Provision of a mirror image of a hand undertaking a motor task (i.e., mirror therapy) elicits behavioural improvements in the inactive hand. A greater understanding of the neural mechanisms underpinning this phenomenon is required to maximise its potential for rehabilitation across the lifespan, e.g., following hemiparesis or unilateral weakness. Young and older participants performed unilateral finger abductions with no visual feedback, with feedback of the active or passive hands, or with a mirror image of the active hand. Transcranial magnetic stimulation was used to assess feedback-related changes in two neurophysiological measures thought to be involved in inter-manual transfer of skill, namely corticospinal excitability (CSE) and intracortical inhibition (SICI) in the passive hemisphere. Task performance led to CSE increases, accompanied by decreases of SICI, in all visual feedback conditions relative to rest. However, the changes due to mirror feedback were not significantly different to those observed in the other (more standard) visual conditions. Accordingly, the unimanual motor action itself, rather than modifications in visual feedback, appears more instrumental in driving changes in CSE and SICI. Therefore, changes in CSE and SICI are unlikely to underpin the behavioural benefits of mirror therapy. We discuss implications for rehabilitation and directions of future research.
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