SPATIAL DIFFERENCES IN THE SITES OF DIRECT AND INDIRECT ACTIVATION OFCORTICOSPINAL NEURONS BY MAGNETIC STIMULATION

Transcranial magnetic stimulation (TMS) over the human motor cortex ev okes multiple descending volleys possibly through activation of differ ent elements within the brain. We have investigated whether such eleme nts can be distinguished spatially. Using a figure of eight coil, TMS was delivered over multiple scalp sites during a low level voluntary c ontraction of the left first dorsal interosseous muscle. At near-thres hold intensity, early or late surface electromyograph (EMG) components (relative to anodal response latency) could be preferentially evoked with the coil aligned in a medio-lateral (ML), antero-posterior (AP), or postero-anterior (PA) orientation. The optimal location of the earl iest component with ML coil orientation was 8 mm medial and 5 mm anter ior compared to a later component with AP orientation. The optimal loc ation for the same latency EMG component mapped using two different co il orientations (AP and ML) was not significantly different. The optim al location of two different late components, one obtained with AP and the other with PA coil orientations, was similar. It is argued that t he earliest TMS-evoked component results from direct activation of cor ticospinal cell axons while later components result from activation of these cells trans-synaptically (indirectly), and that consequently th ere is a substantial spatial separation between these activation sites .