Reversal of Practice-Related Effects on Corticospinal Excitability has No Immediate Effect on Behavioural Outcome

Background: Motor training usually increases the excitability of corticospinal outputs to the trained muscles. However, it is uncertain to what extent the change in excitability is a critical component of behavioral learning or whether it is a non-specific side effect. Objective/hypothesis: We used a depotentiation protocol to abolish the training-induced increase of corticospinal excitability and tested whether this had any immediate effect on the improved motor performance. Methods: We used an index finger abduction task in which behavioral improvement is known to be associated with M1 excitability changes as monitored by the amplitude of motor-evoked potentials produced by single-pulse transcranial magnetic stimulation (TMS). These effects could be reversed by a depotentiation protocol using a short form of continuous theta-burst stimulation (cTBS150). Participants underwent three experimental interventions: 'motor training', 'motor training plus cTBS150' and 'cTBS150'. M1 excitability and TMS-evoked finger movements were assessed before the experimental interventions and 5 min, 15 min, and 30 min thereafter. Motor retention was tested 45 min after the experimental interventions. Results: During training, acceleration of the practiced movement improved. At the end of training, M1 excitability and the acceleration of TMS-evoked index finger movements in the direction of training had increased and the enhanced performance was retained when tested 45 min later. The depotentiation protocol, delivered immediately after the end of training, reversed the excitability changes in M1 but did not affect the acceleration of the TMS-evoked finger movement nor the retention of performance. The depotentiation protocol alone did not modify M1 excitability. Conclusions: The present study indicates that in the short term, increases in corticospinal excitability are not related to immediate changes in behavioral motor outcome.