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UBC Theses and Dissertations

Contributions of a voluntary response to instruction-dependent modulation of the long-latency stretch reflex Forgaard, Christopher James


Successful movement control in a dynamic environment involves generating appropriate and timely motor responses to counter disturbances applied to the body. In the upper-limb, mechanical perturbations elicit responses in stretched musculature at short- (M1: 25-50 ms) and long-latency (M2: 50-100 ms). The M2 response has received a great deal of attention because it can be modified by volition; for instance, increasing when the performer is instructed to resist a perturbation and decreasing when asked to not-intervene/let-go. It remains a matter of contention whether M2 modulation results from a facilitation of the contributing neural circuitry, or from superimposition of a voluntary response. The difficulty in delineating between these alternatives is due to both responses engaging common neural circuitry and the presence of considerable overlap between the voluntary response and M2 in the muscle recordings. This dissertation investigated the contributions of a rapid voluntary response on the modulation of M2. In theme 1, we performed behavioural manipulations that influence volition and observed the corresponding impact on M2. Theme 2 investigated contributions from a startle/StartReact mechanism. The final theme used kinesthetic motor imagery to determine whether the overt initiation of a voluntary response is a pre-requisite for M2 modulation. Taken together, the findings of this dissertation showed that even in the absence of startle, a perturbation could elicit a voluntary response at a latency (75-100 ms) that overlaps M2. Despite the early nature of these rapid voluntary actions, they could not account for all instruction-dependent M2 changes. Irrespective of voluntary latency or magnitude, a general increase to the first half of M2 (50-75 ms) was observed for all active conditions. We suggest that this generic M2 modulation is related to the intention to voluntarily respond, while more sophisticated/flexible modulation observed during the latter portion of M2 is produced in part from voluntary superimposition.

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