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

Abstract

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.

Motor training strengthens corticospinal suppression during movement preparation

“…We had to exclude one subject because we encountered a technical problem during the experiment; hence, analyses were run on the fourteen remaining subjects. Part of the data reported here has been exploited in a separate study (Vassiliadis et al, 2018). All of the data are expressed as mean ± SE.…”

“…The RT was defined as the time interval between the onset of the bridge and the time when the index finger left the outer metal edge of the response device. In order to homogenize the score across subjects, scores on correct trials were individualized according to RTs measured during a familiarization session just before the main experiment (see Vassiliadis et al, 2018; Grandjean et al, 2019 for more details on how the score on correct trials was computed). Incorrect responses were penalized with negative scores displayed in red.…”

“…, double-coil TMS) eliciting MEPs in one or in both hands, respectively. Importantly, in double-coil trials, a 1-ms interval separated the onset of the two pulses, eliciting MEPs in both hands at a near simultaneous time (Algoet et al, 2018; Grandjean et al, 2018; Vassiliadis et al, 2018; Quoilin et al, 2019). This interval was used to avoid direct electromagnetic interference between the two coils (Cincotta et al, 2005), while preventing transcallosal interactions that would occur between motor areas with longer delays (Ferbert et al, 1992; Hanajima et al, 2001).…”

The effects of transcranial direct current stimulation on corticospinal and cortico-cortical excitability and response variability: conventional versus high-definition montages

“…The CV (SD/meanx100) represents the range, or percentage variance that one SD of the data lies about the mean (Atkinson & Nevill, 1998). Its simplicity has led to use in NIBS and TMS literature (eg: (Biabani et al, 2018;Klein-Flugge et al, 2013;Sadnicka et al, 2013;Martin V. Sale et al, 2017;Temesi et al, 2017;Vassiliadis et al, 2018)).…”

M1 disruption delays motor processes but not deliberation about action choices

“…We then determined the resting motor threshold (rMT) at the hotspot. The rMT was defined as the minimal intensity required to evoke MEPs of 50 µV peak-to-peak in the targeted muscle on 5 out of 10 consecutive trials Rothwell et al, 1991;Rossini et al, 1994Rossini et al, , 2015Vassiliadis et al, 2018).…”