Towards assessing corticospinal excitability bilaterally: Validation of a double-coil TMS method

Background: For several decades, Transcranial magnetic stimulation (TMS) has been used to monitor corticospinal excitability (CSE) changes in various contexts. Habitually, single-coil TMS is applied over one primary motor cortex (M1), eliciting motor-evoked potentials (MEPs) in a contralateral limb muscle, usually a hand effector. However, in many situations, it would be useful to obtain MEPs in both hands simultaneously, to track CSE bilaterally. Such an approach requires stimulating both M1 concurrently while avoiding interference between the two descending stimuli.

New method: We examined MEPs obtained at rest using a double-coil TMS approach where the two M1 are stimulated with a 1ms inter-pulse interval (double-coil_1ms). MEPs were acquired using double-coil_1ms (MEP_double) or single-coil (MEP_single) TMS, at five different intensities of stimulation (100, 115, 130, 145 or 160% of the resting motor threshold, rMT). Given the 1ms inter-pulse interval in double-coil_1ms trials, MEP_double were either evoked by a 1st (MEP_double-1) or a 2nd (MEP_double-2) TMS pulse.

Results: All MEP_TYPE (MEP_TYPE=MEP_single, MEP_double-1 and MEP_double-2) were equivalent, regardless of the hand within which they were elicited, the intensity of stimulation or the pulse order.

Comparison with existing method: This method allows one to observe state-related CSE changes for the two hands simultaneously on a trial-by-trial basis.

Conclusion: These results infer the absence of any neural interactions between the two cortico-spinal volleys with double-coil_1ms TMS. Hence, this technique can be reliably used to assess CSE bilaterally, opening new research perspectives for scientists interested in physiological markers of activity in the motor output system.