MODULATION OF MUSCLE RESPONSES EVOKED BY TRANSCRANIAL MAGNETIC STIMULATION DURING THE ACQUISITION OF NEW FINE MOTOR-SKILLS

1. We used transcranial magnetic stimulation (TMS) to study the role of plastic changes of the human motor system in the acquisition of new fine motor skills. We mapped the cortical motor areas targeting the contralateral long finger flexor and extensor muscles in subjects learning a one-handed, five-finger exercise on the piano. In a second experiment, we studied the different effects of mental and physical practice of the same five-finger exercise on the modulation of the cortical motor areas targeting muscles involved in the task. 2. Over the course of 5 days, as subjects learned the one-handed, five-finger exercise through daily 2-h manual practice sessions, the cortical motor areas targeting the long finger flexor and extensor muscles enlarged, and their activation threshold decreased. Such changes were limited to the cortical representation of the hand used in the exercise. No changes of cortical motor outputs occurred in control subjects who underwent daily TMS mapping but did not practice on the piano at all (control group 1). 3. We studied the effect of increased hand use without specific skill learning in subjects who played the piano at will for 2 h each day using only the right hand but who were not taught the five-finger exercise (control group 2) and who did not practice any specific task. In these control subjects, the changes in cortical motor outputs were similar but significantly less prominent than in those occurring in the test subjects, who learned the new skill. 4. In the second experiment, subjects were randomly assigned to a physical practice group, a mental practice group, or a control group. Subjects in each practice group physically or mentally practiced the five-finger piano exercise independently for 2 h daily for 5 days. The control group did not practice the exercise. All subjects had daily TMS mapping of the cortical motor areas targeting the long finger flexor and extensor muscles. 5. Over the course of 5 days, mental practice alone led to significant improvement in the performance of the five-finger exercise, but the improvement was significantly less than that produced by physical practice alone. However, mental practice alone led to the same plastic changes in the motor system as those occurring with the acquisition of the skill by repeated physical practice. 6. We conclude that acquisition of the motor skills needed for the correct performance of a five-finger piano exercise is associated with modulation of the cortical motor outputs to the muscles involved in the task. This rapid modulation may occur through an increase of synaptic efficacy in existing neural circuits (long-term potentiation) or unmasking of existing connections due to disinhibition. 7. Mental practice alone seems to be sufficient to promote the modulation of neural circuits involved in the early stages of motor skill learning. This modulation not only results in marked performance improvement but also seems to place the subjects at an advantage for further skill learning with minimal physical practice.