A comparison of neuroplastic responses to non-invasive brain stimulation protocols and motor learning in healthy adults.

Non-invasive brain stimulation (NBS) techniques can induce neuroplastic changes similar to those associated with motor learning and there is evidence for the involvement of common mechanisms. Whether there are correlations between the changes induced by NBS and those associated with motor learning remains unclear. We investigated whether there was any relationship between an individual's neuroplastic responses to several different NBS protocols (continuous theta-burst stimulation (cTBS); intermittent theta-burst stimulation (iTBS); facilitatory paired associative stimulation (PAS: inter-stimulus interval 25ms)) and whether these responses correlated with the neuroplastic response associated with a motor training (MT) task involving repeated fast-as-possible thumb abductions. Changes in motor evoked potential (MEP) amplitude were used to assess the neuroplastic response to each protocol. MEP amplitude decreased significantly following cTBS, however there was no significant change in MEP amplitude following iTBS, PAS or MT. There were no significant correlations between individuals' neuroplastic responses to any of the NBS protocols tested or between individuals' neuroplastic responses to the NBS protocols and motor learning. These results provide no support for an association between individuals' neuroplastic responses to several plasticity-inducing protocols. Although there is evidence for involvement of common mechanisms in the neuroplastic changes induced by NBS and motor learning, the results of this study suggest (1) the mechanisms mediating TBS-, PAS-, and MT-induced plasticity may only partially overlap, and (2) additional factors, including large intra and inter-subject response variability, may make the demonstration of associations between neuroplastic responses to the various protocols difficult.