Disorganization and reorganization of cognitive and sensorimotor functions in cerebral commissurotomy. Compensatory roles of the forebrain commissures and cerebral hemispheres in man.

Neuropsychological studies of 10 patients with complete cerebral commissurotomy and 2 with frontal commissurotomy showed differences in the nature and severity of pre-operative, initial and long-term post-operative deficits in verbal and nonverbal cognitive, memory, and sensorimotor functions. Neurosurgical findings before and after operation recorded various types of extra-callosal brain damage in all 12 patients, including 4 with gross structural lesions in the right hemisphere before surgery. Our findings revealed that differences in the nature and severity of pre-operative and post-operative deficits varied according to the nature and laterality of associated extra-callosal damage before as well as after operation. Following complete commissurotomy, all 4 patients with pre-operative structural lesions of the right hemisphere revealed a unique pattern of marked, selective impairment of nonverbal cognitive functions strikingly similar to the effects of right hemispherectomy for tumour in adults. The remaining 8 patients showed systematic patterns of greater impairment in verbal and nonverbal cognitive and contralateral sensorimotor functions, depending on the laterality of extra-callosal damage. The significantly higher pre-operative Wechsler Performance IQs of the 4 patients with structural lesions of the right hemisphere suggest that following earlier damage to the right hemisphere, the forebrain commissures contribute to a reorganization of interhemispheric relations to permit restitution of the diminished nonverbal cognitive functions resulting from damage to the right hemisphere. Thus, in addition to other possible roles in integrating the functions of the two hemispheres in the normal brain, the forebrain commissures can also serve as 'standby' or 'reserve' structures for reorganization of compensatory interhemispheric mechanisms following lateralized brain insults. Since some patients showed later postoperative improvement, the extent of ultimate competence and the 'cognitive costs' of commissurotomy can only be defined by continuing follow-up studies. Apart from their practical clinical implications, such studies may also contribute to elucidation of the principles underlying the organization, disorganization and reorganization of human brain functions.