Mapping the rostro-caudal expression of immediate transcranial evoked potentials (i-TEPs) revealed a neural response signature of the pericentral sensorimotor cortex

Background: The combination of transcranial magnetic stimulation (TMS) and electroencephalography (EEG) is powerful but challenging. Specifically, stimulus and muscle artifacts have until recently obscured the immediate neural response to TMS. We have shown that it is possible to record immediate transcranial evoked potentials (i-TEPs) over the left primary sensorimotor hand area as early as 2ms after the TMS pulse (Beck et al., 2024), consisting of a high-frequency component (650-800Hz) and a slower component. Here, we explored whether the i-TEPs reflect a response signature characteristic of the pericentral cortex, using a spatial gradient of rostro-caudal stimulation sites.
Methods: 15 healthy volunteers (10 females; mean age: 27; range: 20-35) participated in the study. TEPs were recorded with 61-channel EEG at a sampling rate of 50 kHz. Single biphasic TMS pulses were delivered at 110% of the resting motor threshold over six frontoparietal cortical sites along a rostro-caudal line. Each stimulus location was spaced 1cm apart in either the rostral or caudal direction with the central locations corresponding to the left primary sensorimotor hand area.
Results: Cluster permutation testing revealed a significant difference among stimulation sites. i-TEPs were maximally expressed close to the central sulcus and gradually decreased in both rostral and caudal directions. Accordingly, motor-evoked potentials (MEPs) were largest pericentrally and decreased to a similar extent to the i-TEPs in the rostral direction but decreased at a faster rate compared to i-TEPs in the caudal direction.
Discussion: Linear rostro-caudal i-TEP mapping revealed a maximal expression over the pericentral sensorimotor stimulation sites, decreasing with distance from the central sulcus. This spatial pattern supports our hypothesis that i-TEPs represent a direct response signature of the pericentral cortex. This study also demonstrates the feasibility of studying i-TEPs over other brain regions in the future.