Cortical motor representation of the ipsilateral hand and arm.

We sought to determine whether motor evoked potentials (MEPs) as well as silent periods could be produced in hand and shoulder muscles by transcranial magnetic stimulation (TMS) of the ipsilateral cerebral hemisphere and, if so, whether their cortical representations could be mapped with respect to those of contralateral muscles. In six normal subjects, we delivered ten stimuli each to a grid of sites 1 cm apart on the scalp. The EMG was recorded and averaged from the contralateral first dorsal interosseous (FDI) and risorius (facial) muscles at rest and the ipsilateral FDI muscle, which was voluntarily contracted. In four of these subjects and an additional subject, we used the same mapping technique and recorded from the deltoid muscle on the right and left sides and the contralateral FDI during activation of the ipsilateral deltoid. In all subjects, the cortical representation of the contralateral risorius was anterolateral to that of the FDI. The contralateral deltoid could be activated in only three subjects. In them, its representation was slightly medial to that of the FDI. All subjects had at least one scalp site where TMS produced MEPs in the ipsilateral FDI. Two subjects had rich ipsilateral hand representations with multiple ipsilateral MEP sites. Both had ipsilateral MEP sites near the representation of the contralateral FDI, but the largest ipsilateral MEPs occurred with TMS at more lateral sites, which were near the representation of the contralateral risorius. In these subjects, the ipsilateral deltoid was preferentially activated at sites medial and posterior to those activating the contralateral muscle. Ipsilateral TMS also produced silent periods in the FDI in all subjects. These silent periods were much more frequent than the ipsilateral MEPs and tended to occur with TMS near the representation of the contralateral FDI. The excitatory cortical representation of the ipsilateral arm muscles is accessible to TMS in normal subjects and is different from that of the homologous contralateral muscles. The hand may have two ipsilateral representations, one of which produces silent periods and the other MEPs at the same stimulus intensity.