Magnetic stimulation mapping of motor cortex: factors contributing to map area

Transcranial magnetic stimulation (TMS) can evoke an electromyographic response in muscles of the hand with stimulation at a large number of scalp sites widely spaced over the contralateral primary motor area of the brain. To determine the extent to which this is due to current spread from the stimulating coil to a smaller region of excited cortex, excitability curves of motor evoked potential amplitude vs. stimulus intensity were measured at multiple scalp sites. It was found that these curves were of very similar shape, but with different offsets along the stimulus intensity axis. This could be explained on the basis of current flow to a small excitable region of cortex located at some depth in the brain. It is concluded that the surface area of TMS maps is primarily determined by current spread and by the relationship between the position of the coil on the scalp and the depth of the motor output region in the cortex, and does not necessarily provide a true indication of the spatial extent of the motor output region projecting to the target muscle.