Lateral asymmetries in visual perception: a review of tachistoscopic visual half-field studies.

Experiments using the visual half-field paradigm are reviewed with special reference to the three theories which purport to account for perceptual asymmetries: the left-to-right internal scanning mechanism theory, the direct anatomical access theory, and the attentional bias theory. Recent evidence has clearly shown that the direct anatomical access theory can account for virtually all lateral asymmetries in the perception of verbal and spatial stimuli. The results of the studies reviewed strongly support a model of brain function which suggests that the simultaneous processing of verbal and spatial stimuli does not constrict the attention of either hemisphere. The difficulty in separating the factors of hemispheric processing and stimulus characteristics is discussed. It is suggested that a reliable conclusion about the role of the cerebral hemispheres can only be made when a better description of the operating characteristics of the hemispheres is available.