The Roles of Relevance and Expectation for the Control of Attention in Visual Search

Representations of target-defining features (attentional templates) control the allocation of attention during visual search. We investigated whether template-guided attentional selectivity is sensitive not only to the relevance of visual features, but also to expectations about their probability. Search displays could contain a target in an expected (80%) or unexpected (20%) color. They were preceded by spatially uninformative cues that matched either the expected or unexpected target color. These color cues attracted attention, reflected by behavioral spatial cueing effects and by cue-elicited N2pc components obtained via EEG measured during task performance. Critically, these attentional capture effects were identical for both color cues, suggesting that preparatory attentional templates only reflect relevance, and are insensitive to expectations about target color probabilities. In contrast, RTs and N2pc components to search targets in the unexpected color were delayed, showing that expectations modulated the speed of attentional target selection within search displays. This dissociation between the effects of relevance and expectation on attentional preparation versus target selection suggests that these 2 parameters for attentional control are represented differently. Task relevance is likely to be specified at the level of individual features, whereas expectations could be represented in an object-based fashion.