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Looking beyond the motor cortex : examining the potential of the primary sensory cortex as a target for repetitive transcranial magnetic stimulation after stroke Brodie, Sonia Mae


Stroke is the leading cause of chronic adult disability, and standard post-stroke therapies may not be sufficient for individuals to reach their full recovery potential. When paired with skilled motor practice, non-invasive brain stimulation techniques such as repetitive transcranial magnetic stimulation (rTMS) may enhance motor recovery by transiently modulating cortical excitability, effectively priming the brain to facilitate mechanisms of motor learning. Depending on the pulse frequency, rTMS may be used to increase cortical excitability in the damaged hemisphere, or decrease it in the undamaged hemisphere, with the goal of re-establishing a normal interhemispheric balance. While theoretically promising, the majority of studies considering the effects of rTMS over the primary motor cortex (M1) have shown relatively small effect sizes and high inter-individual variability. Improved effect sizes may be produced by 1) finding the optimal cortical target for stimulation, rather than defaulting to M1, and 2) choosing an appropriate sample that will optimally benefit from the intervention. In the following thesis, we will explore the potential of the primary sensory cortex (S1) as an alternative target for rTMS intervention, and the anatomical and physiological variables that may help to identify who may best benefit from this intervention. First, we describe a randomized, single blind experiment comparing the impact of active versus sham rTMS over S1 paired with practice of a skilled visuomotor reaching task in individuals with chronic stroke. Second, we describe a retrospective analysis of the participants from the first experiment, to determine whether individual differences in morphology of the underlying sensory cortex might be predictive of rTMS responsiveness. Third, we describe an exploratory study using a paired median nerve somatosensory evoked potential paradigm using electroencephalography in healthy individuals, to elucidate the neurophysiological mechanism of interhemispheric inhibition between S1s. We conclude that S1 should be considered as a viable target for future rTMS trials as an adjunct therapy to rehabilitation after stroke.

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