Lateral Differences in Normal Man and Lateralization of Brain Function

Abstract

The historical development of work on lateral differences (LDs) is described and an evaluation is attempted of the present state of the study and of the conceptual and methodological problems which it encounters. Contemporary work is mainly motivated by the notion that LDs reflect hemispheric specialization and thus provide a means of studying hemisphere function in easily available normal subjects. Work on LDs in visual recognition has first been conducted, however, within a completely different conceptual framework. Right visual field advantage (RVFA) for words and letters was first thought of as reflecting the left‐to‐right direction of latin writing which produces opportunity for differential perceptual learning in the two hemifields or, as was later considered, creates left‐to‐right scanning habits. On the contrary, right ear advantage (REA) in dichotic listening was, from its discovery by Kimura, shown to be linked to lateralization of speech control in the left hemisphere. The possibility that visual field effects could also be related to hemispheric specialization was then examined by looking for effects of handedness and for correlations between LDs and pathological data. After considerable initial resistance, the notion has become widely accepted. While the early work tended to deal with broad categories of tasks or of stimulating materials, more analytical approaches have been developed and investigators have tried to specify the component operations which are responsible for observed LDs. It has been shown that a same task can give rise to different patterns of asymmetry according to the particular operating mode which is adopted. Chronometric methods have been used to isolate processing stages with different lateralizations, and specific hypotheses have been advanced concerning levels at which lateralization originates. Regarding the mechanism of LDs, the most often considered interpretation is based on the notion of direct access, i.e. of an advantage associated with primary projection in the competent hemisphere. According to whether localization of the critical operations is strict or relative, one of two alternative versions of direct access, called respectively the callosal relay and efficiency models, apply. Other mechanisms based on hemispheric activation or priming and on interference between operations competing for hemispheric processing capacity have also been considered and presumably contribute, beside direct access, to observed LDs. There are also other sources for LDs than brain asymmetry, a fact which tends to be overlooked with the current focusing of interest on hemispheric interpretations. Even in cases where the role of hemispheric specialization has been established, other determinants can play amplifying or masking roles, as has been shown for auditory laterality effects. Current attempts at providing simple dichotomous general characterizations of hemisphere function, such as the one in terms of holistic vs. analytic processing, are described. The opinion is offered that such attempts are premature and that they reflect an underestimation of the complexity of the problem. It is argued (1) that the present catalogue of registered LDs is both too narrow and too subject to selective biases to provide a basis for ambitious generalizations; (2) that a convincing account of lateralization would require an understanding of its evolutionary origin; (3) that it would also require a more advanced understanding of the various mechanisms, including interhemispheric interactions, intervening between hemisphere competence and performance.