Stimulus-intensity dependence of the EEG-response to transcranial magnetic stimulation (TMS)

Introduction: Transcranial magnetic stimulation (TMS) elicits an EEG response which has been suggested recently as a direct marker of cortical excitability. We examined the development of the EEG response to TMS in children and its relationship to peripheral indexes of cortical excitability (motor evoked potential, MEP amplitude).

Methods: We applied single pulse TMS to the non-dominant motor cortex of 6–10-year-old children and adult subjects and determined the TMS-evoked N100-amplitude in DC EEG recordings. Amplifier deblocking was employed. A forewarned reaction time task served to investigate the influence of sensory attention and motor preparation on the EEG response to TMS. The study was approved by the local ethics committee and performed according to the Declaration of Helsinki.

Results: We found striking changes in the TMS-evoked EEG response with increased stimulation intensities: Children showed a giant TMS-evoked N100 amplitude at motor threshold stimulation intensities of over 100µV, which was easily detectable even in single trials. This amplitude correlated positively with absolute stimulation intensity and negatively with the age of the subject. It differed clearly and highly significantly from the artifact induced by TMS when we stimulated the electrodes on a head dummy. Its latency was over 100 ms and its amplitude was independent from MEP amplitude. During sensory attention and motor preparation, the amplitude of the TMS-evoked N100 was reduced.

Conclusion: TMS-evoked N100 amplitude seems to reflect inhibitory systems because it was independent from MEP amplitude and was reduced during the preactivation of cortical motor areas during motor preparation. Its latency of over 100 ms casts doubts on a generation by direct cortical excitation by the TMS pulse. We would rather propose parallels between the inhibitory N100 after TMS (provoking massive synchronous excitation) and the wave-component of the typical epileptic spike wave complex. Future research will show, whether high stimulation intensity TMS-evoked N100 could serve as a model for epileptic spike wave activity and / or as a diagnostic tool for thalamo-cortical/intracortical inhibition.

References: Bender S, Basseler K, Sebastian I, Resch F, Kammer T, Oelkers-Ax R, Weisbrod M. Electroencephalographic response to transcranial magnetic stimulation in children: Evidence for giant inhibitory potentials. Ann Neurol. 2005 Jul;58(1):58–67.