Muscle-specific modulation of indirect inputs to primary motor cortex during action observation

The recruitment of indirect waves within primary motor cortex during motor imagery: A directional transcranial magnetic stimulation study

Motor imagery (MI) refers to the mental simulation of an action without overt movement. While numerous transcranial magnetic stimulation (TMS) studies provided evidence for a modulation of corticospinal excitability and intracortical inhibition during MI, the neural signature within the primary motor cortex is not clearly established. In the current study, we used directional TMS to probe the modulation of the excitability of early and late indirect waves (I-waves) generating pathways during MI. Corticospinal responses evoked by TMS with posterior-anterior (PA) and anterior-posterior (AP) current flow within the primary motor cortex evoke preferentially early and late I-waves, respectively. Seventeen participants were instructed to stay at rest or to imagine maximal isometric contractions of the right flexor carpi radialis. We demonstrated that the increase of corticospinal excitability during MI is greater with PA than AP orientation. By using paired-pulse stimulations, we confirmed that short-interval intracortical inhibition (SICI) increased during MI in comparison to rest with PA orientation, whereas we found that it decreased with AP orientation. Overall, these results indicate that the pathways recruited by PA and AP orientations that generate early and late I-waves are differentially modulated by MI.

Intracortical facilitation and inhibition in human primary motor cortex during motor skill acquisition

The primary motor cortex (M1) is critical for movement execution, but its role in motor skill acquisition remains elusive. Here, we examine the role of M1 intracortical circuits during skill acquisition. Paired-pulse transcranial magnetic stimulation (TMS) paradigms of short-interval intracortical facilitation (SICF) and inhibition (SICI) were used to assess excitatory and inhibitory circuits, respectively. We hypothesised that intracortical facilitation and inhibition circuits in M1 would be modulated to support acquisition of a novel visuomotor skill. Twenty-two young, neurologically healthy adults trained with their nondominant hand on a skilled and non-skilled sequential visuomotor isometric finger abduction task. Electromyographic recordings were obtained from the nondominant first dorsal interosseous (FDI) muscle. Corticomotor excitability, SICF, and SICI were examined before, at the midway point, and after the 10-block motor training. SICI was assessed using adaptive threshold-hunting procedures. Task performance improved after the skilled, but not non-skilled, task training, which likely reflected the increase in movement speed during training. The amplitudes of late SICF peaks were modulated with skilled task training. There was no modulation of the early SICF peak, SICI, and corticomotor excitability with either task training. There was also no association between skill acquisition and SICF or SICI. The findings indicate that excitatory circuitries responsible for the generation of late SICF peaks, but not the early SICF peak, are modulated in motor skill acquisition for a sequential visuomotor isometric finger abduction task.