Intrahemispheric Processing and Subcortical Transfer of Non-Verbal Information in Subjects with Complete Forebrain Commissurotomy

Subjects who had undergone complete surgical division of the forebrain commissures for treatment of intractable epilepsy were tested on a variety of cognitive and perceptual tasks. It was found that the right hemisphere performs as well as the left on a test of abstract concept comprehension when the stimulus materials are presented in a non-verbal format. In light of evidence of a selective right hemisphere deficiency for processing abstract words, this result is taken to imply a dissociation of language and cognition at a high level. A second experiment involved the nature of information which can cross subcortically between the cerebral hemispheres. With stimuli presented to opposite visual hemi-fields for prolonged durations, three commissurotomy subjects were able to make matches which convincingly demonstrated interhemispheric transfer and integration of cognitive information, including concrete and abstract concepts. Transfer between the hemispheres was equally successful in the two directions, though the pathway originating in the right and terminating in the left hemisphere may be more sensitive to some affective and semantic components of the stimulus. The information relayed subcortically is neither verbal nor imagic in nature, but appears to involve contextual or connotative associations of the stimulus. Implications for the evolution and development of non-verbal thought include the possible existence of a common bilateral cognitive system which permits interhemispheric communication of complex, if imprecise, associations that are distinct from the more specific verbal and visuospatial constructs of the left and right hemispheres, respectively. Finally, differences in the ability of the two hemispheres to perceive figure and background were described for four commissurotomy subjects. While the left hemisphere preferentially identified figures from briefly-presented picture compositions, the right hemisphere was equally adept at recognizing both figure and ground. The right hemisphere was also more sensitive to background influences on object perception, and was furthermore able to use "natural" gradient and perspective cues in evaluating an object's size and position in a field. In sum, the results demonstrate (1) the richness and complexity of non-verbal information and its place in human thought processes, and (2) the sophistication of the right hemisphere as a perceptual and cognitive system.