An EEG signature of immediate motor cortex activity evoked by single-pulse TMS

Background
Transcranial evoked potentials (TEPs) are widely measured with electroencephalography (EEG) to study the cortical responses to transcranial magnetic stimulation (TMS). Immediate transcranial evoked potentials (i-TEPs) evoked a few milliseconds after TMS may constitute a valuable direct marker for functional engagement of the targeted cortex, but i-TEPs have so far been obscured by pulse and muscular artifacts.
Methods
In 25 healthy individuals, we recorded i-TEPs evoked by a single biphasic TMS pulse targeting the primary motor hand area M1-HAND. Sampling EEG at 50 kHz enabled us to reduce the duration of the TMS pulse artifact to a few milliseconds, while minor adjustments of TMS position enabled us to avoid cranial muscular twitches during the experiment. In a series of follow-up experiments we explored how i-TEPs responded to well-known experimental manipulation including paired pulse TMS. We furthermore explored the cortical origin of i-TEPs and conducted control experiments to exclude an artefactual origin.
Results
All participants showed an early positive EEG deflection starting after app. 2 ms followed by a series of superimposed peaks with an inter-peak interval of 1.1–1.4 ms in electrodes surrounding the stimulated sensorimotor region. This multi-peak i-TEP response was only evoked by TMS engaging the peri-central region and was modified by changes in stimulation intensity and current direction. All follow-up and control experiments supported a motor cortical origin of the i-TEPs.
Discussion
Single-pulse TMS of the M1-HAND evokes an immediate local multi-peak response at the cortical site of stimulation indicating a repetitive synchronized excitation of pyramidal neurons in the targeted precentral cortex. This new direct read-out of TMS evoked motor cortex activity can be broadly applied to understand motor physiology. It remains to be explored if similar immediate response signatures can be unveiled with TMS-EEG outside of motor cortex