Synchronizing hand and foot movements under reactive versus predictive control results in differential timing structures between the responses. Under reactive control, where the movement is externally triggered, the electromyographic (EMG) responses are synchronized, resulting in the hand displacement preceding the foot. Under predictive control, where the movement is self-paced, the motor commands are organized such that the displacement onset occurs relatively synchronously, requiring the EMG onset of the foot to precede that of the hand. The current study used a startling acoustic stimulus (SAS), which is known to involuntarily trigger a prepared response, to investigate whether these results are due to differences in the pre-programmed timing initiation structure of the responses. Participants (n=17) performed isolated and synchronous movements of the right heel and right hand under both reactive and predictive modes of control. The reactive condition involved a simple reaction time (RT) task where participants performed the required movement in response to a visual go-signal. The predictive condition involved an anticipation-timing task where participants initiated the required movement coincidently with a sweeping clock hand reaching a target. On a subset of trials, a SAS (114 dB) was presented 150 ms prior to the imperative stimulus. Results from the SAS trials revealed that while the differential timing structures between the responses was maintained under both reactive and predictive control, the EMG onset asynchrony under predictive control was significantly smaller following the SAS. Additionally, there was no difference in the effect of the SAS when the movements were performed in isolation versus synchronously. Together, these results suggest that the timing between the responses, which differs between the two control modes, is pre-programmed; however, under predictive control, an increase in cortical activation from the SAS may have shortened the between-limb delay.

abstract: Stroke is a debilitating disorder and 75% of individuals with stroke (iwS) have upper extremity deficits. IwS are prescribed therapies to enhance upper-extremity mobility, but current most effective therapies have minimum requirements that the individuals with severe impairment do not meet. Thus, there is a need to enhance the therapies. Recent studies have shown that StartReact -the involuntary release of a planned movement, triggered by a startling stimulus (e.g., loud sound)- elicits faster and larger muscle activation in iwS compared to voluntary-initiated movement. However, StartReact has been only cursorily studied to date and there are some gaps in the StartReact knowledge. Previous studies have only evaluated StartReact on single-jointed movements in iwS, ignoring more functional tasks. IwS usually have abnormal flexor activity during extension tasks and abnormal muscle synergy especially during multi-jointed tasks; therefore, it is unknown 1) if more complex multi-jointed reach movements are susceptible to StartReact, and 2) if StartReact multi-jointed movements will be enhanced in the same way as single-jointed movements in iwS. In addition, previous studies showed that individuals with severe stroke, especially those with higher spasticity, experienced higher abnormal flexor muscle activation during StartReact trials. However, there is no study evaluating the impact of this elevated abnormal flexor activity on movement, muscle activation and muscle synergy alterations during voluntary-initiated movements after exposure to StartReact. This dissertation evaluates StartReact and the voluntary trials before and after exposure to StartReact during a point-to-point multi-jointed reach task to three different targets covering a large workspace. The results show that multi-jointed reach tasks are susceptible to StartReact in iwS and the distance, muscle and movement onset speed, and muscle activations percentages and amplitude increase during StartReact trials. In addition, the distance, accuracy, muscle and movement onsets speeds, and muscle synergy similarity indices to the norm synergies increase during the voluntary-initiated trials after exposure to StartReact. Overall, this dissertation shows that exposure to StartReact did not impair voluntary-initiated movement and muscle synergy, but even improved them. Therefore, this study suggests that StartReact is safe for more investigations in training studies and therapy. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2020

When exploring movement production, motor control researchers have been interested in investigating the relative contributions to different types of movement. In a research setting, a startling acoustic stimulus (SAS) can be used as a tool to explore the neural processes that are occurring when preparing and initiating a movement. Additionally, suprathreshold TMS is another tool which can induce a suppression of the cortical region of the brain, resulting in RT delays which provides us with the ability to assess the corticospinal contributions to a particular movement. The aim of the current study was to investigate potential differences in the planning and execution of bimanual versus unimanual wrist extension movements. It was of particular interest as to whether bimanual coupling occurs at the cortical level or in lower parts of the output pathway (reticulospinal). Participants (N=6) were instructed to complete a unimanual or bimanual wrist extension following a control go-signal or a SAS. For subset of trials, in order to explore the level of corticospinal excitability of the movement, suprathreshold TMS was applied over the left M1 during the task to induce a cortical silent period (CSP). Results revealed that theimpact of TMS on response initiation was not significantly different for unimanual task versus a bimanual task. Furthermore, the SP (silent period) only had an impact on the right limb and not the left during the bilateral task. Lastly, SAS did lead to shorter RTs for both the unimanual and bimanual wrist extension task, but the RT delay induced by TMS in the right limb was not shorter in SAS trials compared to control. The findings of the present study suggest that bimanual coupling may be occurring at the cortical level and in lower parts of the output pathway as there may be correlated neural activity in the two hemispheres occurring during bimanual wrist extension movements.