Immediate transcranial evoked potentials (i-TEPs) are evoked by magnetic stimulation of the motor cortical leg representation

Background: Cortical responses to TMS can be recorded using electroencephalography (EEG). Recently, we discovered immediate TMS-evoked potentials (i-TEPs) approx. 2 ms following stimulation of the hand representation of the primary motor cortex (M1-HAND) [1] that potentially could reflect direct TMS-evoked excitation and the cortical source of indirect corticospinal volleys (I-waves). However, it is unknown if i-TEPs, like I-waves, can be evoked from the leg representation of the primary motor cortex (M1-LEG).
Methods: Single biphasic TMS pulses were delivered over the right m. tibialis anterior hotspot in healthy volunteers at subthreshold and suprathreshold intensities. EEG was recorded from 61 scalp electrodes at 50 kHz, which enabled us to detect EEG responses approx. 2 ms after the TMS pulse. Data obtained from stimulation over M1-LEG was compared to previously acquired data from stimulation over M1-HAND.
Results: TMS applied to M1-LEG elicited an i-TEP consisting of a slower positive EEG deflection starting after approx. 2 ms with a series of superimposed high-frequency peaks. Immediate M1-LEG responses were observed in multiple electrodes overlying the stimulated sensorimotor region and scaled with stimulation intensity. The absolute stimulation intensity needed to evoke i-TEPs from M1-LEG was higher than for evoking i-TEPs from M1-HAND. i-TEPs following stimulation of M1-LEG also appeared to be more prominent than those elicited after TMS of M1-HAND.
Discussion: TMS of M1-LEG evokes i-TEPs that share some, but not all, characteristics of i-TEPs elicited by TMS of M1-HAND. Immediate cortical responses from co-stimulated M1 regions representing axial and proximal arm muscles may have contributed to i-TEPs after stimulation of M1-LEG and possibly to the overall larger response size compared to i-TEPs observed after stimulation of M1-HAND. Our results corroborate the notion that i-TEPs could reflect a direct marker of motor cortical excitability.