Rostro-caudal TMS mapping of immediate transcranial evoked potentials reveals a pericentral crescendo-decrescendo pattern

Background

We recently demonstrated that single-pulse TMS of the primary sensorimotor hand area (SM1_HAND) elicits an immediate transcranial evoked potential (iTEP). This iTEP response appears within 2–8 ms post-TMS, featuring high-frequency peaks superimposed on a slow positive wave. Here, we used a linear TMS-EEG mapping approach to characterize the rostro-caudal iTEP expression and compared it to that of motor-evoked potentials (MEPs).

Methods

In 15 healthy young volunteers (9 females), we identified the iTEP hotspot in left SM1_HAND. We applied single biphasic TMS pulses at an intensity of 110 % of resting motor threshold over six cortical sites along a rostro-caudal axis (2 cm rostral to 3 cm caudal to the SM1_HAND hotspot). We analyzed site-specific iTEP and MEP responses.

Results

iTEP magnitude decreased rostrally and caudally from the SM1_HAND hotspot. MEPs exhibited a similar rostro-caudal crescendo-decrescendo pattern. While iTEP and MEP response profiles were similar, normalized iTEP amplitudes decayed less rapidly at the first postcentral site.

Discussion

These findings support the idea that pericentral iTEPs reflect a direct response signature of the pericentral cortex, possibly involving a synchronized TMS-induced excitation of cortical pyramidal tract neurons. Similar but non-identical rostro-caudal patterns suggest that iTEPs and MEPs may arise from overlapping but distinct neuronal populations.