Time course of changes in corticospinal excitability induced by motor imagery during action observation combined with peripheral nerve electrical stimulation.

While previous studies assessed corticospinal excitability changes during and after motor imagery (MI) or action observation (AO) combined with peripheral nerve electrical stimulation (ES), we examined, for the first time, the time course of corticospinal excitability changes for MI during AO combined with ES (AO-MI + ES) using transcranial magnetic stimulation to measure motor evoked potentials (MEPs) in healthy individuals. Fourteen healthy volunteers participated in the following three sessions on different days: AO-MI alone, ES alone, and AO-MI + ES. In the AO-MI task, participants imagined squeezing and relaxing a ball, along with the respective actions shown in a movie, while passively holding the ball. We applied ES (intensity, 90% of the motor threshold) to the ulnar nerve at the wrist, which innervates the first dorsal interosseous (FDI) muscle. We assessed the FDI muscle MEPs at baseline and after every 5 min of the task for a total of 20 min. Additionally, participants completed the Vividness of Movement Imagery Questionnaire-2 (VMIQ-2) at the beginning of the experiment. Compared to baseline, AO-MI + ES significantly increased corticospinal excitability after 10 min, while AO-MI or ES alone had no effect on corticospinal excitability after 20 min. Moreover, the AO-MI + ES-induced cortical excitability changes were correlated with the VMIQ-2 scores for visual and kinaesthetic imagery. Collectively, our findings indicate that AO-MI + ES induces cortical plasticity earlier than does AO-MI or ES alone and that an individual's imagery ability plays an important role in inducing cortical excitability changes following AO-MI + ES.