Is cerebral asymmetry of function a dynamic process? Implications for specifying degree of lateral differentiation.

It is proposed that the product-moment correlation between a subject's trial-to-trial performance and sensory half-field in which a stimulus appears be employed as an index of functional differentiation of the cerebral hemispheres, both when reaction time is the dependent variable and when accuracy is the dependent variable. This proposal is based on a model of hemispheric functioning in which the difference in competencies of the two sides of the brain, rather than being a static and unchanging dimension of an individual subject, is seen to vary from moment to moment, a variability that is just as characteristic of the cerebral hemispheres and their relationship as is the mean level of performance for each side. The relative degrees of asymmetry inferred for different subjects differ for laterality indices based on the static assumption verus correlational indices based on the dynamic assumption.