Similarity between concurrent visual discriminations: dimensions and objects.

Accuracy is often reduced when two visual discriminations must be made concurrently ("divided attention"). According to a hypothesis originally proposed by Treisman (1969) and Allport (1971), this result should depend on the similarity of required discriminations. When discriminations concern different visual dimensions, they should be made in somewhat separate visual subsystems, reducing interference between them. This prediction was tested in two experiments, involving discriminations of shape, size, orientation, and spatial frequency. In different conditions of divided attention, concurrent discriminations concerned either the same or different dimensions, and either one or two objects. The results showed that performance depends only on the number of relevant objects, not on the number or similarity of required discriminations. They suggest that selective attention to an object is a coordinated state in which the outputs of multiple visual subsystems are made concurrently available for control of behavior.