Scalp-recorded potentials evoked by TMS

Nakajima, Y., and Kohno, Y.
National Rehabilitation Center for Persons with Disabilities, Department of Rehabilitation of Sensory Functions, Tokorozawa, Japan

Objective: To establish the physiological basis of cerebral potentials evoked by using transcranial stimulation (TMS), this study was conducted.

Methods: Magnetic stimulus was transcranially delivered over the primary motor area of the right thumb in normal subjects. Evoked potentials were recorded at F4, C4, and P4 with a reference of the left ear.

Results: Two potentials were evoked; the earlier negative potential had a latency of 14 msec (N14) and the later negative one had a latency of around 100msec (N100). Both potentials showed the largest amplitude at C4. The N100 component was increased in amplitude with increasing the stimulus intensity. In addition, it was increased in amplitude when the subjects counted the skin sensation induced magnetic stimulus.

Discussions & Conclusions: To identify the N14 component that may be the transcallosal response, contamination of the blink reflex or EMG of the temporal muscle should be eliminated. The origin of N100 component is complicated because somatosensory and auditory pathways are stimulated at the same time by TMS. Then, we discuss on the possible N100 component evoked by cortical stimulation.