Pierre Vassiliadis

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-coil1ms). MEPs were acquired using double-coil1ms (MEPdouble) or single-coil (MEPsingle) 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-coil1ms trials, MEPdouble were either evoked by a 1st (MEPdouble-1) or a 2nd (MEPdouble-2) TMS pulse. RESULTS All MEPTYPE (MEPTYPE=MEPsingle, MEPdouble-1 and MEPdouble-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-coil1ms 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.

Using a Double-Coil TMS Protocol to Assess Preparatory Inhibition Bilaterally

Transcranial magnetic stimulation (TMS) applied over the primary motor cortex (M1), elicits motor-evoked potentials (MEPs) in contralateral limb muscles which are valuable indicators of corticospinal excitability (CSE) at the time of stimulation. So far, most studies have used single-coil TMS over one M1, yielding MEPs in muscles of a single limb—usually the hand. However, tracking CSE in the two hands simultaneously would be useful in many contexts. We recently showed that, in the resting state, double-coil stimulation of the two M1 with a 1 ms inter-pulse interval (double-coil1 ms TMS) elicits MEPs in both hands that are comparable to MEPs obtained using single-coil TMS. To further evaluate this new technique, we considered the MEPs elicited by double-coil1 ms TMS in an instructed-delay choice reaction time task where a prepared response has to be withheld until an imperative signal is displayed. Single-coil TMS studies have repetitively shown that in this type of task, the motor system is transiently inhibited during the delay period, as evident from the broad suppression of MEP amplitudes. Here, we aimed at investigating whether a comparable inhibitory effect can be observed with MEPs elicited using double-coil1 ms TMS. To do so, we compared the amplitude as well as the coefficient of variation (CV) of MEPs produced by double-coil1 ms or single-coil TMS during action preparation. We observed that MEPs were suppressed (smaller amplitude) and often less variable (smaller CV) during the delay period compared to baseline. Importantly, these effects were equivalent whether single-coil or double-coil1 ms TMS was used. This suggests that double-coil1 ms TMS is a reliable tool to assess CSE, not only when subjects are at rest, but also when they are involved in a task, opening new research horizons for scientists interested in the corticospinal correlates of human behavior.