The time course of target template activation processes during preparation for visual search

Abstract

Search for target objects in visual scenes is guided by mental representations of target features (attentional templates). However, it is unknown when such templates are activated during each search episode and whether this can be controlled by temporal expectations. We used electrophysiological measures to track search template activation processes in real time. In three experiments, female and male humans searched for a colour-defined target object in search displays where targets were accompanied by distractors in different nontarget colours. Brief task-irrelevant colour singleton probes that matched the target template were flashed rapidly (every 200 ms) throughout each block. Probes presented at times when the target template is active should capture attention, while probes presented at other times should not. To assess this, N2pc components were measured as markers of attentional capture, separately for probes at each successive temporal position between two search displays. Results demonstrated that search templates were active from about 1000 ms prior to the arrival of the next search display, and were de-activated after each search episode, even when the preceding search display did not contain a target object. Templates were activated later when the predictable interval between search displays was increased. Results demonstrate that search templates are not continuously active, but are transiently activated during the preparation for each new search episode. These activation states are regulated in a top-down fashion by temporal expectations about when an attentional template will become task-relevant.