Abstract
The study of covert orienting has been an important impetus for the field of cognitive neuroscience. Seminal reaction-time studies demonstrated that a suddenly appearing visual stimulus attracts attention involuntarily, but the neural processes associated with visually guided attention orienting have been difficult to isolate because they are intertwined with sensory processes that trigger the orienting. Here, we developed a framework for disentangling orienting activity from purely sensory activities using scalp recordings of event-related potentials (ERPs). The working hypothesis was that sensory processing of a lateral abrupt onset would drive timing differences between visual evoked activities (e.g., P1 and N1 peaks) recorded contralateral and ipsilateral to the stimulus (because of projections from eye to contralateral visual cortex and the callosal projections connecting the two brain hemispheres), while covert orienting to the stimulus would drive amplitude differences between contralateral and ipsilateral ERPs. We tested this hypothesis by comparing ERPs elicited by lateral visual stimuli under two conditions: one in which participants discriminated a feature of the lateral stimulus (attend lateral), and one in which participant responded to some other, non-lateralized stimulus (attend other). It was presumed that covert orienting would be necessary in the attend-lateral condition but would be minimized in the attend-other condition. We identified an early positive ERP deflection over the ipsilateral visual cortex that was associated with the covert orienting of visual attention. Across five experiments, this ipsilateral visual orienting activity (VOA) was linked with behavioral measures of orienting (i.e., was larger when the stimulus was detected rapidly than when it was detected more slowly), and its onset occurred prior to unrestrained eye movements towards the targets. The VOA appears to be a specific neural index of the visually guided orienting of attention to a stimulus that appears abruptly in an otherwise uncluttered visual field.