Uneven interhemispheric connections between left and right primary sensori-motor areas.

To clarify the characteristics of interhemispheric connections, we investigated cortico-cortical evoked potentials (CCEP) in human. Fourteen patients with temporal lobe epilepsy who underwent invasive EEG monitoring with bilaterally implanted subdural electrodes were studied. Electric pulse stimuli were given in a bipolar fashion at two adjacent electrodes on and around the motor area (MA) or sensory area (SA), and CCEP responses were recorded by averaging electrocorticograms from the contralateral hemisphere. Seventy-two pairs of electrodes were stimulated, and 468 recordings were analyzed. Fifty-one of 468 recordings demonstrated CCEP responses. Of 51 responses, 16 consisted of an initial positive triphasic wave (Type 1), 27 had an initial negative biphasic wave (Type 2), and 8 showed an initial positive biphasic wave (type 3). The mean latencies of the earliest peaks were 13.1, 28.9, and 29.4 ms in Types 1, 2, and 3 responses, respectively. The responses were more frequently evoked by stimulating facial MA (f-MA) and nonfacial MA (nf-MA) than by stimulating SA or noneloquent area. In both f-MA and nf-MA stimulation, the responses were more frequently recorded at the contralateral f-MA than at the contralateral nf-MA or other areas. SA stimulation never evoked CCEP responses at the contralateral MA or SA. The amplitudes were maximal when f-MA was stimulated and responses recorded at the contralateral f-MA. These findings suggest that the interhemispheric connections are uneven. Both f-MA and nf-MA send dense interhemispheric connections to the contralateral f-MA. SA may have no or only rare direct connection with the contralateral MA or SA.