Role of the corpus callosum in the somatosensory activation of the ipsilateral cerebral cortex: an fMRI study of callosotomized patients

To verify whether the activation of the posterior parietal and parietal ope rcular cortices to tactile stimulation of the ipsilateral hand is mediated by the corpus callosum, a functional magnetic resonance imaging (fMRI, 1.0 tesla) study was performed in 12 control and 12 callosotomized subjects (th ree with total and nine with partial resection). Eleven patients were also submitted to the tactile naming test. In all subjects, unilateral tactile s timulation provoked a signal increase temporally correlated with the stimul us in three cortical regions of the contralateral hemisphere. One correspon ded to the first somatosensory area, the second was in the posterior pariet al cortex, and the third in the parietal opercular cortex. In controls, act ivation was also observed in the ipsilateral posterior parietal and parieta l opercular cortices, in regions anatomically corresponding to those activa ted contralaterally. In callosotomized subjects, activation in the ipsilate ral hemisphere was observed only in two patients with splenium and posterio r body intact. These two patients and another four with the entire splenium and variable portions of the posterior body unsectioned named objects expl ored with the right and left hand without errors. This ability was impaired in the other patients. The present physiological and anatomical data indic ate that in humans activation of the posterior parietal and parietal opercu lar cortices in the hemisphere ipsilateral to the stimulated hand is mediat ed by the corpus callosum, and that the commissural fibres involved probabl y cross the midline in the posterior third of its body.