Hemispheric interactions in simple reaction time.

Fifty-eight normal subjects carried out a simple reaction-time task in which they responded unimanually to stimuli presented either singly in the left visual field, singly in the right visual field, or in both visual fields at once. The stimuli were white against a dark background on half the trials and gray against an equiluminant yellow background on the other half. For the unilateral stimuli, the difference in RT between crossed and uncrossed hand-field combinations (crossed-uncrossed difference, or CUD) yielded a measure of interhemispheric transfer time. The difference in RT between bilateral and unilateral stimuli was also measured in different ways to provide measures of redundancy gain (RG). The CUD and RG were uncorrelated, suggesting that they depended on independent processes. Component analysis further suggested that there may have been different processes underlying RG, depending on whether the stimuli were brighter than the background, or equiluminant with it.