Differences of the ipsilateral silent period in small hand muscles.

The ipsilateral silent period (iSP) is thought to depend on activity transmitted by the corpus callosum but ipsilateral corticospinal pathways may also contribute. Because the presence of ipsilateral corticospinal pathways differs between small hand muscles, we compared the iSP in the first dorsal interosseous (FDI) and abductor pollicis brevis (APB) muscles. The iSP was elicited in 20 healthy subjects by focal transcranial magnetic stimulation of one primary motor cortex during maximal voluntary contraction of the ipsilateral target muscle. The iSP duration was significantly longer in the FDI than APB because of an irregularly occurring second phase of inhibition in the FDI that was absent in the APB. Although the first phase of inhibition is transmitted by the corpus callosum, we provide evidence that the second phase is mediated through ipsilateral corticospinal pathways. Therefore, for specific assessment of callosal conduction, the iSP should be measured in the APB rather than FDI.