying visual search are distributed over an extensive networ k of brain regions, with varying roles for different parts of the network a s the dynamics of top- down vs. bottom-up influences shift. The conjunction of bottom-up processing with top-down attentional suppression of an irrele vant singleton could account for activity found in right primary visual cor tex (V1). The conjunction of bottom-up processing with top down attentional set could explain activity noted in the right superior temporal gyrus/insu lar cortex. The left lateral cerebellum appears to play a role in attention , either in signaling popout or in switching attention repeatedly between m ultiple visual attributes. Loci in left parietal cortex (parietal operculum /superior temporal gyrus, parieto-occipital fissure and precuneus) are impl icated in attention-demanding search for a target shape. Returning to behav ioral experiments, we find that, when multiple feature singletons compete f or attention, interference between them is strongest for features closely r elated to the distinguishing target feature. This competition appears to be feature-level rather than object-level, and is characterized by a varying degree of specificity for different features. Task complexity modulates int erference effects, even for abrupt visual onsets, which are often considere d to capture attention involuntarily. Overall, our observations converge on the conclusion that visual search is extremely flexible and subject to con siderable specificity of top-down control, although such specificity is cle arly not absolute.