Abstract
A longstanding issue in the study of visual processing is whether attention is allocated in serial or in parallel during difficult visual search tasks. In a previous study, we provided electrophysiological evidence indicating that attention is allocated in a serial fashion. However, these data could also be explained by a parallel system that continuously adjusts the distribution of attention as information accumulates. In the present study, event-related potentials (ERPs) were recorded while subjects performed visual search tasks. As in our previous study, we examined the “N2pc” component, an index of the focusing of attention. However, whereas our previous experiment used the N2pc component to monitor the relative allocation of attention among multiple items in the search arrays, the present study was designed to measure the time course of the processing of individual items in the search arrays. By varying the eccentricity of the N2pc-eliciting item, we varied whether this item would be the first or second item processed in the array. Attention was observed to discretely switch between items, being disengaged at one location before being reallocated to a different object location. These results support a serial model of attentional allocation during visual search, and they are difficult to reconcile with parallel models of attention.