Neurophysiological and neuropsychological methods are used to investigate the influence of one hemisphere on the activity of the other one. The findings concern contralateral motor irradiation, mirror movements and contralateral transmission of evoked potentials. At transcallosal transmission of information both quality and quantity of the information as well as the functional specification of the hemispheres is called into play. In derangements of the corpus callosum various disconnection syndromes develop, determined by the extent and localization of the lesion. Examination of patients with commissurotomy revealed a remaining partially operative interaction between the two hemispheres. The question of the relationship between conscious subjective experience and cerebral activity remains a philosophical problem. (Ref. 49).