The role of mirror-visual feedback in modulating bilateral corticospinal excitability and bilateral performance gains following unilateral training in younger and older adults

The aim of the research described in this study is to improve the understanding of how visual feedback can be employed to modulate corticospinal excitability and performance gains in younger and older adults. Adopting the clinical approach of mirror training, it was my interest to further elucidate the effects of mirror-visual feedback on neurophysiological and behavioural changes during a motor training when compared to more standard visual feedback (i.e., focussing on the hand executing the motor task). Such knowledge is necessary to further expand the use of this promising technique to treat various neurological disorders across the lifespan. The thesis consists of six chapters. The first chapter outlines the motivation for the conducted research, while the last chapter provides the reader with an overall discussion of the results found in chapters 3 - 6. Chapter 2 reviews current literature about theories of mirror training and the phenomenon of cross-limb transfer. It furthermore familiarises the reader with the mechanisms underlying the techniques used for the majority of the present research. Moreover, the background provided in this chapter provides the conceptualisation and rationale for the current research project. Accordingly, the chapter is written on the basis of published research at the commencement of the present thesis. Relevant research that has been published during the last three years (i.e., during the period that the present thesis has been undertaken) will be discussed in the appropriate experimental chapters (3-5), and in the overall discussion (chapter 6). Chapter 3 is a report of a study that investigated the degree to which mirror-visual feedback of a moving limb was able to elicit specific neurophysiological changes in the ipsilateral hemisphere when compared to more standard visual feedback conditions in younger and older adults. Unlike previously reported, it was found that mirror-visual feedback, irrespective of age, did not lead to more pronounced changes in ipsilateral corticospinal excitability or intracortical inhibition compared to when the visual focus was on the active or passive limb. Based on these results, it was concluded that enhanced corticospinal excitability increases in the hemisphere ipsilateral to a moving limb cannot be considered the primary mechanism underlying mirror-visual feedback-related behavioural changes. Chapter 4 reports the results of a study that aimed to consolidate previous findings of mirror-visual feedback-induced augmented cross-limb transfer in younger adults in a complex motor learning task and to further investigate the underlying mechanism(s) of such mirror-specific behavioural changes. Unlike in previous studies, the results did not demonstrate a beneficial effect of mirror-visual feedback on cross-limb transfer. Moreover, and contrary to previously proposed, the findings suggest mirror-visual feedback-induced cross-limb transfer to most likely occur due to a combination of mechanisms of traditional‚ÄövÑvp motor transfer and action observation. Chapter 5 reports the results of a study that addressed cross-limb transfer within the ageing population, with the aim to investigate the degree to which mirror-visual feedback is able to augment cross-limb transfer within a simple ballistic motor task in general and in older adults more specifically. Even though the results did not show either behavioural or neurophysiological effects specific to the provision of mirror-visual feedback, it was shown that younger and older adults were both able to demonstrate similar amounts of cross-limb transfer. Interestingly, cross-limb transfer was associated with different parameters in the two age groups. While transfer in the younger adults was found to be predominantly associated with the performance gains in the trained hand, it was the amount of mirror muscle activity in the inactive hand exhibited during learning that was related to subsequent transfer in the older adults. The results thus demonstrate a preserved ability of cross-limb transfer in older adults, and further suggest bilateral activation, often reported within the context of unilateral motor tasks in the ageing population, to be a compensatory and helpful mechanism with regards to transfer processes.