Unravelling the modulation of intracortical inhibition during motor imagery: An adaptive threshold-hunting study

Abstract

Motor imagery (MI) is the mental simulation of an action without any apparent muscular contraction. By means of transcranial magnetic stimulation, few studies revealed a decrease of short-interval intracortical inhibition (SICI) within the primary motor cortex. However, this decrease is ambiguous, as one would expect greater inhibition during MI to prevent overt motor output. The current study investigated the extent of SICI modulation during MI through a methodological and a conceptual reconsideration of i) the importance of parameters to assess SICI (Exp.1) and ii) the inhibitory process within the primary motor cortex as an inherent feature of MI (Exp.2). Participants performed two tasks: 1) rest and 2) imagery of isometric abduction of the right index finger. Using transcranial magnetic stimulation, motor evoked potentials were elicited in the right first dorsal interosseous muscle. An adaptive threshold-hunting paradigm was used, where the stimulus intensity required to maintain a fixed motor evoked potential amplitude was quantified.To test SICI, we conditioned the test stimulus with a conditioning stimulus (CS) of different intensities. Results revealed an Intensity by Task interaction showing that SICI decreased during MI as compared to rest only for the higher CS intensity (Exp.1). At the lowest CS intensities, a Task main effect revealed that SICI increased during MI (Exp.2). SICI modulation during MI depends critically on the CS intensity. By optimising CS intensity, we have shown that SICI circuits may increase during MI, revealing a potential mechanism to prevent the production of a movement while the motor system is activated.

Influence of Voluntary Contraction Level, Test Stimulus Intensity and Normalization Procedures on the Evaluation of Short-Interval Intracortical Inhibition

“…This choice was made to avoid any fatigue effect that might have potentially confounded the results obtained. Moreover, both SICI MEPTEST and SICI Mmax modulations also depend on CS intensity and interstimulus interval (ISI) duration [ 6 , 12 , 51 , 52 , 53 ], two fixed parameters whose effects were not assessed in the current study. The CS intensity was fixed at 70% of MT calculated according to each levels of contraction.…”

The recruitment of indirect-waves within primary motor cortex during motor imagery: A directional Transcranial Magnetic Stimulation study

“…This led to a MEP target of 0.251 ± 0.141 mV amplitude (see Table 1 for individual values). Generally, a non-personalized fixed 0.2 mV MEP target amplitude is selected in studies using the adaptive threshold-hunting technique, corresponding approximately to 109% rMT (Fisher et al ., 2002; Awiszus, 2003; Vucic et al ., 2006; Menon et al ., 2015; Cirillo & Byblow, 2016; Cirillo et al ., 2018; Samusyte et al ., 2018; Van den Bos et al ., 2018; Neige et al ., 2020). However, in the current study, a subject-specific MEP target was chosen since 1) a huge between-subject variability in the intrinsic excitability of the corticospinal pathway exists, 2) a TS delivered at a lower intensity (i.e., below 110% rMT) could fail to evoke late I-waves, and limits SICI magnitude (Garry & Thomson, 2009) and 3) a TS delivered at a higher intensity can also elicit early I-waves when using an AP current direction, therefore limiting the interpretation differences obtained between PA and AP findings.…”

“…This allows to assess SICI modulation at rest vs. during MI, with PA and AP currents direction. The CS intensity was fixed at 60% rMT PA for SICI PA and 60% rMT AP for SICI AP , based on a previous study showing that higher CS intensities could lead to the unwanted recruitment of excitatory interneurons during MI, biasing the result interpretation (Neige et al ., 2020). The inter-stimulus interval (ISI) between CS and TS was set at 3 ms, to induce the greatest inhibition when using AP current direction (Kujirai et al ., 1993; Cirillo et al ., 2018).…”

“…We used single-pulse and paired-pulse TMS to probe corticospinal excitability and short-interval intracortical inhibition (SICI), respectively. Interestingly, SICI affects mainly later I-waves that are mainly targeted by AP orientation (Nakamura et al ., 1997; Hanajima et al ., 1998; Cirillo & Byblow, 2016; Wessel et al ., 2019), and SICI increases during MI, but only observed with PA orientation (Neige et al ., 2020).…”

“…Transcranial magnetic stimulation studies (TMS) provided evidence of the activation of the corticospinal pathway during MI, when compared to rest (Yahagi & Kasai, 1999; Grosprêtre et al ., 2016). This activation is classically marked by an increase in the amplitude of the motor evoked potentials (MEPs) evoked by single-pulse TMS and recorded in the specific muscle involved in the imagined movement (Yahagi & Kasai, 1999; Lebon et al ., 2012; Grosprêtre et al ., 2016; Neige et al ., 2020, 2021). According to Di Lazzaro and Ziemann (Di Lazzaro & Ziemann, 2013), the axons of the more superficial pyramidal neurons (P2/P3) are conceivably the most excitable neural elements to low-threshold TMS, due to their superficial location close to the stimulating coil.…”