Perceptual selectivity is task dependent: evidence from selective search.

Four visual search experiments examined the allocation of attention in a multi-item display with a salient stimulus difference. If, irrespective of its relevance to performance, an item with a unique property attracts attention, it can be said that the unique property automatically captures attention. Alternatively, if capturing attention depends on the relevance of the unique property it would suggest that subjects are capable of engaging in active filtering. In the experiments, subjects searched visually for a horizontal line segment positioned in either one of the 4, 8 or 16 surrounds, arranged in an imaginary circle around the fixation point. In each trial, there was always one surround that differed from the others. The data showed that attention was not automatically attracted to the exceptional form (experiment 1) or exceptional color (experiment 2). The control conditions showed that form and color differences could serve as the basis for active allocation of attention to the unique item. Attention capturing is occasionally observed when the surround changes abruptly in form (experiment 3) but not when it changes abruptly in color (experiment 4). Implications of these findings are discussed in relation to theories of attention that distinguish between a first, parallel stage of perceptual processing followed by a second stage of limited capacity. The results are compatible with the notion that pre-attentive processing is under strategic control and can be stopped by filtering on a spatial basis. The results are related to the specific properties of the transient channels in the visual system.