Lee, Lucy; (2004) Imaging the effects of 1 Hz repetitive transcranial magnetic stimulation during motor behaviour. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Repetitive Transcranial Magnetic Stimulation (rTMS) can be used to induce temporary alterations in the excitability of the brain in healthy subjects. For some motor behaviour it has been possible to impair or improve performance following rTMS, but for most simple tasks performance is unaltered. This suggests that the motor system is able to compensate, to some extent, for the changes in excitability induced by rTMS. Potentially this makes rTMS a useful tool for studying reorganisation in the healthy motor system, and may provide insights into adaptive mechanisms after injury such as ischaemic stroke. The work presented in this thesis examines rTMS-induced changes in regional excitability following 1Hz rTMS to the primary motor cortex, and potential compensatory mechanisms during various motor tasks. The results of three functional neuroimaging experiments reveal significant changes in movement-related responses and coupling with the motor system following rTMS. The results of a behavioural experiment suggest that the increases in movement-related responses in the right premotor cortex have a functional role in maintaining motor performance following 1Hz rTMS to left primary motor cortex. Analyses of effective connectivity suggest that the influence of the right premotor cortex in maintaining motor performance after rTMS is mediated via increased transcallosal connections from right to left premotor cortex, as opposed to non-homologous connections from right premotor to left motor cortex. Increased activity in motor areas not normally engaged in task performance may contribute to compensatory mechanisms during altered cortical excitability. Analyses of effective connectivity suggest that operational remapping of motor networks may also occur, and this may also contribute to compensatory mechanisms for rTMS-induced reductions in cortical excitability. Mapping these patterns of reorganisation in the motor system may provide a useful method to study acute compensatory plasticity of the human brain and may help to understand how the brain reacts to more permanent lesions. Establishing the functional relevance of increased activity in areas not normally engaged in task performance using TMS may play a key role in rehabilitation and provide a mechanistic understanding of compensatory mechanisms in stroke patients.

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