Abstract
During naturalistic viewing, goal-directed eye movements allow humans to foveate on locations containing potentially relevant visual information. These saccadic explorations of the visual field are thought to be guided by cortical top-down signals into visual areas, essential for successfully processing visual information. However, little is known regarding when and how these putative cortical signals inform goal-related saccadic eye-movements. To address this question, we examined fixation-locked phase synchrony from dense-array EEG while 30 participants visually searched nature scenes for a target animal. Scenes flickered at 30 Hz, to invoke a steady-state visually evoked potential (ssVEP) as a measure of primary visual cortical activity. We found differences in ssVEP phase-locking preceding fixations to targets, compared with control fixations to locations that did not contain a target. In particular, phase locking at the steady-state frequency remained high across a 400 ms window surrounding control fixations, suggesting consistent synchronous activation of visual cortex during control fixations by the flickering stimulus. However, 150 ms before fixation on a target, phase locking at the steady-state frequency was interrupted, suggesting less synchronous activity related to incoming visual information immediately preceding a successful saccadic eye movement. During this same time window preceding target fixations, phase locking, inter-site phase locking, and power in the alpha range increased relative to control fixations, suggesting increased top-down signaling before successful goal-related saccades. Broad band gamma activity increased 60 ms before successful target saccades, but sharply decreased immediately following fixations for another 60 ms. This gamma pattern was reversed for control fixations, showing a sharp decrease 60 ms before fixation, and a slight increase following fixations. Together, these results suggest that oscillatory signaling within visual cortex as well as between visual and extra-visual areas may mediate saccadic behavior during goal-directed visual search.
Meeting abstract presented at VSS 2016