Abstract
Use-dependent plasticity, a mechanism underlying motor learning, can be induced by physical practice or action observation. In the current study, we tested whether motor imagery, defined as the mental representation of the action without sensory inputs, could induce use-dependent plasticity as well. By means of transcranial magnetic stimulation (TMS) over the left primary motor cortex, we evoked isolated thumb movements in the right hand and assessed corticospinal excitability in the flexor and extensor pollicis brevis muscles. We measured the mean TMS-induced movement direction before and after an acute session of motor imagery practice in two experiments. In a first experiment, participants in the imagery group were instructed to repeatedly imagine their thumb moving in a direction deviated by 90° from the pre-test movement. The imagery group, but not the control group, deviated the post-training TMS-induced movements towards the training target direction (+34° ±55° and −13° ±53°, respectively). Interestingly, the deviation magnitude was driven by the corticospinal excitability increase in the agonist muscle. In a second experiment, we found that post-training TMS-induced movements were proportionally deviated towards the trained direction: we observed a significant deviation of +36° ±13° and +89 ±19° for the 60° and 110° groups, respectively, but not for the 0° group (−7 ±4°). Also, the 110° group deviation returned to baseline when tested after 30 and 60 minutes. These findings demonstrate that motor imagery induces use-dependent plasticity and, this neural process is accompanied by corticospinal excitability increase in the agonist muscle.
