Immediate Transcallosal Reorganization of the Somatosensory Cortex following Ipsilateral Anesthesia

Long-term as well as immediate short-lasting cortical reorganization of the somatosensory cortex following an alteration of the peripheral sensoric input is well established. Sustained attention to the peripheral stimulation modulates the reorganization process. In this study we investigate if a temporary deafferentiation of the contralateral hand results in a different topical arrangement of the somatosensory evoked potentials (SEP) following digit stimulation. SEPs were recorded following electric stimulation of digit D1 and D5 at the left hand before and after anesthesia was applied to D2– D4 at the right hand. Simultaneously to anesthesia the subject was asked to turn his/her attention (i) to the right, (ii) to the left or (iii) to neither hand. Source reconstruction of the N20 component was based on a 96-channel arrangement. Location and 3D-distance of the reconstructed electric sources representing D1 and D5 were compared before and after anesthesia. Before anesthesia we found the expected somatotopic organization of the somatosensory cortex. After anesthesia a significant enlargement of the 3D-distance could be shown for the three different conditions. In particular the cortical representation of the thumb shifted towards the front, along the gyrus postcentralis. A significance of this shift could be demonstrated for the conditions (iii) without spending attention to the stimulation and (ii) when turning attention to the hand being electrically stimulated. The topical reorganization of the somatosensory cortex occurs immediately after interrupting the sensoric input from the hand contralateral to the stimulated side caused by a transcallosal mediated disinhibition. This effect can be modulated by shifting spatial attention.