Non-Invasive Stimulation of the Motor System: Methodology and Application for the Study of Motor Control

Transcranial magnetic stimulation (TMS) and peripheral nerve stimulation (PNS) are techniques used to study the neurophysiology of the motor system. The primary aim of this thesis is to investigate the reliability and validity of these techniques and their relevance to investigate the neural substrates of motor control in healthy individuals. When delivering TMS and PNS in combination, a method called TMS-conditioning of the monosynaptic reflex, it is possible to selectively assess the excitability of cortical, spinal and peripheral circuits. However, the intersession reliability of this method when stimulating forearm muscles was never investigated. In the first study, it was demonstrated that the method produced reliable results over the course of three sessions. In the second study, the effects of auditory activation and stimulus expectation on TMS motor-evoked potentials were examined. Masking (P = 0.02) or attenuating (P = 0.004) the sound produced by TMS and informing the participant of the time of stimulation (P = 0.049) decreased the responses recorded from forearm muscles. This suggests that part of the activity elicited by TMS is conducted through non-corticospinal pathways. Finally, the objective of the last experimental chapter was to investigate the acute effects of unilateral skill and strength training on the performance and neural circuits of the contralateral untrained limb. The results showed that a single session of unimanual skill (force-matching) training successfully increased skill in the untrained limb (F1, 9 = 10.266, P = 0.011), but a single session of unimanual strength training did not affect the untrained limb (F1, 9 = 3.069, P = 0.114). However, the excitability of the untrained motor cortex increased after strength and skill training (F1,9 = 15.224, P = 0.004), without any changes in spinal and peripheral excitability. This demonstrates that both training modalities induce long-lasting effects in the untrained motor cortex.