Spatial differences in the sites of direct and indirect activation of corticospinal neurones by magnetic stimulation

Transcranial magnetic stimulation (TMS) over the human motor cortex evokes multiple descending volleys possibly through activation of different elements within the brain. We have investigated whether such elements can be distinguished spatially. Using a figure of eight coil, TMS was delivered over multiple scalp sites during a low level voluntary contraction of the left first dorsal interosseous muscle. At near-threshold 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 earliest component with ML coil orientation was 8 mm medial and 5 mm anterior compared to a later component with AP orientation. The optimal location for the same latency EMG component mapped using two different coil orientations (AP and ML) was not significantly different. The optimal location of two different late components, one obtained with AP and the other with PA coil orientations, was similar. It is argued that the earliest TMS-evoked component results from direct activation of corticospinal cell axons while later components result from activation of these cells trans-synaptically (indirectly), and that consequently there is a substantial spatial separation between these activation sites.