Gazarian, K; (2017) Engagement of the Corticospinal System During Action Observation: comparing the intact and damaged motor system. Doctoral thesis , UCL (University College London).

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Abstract

Stroke is the commonest cause of physical disability in the world and hemiparesis resulting from injury to the corticospinal system is a major contributor to that. Restitution of motor function is at least in part dependent on plasticity mechanisms, which might be aided by a number of interventions. For example, Action observation treatment (AOT) has been proposed to be a useful adjunct to motor treatment. Watching other people perform actions engages the motor system of the observer in the way that mirrors activity during action execution. Implemented together with motor training AOT might augment plasticity mechanisms in surviving brain regions leading to greater motor improvements. To date, however, outcomes of AOT have been variable and it is unclear which factors drive AOT related benefits in patients. One possible factor may be patients’ ability to execute observed hand actions. This thesis examined whether the ability to execute hand actions is necessary for engagement of the motor system during observation of those actions. Transcranial magnetic stimulation was used to assess excitability in the intact and damaged corticospinal system by measuring motor evoked responses in hand muscles. Functional magnetic resonance imaging was employed to examine magnitude and patterns of cortical activity during action observation in the intact and damaged motor system. There was no relationship between the ability to execute hand actions (i.e. impairment) and engagement of the corticospinal system during observation of those actions. Instead, findings indicate that pre-stroke dominance of the paretic hand determines the response to action observation. Firstly, activity during action observation was greater in the dominant affected hand. Secondly, altered dexterity in the non-dominant hand (when dominant hand was impaired) led to changes in contralateral neural representations and in magnitude of brain activity during action observation. Finally, during action observation, corticospinal system excitability was markedly reduced with age in the non-dominant hand muscle required for dexterous execution in healthy individuals. These findings suggest that response to AOT is likely to be variable among patients: modulated by dominance of the impaired hand, use of the unimpaired hand and age of the patient.

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