Parallel processes within the 'spot-light' of attention.

Abstract

Human ability to identify simultaneously two targets in the visual field is severely limited. Previous studies have shown that orientation identification of two targets takes twice the time needed for one target. Here we asked whether this seriality is imposed by the decision requirement of the task or by such stimulus properties as target spatial separation and similarity. Observers had to identify the orientations (vertical vs horizontal) of two Gabor patches presented at random positions. Performance on this double-task experiment was compared with performance on each of the tasks when carried out alone. We varied the spatial separation between the two targets for targets having identical or different spatial-frequencies and found that the orientation of two targets having different frequencies could be identified in parallel when occupying the same spatial position but not when separated in space by 4 deg of visual angle or more. Targets having the same frequency could be identified in parallel even when separated by 8 deg, demonstrating that decision factors do not impose seriality. This result can be taken as evidence for the existence of a grouping process operating prior to orientation identification. This grouping process operates according to classical Gestalt rules (proximity, similarity) and enables parallel attentive processing of large input chunks.