An event-related optical topography study of cortical activation induced by single-pulse transcranial magnetic stimulation.

To visualize cortical activations during transcranial magnetic stimulation (TMS), it is necessary to measure those activations at high spatiotemporal resolution while preventing interference with the magnetic property of a coil. One suitable method that satisfies these demands is optical topography (OT), which has been used in cortical activation studies. In the present study, single-pulse TMS was applied to the left primary motor area, and cortical responses at the stimulation site were measured simultaneously with event-related OT. When TMS was applied at 110% motor threshold (MT), we observed significant oxyhemoglobin increases that were both time-locked and correlated with the hemodynamic basis function. Moreover, when TMS was applied at 90% MT, significant oxyhemoglobin increases were detected even though there were no motor-evoked potentials. These results demonstrate that OT can directly measure cortical responses to subthreshold single-pulse TMS, independent of the afferent feedback from the peripheral neuromuscular activity.