Functional significance of the corpus callosum based on the analysis of rhythmic capabilities in the split-brain patients.

We carried out our Systematic Rhythm Test for the Brain-Damaged Patients named "The Asian Rhythm Test" on our four patients with lesions of the corpus callosum, and analyzed their rhythmic capabilities--their performances in rhythm perception (auditory distinction) and in rhythm expression (rhythm-tapping)--in accordance with the neuropsychological model proposed in 1980 by us. All our partial split-brain patients showed the abnormal one-ear inferiority (superiority) and the one-hand inferiority under our testing conditions. The patients with lesions of the anterior corpus callosum showed the interhemispheric (the left-to-right) transfer-dysfunction of motor-programming information, while the patient with the lesion of the posterior corpus callosum showed the interhemispheric transfer-dysfunction of auditory information. The patients with lesions including the truncus showed the interhemispheric integration-loss in the process of rhythm perception and its expression. And moreover, we could see the specific auditory interruption between the ipsilateral and the contralateral systems in the patient with the lesion including the anterior commissure. Based on these our findings, the roles of the corpus callosum and the anterior commissure were discussed.