Mechanisms underlying cross education of motor function

Ruddy, Kathy Louise
PhD Thesis, Queen’s University Belfast, 2014

Cross education is the process whereby training one limb gives rise to enhancements in performance of the opposite, untrained limb. It is widely believed that cross education arises due to crossed facilitation of corticospinal pathways during unilateral movement, manifested by a change in excitability measured using transcranial magnetic stimulation over the primary motor cortex (M1) corresponding to the untrained limb. In Chapter 2 we manipulated the visual feedback that was available during training; a factor known to modulate the degree of crossed facilitation. Transfer of performance to the untrained limb was elevated in a group who trained with mirrored visual feedback of the moving limb. However, the same results were not replicated in a later experiment with a larger sample size. The noteworthy finding from this investigation was that despite large performance improvements in the untrained limb, the excitability of corticospinal projections to the relevant task agonist muscle remained unchanged. Changes in muscle activation dynamics in the untrained limb following training were found to be statistically associated with transfer of performance. These modifications, in the absence of a change in the state of M1 pathways, suggests that the neural adaptations that subserve transfer of this task, are located in motor regions ‘upstream’ from M1. In order to develop more specific hypotheses, Chapter 3 was an investigation of transcallosal white matter fibre pathways between motor cortical regions. This revealed that celiain regions demonstrate superior structural connectivity with the contralateral hemisphere. The investigation culminated in Chapter 4 with a multimodal neuroimaging study. It was revealed that the structural organisation of white matter pathways connecting bilateral supplementary motor area (SMA) was a remarkably good predictor of transfer. The results are interpreted in the context of theories regarding the functional role of SMA, and an emphasis on the task-dependancy of the results is maintained.