Optimal transcranial magnetic stimulation sites for the assessment of motor function

The optimal placement sites for eliciting motor evoked potentials from the abductor digiti minimi and abductor hallucis muscles by means of transcranial magnetic stimulation were determined using a commercially available circular coil. Fifty volunteers were used for the study. The ability to elicit responses was found to be strongly dependent upon the scalp placement of the stimulator coil. The effects of altering the direction of current flow in the coil were tested on two different sets of volunteers: clockwise in one and counterclockwise in the other. It influenced only the locations of the sites which were optimal for eliciting responses from the abductor hallucis muscles and not those which were optimal for eliciting responses from the abductor digiti minimi muscles. Response latencies were found to be significantly dependent only upon volunteers' heights and not on their sex, age, or weight or the stimulus intensities used to elicit responses. No previous studies have defined the optimal scalp placements for eliciting responses from the lower extremities. This information may have clinical importance for making reliable assessments of patients with significantly impaired motor function.