Saliency effects across dimensions in visual search.

In previous work it has been shown that search for an orientation target can only be done fast, and in parallel, when the target's orientation differs sufficiently from that of neighbouring lines. Targets with an increased local orientation contrast appear as salient and are immediately detected. In the present study, the effect of saliency from other stimulus dimensions was investigated. Five subjects were asked to look for vertical lines in texture-like displays, which were presented either embedded in the local orientation flow ("non-salient" target presentation) or at an increased feature contrast ("salient" target presentation). Saliency was obtained from local differences in orientation ("within dimension" saliency) or other visual cues ("cross dimensions" saliency), such as colour, luminance, motion, and disparity. Targets displaying cross-dimensional saliency were always non-salient in the orientation domain. The results show that subjects were slower in detecting targets from non-salient than from salient presentations. The effect of saliency on visual search is not feature-specific and, in particular, is not restricted to the stimulus dimension in which target features are searched for.