Using transcranial magnetic stimulation (TMS), we investigated how short-interval intracortical inhibition (SICI) was involved with transient motor cortex (M1) excitability changes observed just before the transition from muscle contraction to muscle relaxation. Ten healthy participants performed a simultaneous relaxation task of the ipsilateral finger and foot, relaxing from 10% of their maximal voluntary contraction (MVC) force after the go signal. In the simple reaction time (RT) paradigm, single or paired TMS pulses were randomly delivered after the go signal, and motor evoked potentials (MEPs) were recorded from the right first dorsal interosseous (FDI) muscle. We analyzed the time course prior to the estimated relaxation reaction time (RRT), defined here as the onset of voluntary relaxation. SICI decreased in the 80–100 ms before RRT, and MEPs were significantly greater in amplitude in the 60–80 ms period before RRT than in the other intervals in single-pulse trials. TMS pulses did not effectively increase RRT. These results show that cortical excitability in the early stage, before muscle relaxation, plays an important role in muscle relaxation control. SICI circuits may vary between decreased and increased activation to continuously maintain muscle relaxation during or after a relaxation response. With regard to M1 excitability dynamics, we suggest that SICI also dynamically changes throughout the muscle relaxation process.

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Although motor imagery enhances the excitability of the corticospinal tract, there are no peripheral afferent inputs during motor imagery. In contrast, peripheral nerve electrical stimulation (ES) can induce peripheral afferent inputs; thus, a combination of motor imagery and ES may enhance the excitability of the corticospinal tract compared with motor imagery alone. Moreover, the level of stimulation intensity may also be related to the modulation of the excitability of the corticospinal tract during motor imagery. Here, we evaluated whether a combination of motor imagery and peripheral nerve ES influences the excitability of the corticospinal tract and measured the effect of ES intensity on the excitability induced during motor imagery. The imagined task was a movement that involved touching the thumb to the little finger, whereas ES involved simultaneous stimulation of the ulnar and median nerves at the wrist. Two different ES intensities were used, one above the motor threshold and another above the sensory threshold. Further, we evaluated whether actual movement with afferent input induced by ES modulates the excitability of the corticospinal tract as well as motor imagery. We found that a combination of motor imagery and ES enhanced the excitability of the motor cortex in the thenar muscle compared with the other condition. Furthermore, we established that the modulation of the corticospinal tract was related to ES intensity. However, we found that the excitability of the corticospinal tract induced by actual movement was enhanced by peripheral nerve ES above the sensory threshold.

Abstract.[Purpose] The aim of this study was to investigate whether the performance of a combination of observation and motor imagery of chopstick use (complex task) increased corticospinal excitability more than the performance of observation alone. [Subjects and Methods] We recruited 10 healthy subjects with no history of neurological diseases. Corticospinal excitability was assessed with the participants seated in front of a computer screen performing three tasks: (1) control, the subjects were instructed to relax; (2) OBS, the subjects were told to observe an action depicted in the video, and (3) OBS + IMG, the subjects were told to imagine performing an action depicted in a video. During tasks (2) and (3), a video was displayed on the computer screen showing the hand of a male subject using chopsticks to move small items of food from one dish to another (first person perspective). Imagery was performed kinesthetically.[Results] The MEP amplitude in the first dorsal interosseous was significantly increased during OBS+IMG relative to that in the control condition, but not that in the OBS condition. The MEP amplitude in the thenar muscles was significantly different between OBS and OBS+IMG. [Conclusion] These results suggest that the combination of observation and motor imagery of a complex task may be more effective than observation alone for motor rehabilitation purposes.