The visual system is generally limited in both the quantity and quality of information it can process at any given time. Attentional selection, the brain's mechanism to cope with this wealth of information, enhances the processing of particular stimuli, relative to less important ones, in order to serve perception and behavior. Behavioral evidence suggests that the selection of visual stimuli for detailed investigation depends on the interaction of stimulus-driven (exogenous) and goal-driven (endogenous) attentional mechanisms. This work aimed to better characterize the neural basis of exogenous and endogenous mechanisms governing attentional selectivity in visual search tasks, and to explore the relationship between these two modes in a normal human population.; Employing behavioral techniques, we investigated the possible role of the magnocellular (M) visual subsystem in mediating stimulus-driven processes during a visual search task and found that selectively impairing the M system did not affect search performance, suggesting that visual search does not necessarily depend on processing by this visual sub-system. We then employed positron emission tomographic (PET) imaging to explore the neural activity elicited by varying the relative influence of top-down and bottom-up attentional factors in visual search. Our findings indicate that the neural processes underlying visual search are distributed over an extensive network of brain regions, with varying roles for different parts of the network as the dynamics of top-down vs. bottom-up influences shift. For instance, the conjunction of bottom-up processing with top-down selectivity may account for activity in the right superior temporal gyrus/insular cortex. The left lateral cerebellum appears to play a role in attention, either in signaling popout or in switching attention repeatedly between multiple visual attributes. A final set of behavioral studies focuses on interactions between feature singletons in visual search. Previous studies have resulted in often conflicting results, with no emerging consensus regarding the specificity of top-down selectivity. The present studies show that, during search for a singleton target, multiple non-target singletons compete for attention, and interference between them is strongest for features closely related to the distinguishing target feature. Overall, these observations converge on the conclusion that visual search is extremely flexible and subject to considerable specificity of top-down control, although such specificity is not necessarily absolute. Additionally, the process of attentional selection involves competitive neuronal interactions, consistent with the biased competition model of Desimone and Duncan (1995) in which the enhancing effect of attention on neuronal responses is best understood as an emergent property of many neural systems working to resolve competition among all of the stimuli in the visual field for control over behavior. Further studies must focus on understanding the host of issues that comprise attentional templates that generate top-down bias to influence bottom-up interactions. By addressing the phenomenon with various experimental paradigms and multiple techniques, the work as a whole contributes to correlating the emerging behavioral profile of attentional selectivity to its neural foundations.
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