Stroop effects within and between the cerebral hemispheres: studies in normals and acallosals.

Interference and facilitation of tachistoscopically presented colour stimuli by adjacent incongruent and congruent colour-words (the Stroop effect) was examined in the right and left visual fields, and centrally, in normal subjects and three acallosals. In the central condition, the word and colour were each separated across the fovea. Normal subjects showed a small left hemisphere advantage for colour naming. The combined Stroop effect (CSE), that is the reaction time difference between incongruent and congruent pairs, was the same in both visual fields but reduced centrally. Furthermore, in the central condition, the CSE was the same regardless of the side on which the word or colour appeared. The reduced CSE in the central condition was exaggerated in one of the acallosals, suggesting that the corpus callosum, by acting as a partial barrier, mediates the interhemispheric Stroop effects. The other acallosals while providing some tentative support for this, showed evidence of lateralized dysfunctions to that their performance was less easily interpreted. The methodology described would appear to have applications in studying the functional characteristics of the corpus callosum in a variety of clinical groups.