Symmetry and selective attention: a dissociation between effortless perception and serial search.

It is widely assumed that symmetry is an important visual primitive, probably encoded without the need for attention. Julesz's (1981) definition of effortless perception, which states that any stimulus property perceived for exposure durations of 160 msec or less is detected preattentively, contributed greatly to this belief. Single pattern studies confirm that symmetry is detected within this limit. In the present study, however, Julesz's operationalization is compared with the multiple pattern visual search task, to see whether symmetry as a wholistic property is detected in parallel. The results show that symmetry detection times are highly dependent on the number of distractor patterns. The findings are similar for dot patterns, wire polygons, solid block shapes, and simple parentheses. We conclude that symmetry detection per se requires selective attention, but that some related grouping or segmentation mechanism may operate preattentively.