Visual search alpha: A novel window into lateralized visual attentional processes

Visual search paradigms evoke the attentional processes used in the identification and processing of a target stimulus in an array of distracting items. A widely studied ERP component elicited in visual search is the N2pc, which is thought to reflect the shifting and focusing of spatial attention. Spatial attention has also been linked to modulations in oscillatory alpha (8-14 Hz), whose decreases in amplitude are thought to reflect increases in local cortical activation. Endogenous control of spatial attentional can trigger lateralized changes in alpha, e.g. when participants are cued to spatially attend to one visual hemifield in preparation for an upcoming stimulus. Here we report novel findings for lateralized alpha activity during visual search, when subjects can only reactively shift their attention to a target after the onset of the array. This “visual-search alpha” appears as a decrease in alpha power ~300-600 ms after array/target onset over parietal-occipit al scalp contralateral to the target. We tracked visual-search alpha across three independent visual-search datasets to examine its modulation as a function of: a week-long training regimen (n=13), target-reward associations (n= 24), and target-discrimination difficulty (n=18). Results across all three datasets indicated that this component reflects a target-associated increase in excitation sensitive to both top-down and bottom-up factors, while providing a unique window into attentional processes during visual search. More precisely, we suggest that while the N2pc reflects the deployment of attention at the target location, the alpha reflects attentionally driven focused processing of target-feature information in the face of potential distractor stimuli.