Romeo Chua

In this thesis the neural correlates associated with reaches under varying conditions of visual feedback, delay and movement difficulty were examined. The events of interest, wherein these correlates are observed were: 1) the encoding of a target (P3); and 2) the execution of a reaching movement towards the target (motor MP & N4). In study 1, the neural correlates associated with variations in task difficulty and target visible vs. target occluded were examined. The results of target encoding showed that P3 was greater when observed prior to reaches without than with vision of the target. Results of MP and N4 both varied based on task difficulty, as reaches to the far target yielded larger amplitudes than reaches to the close target. In study 2, the effects of visual feedback, delay condition, and presentation schedules (i.e., blocked vs. randomized) on target encoding and movement related cortical potentials was examined. For target encoding, component P3 failed to yield any significant differences across all vision, delay and presentation schedules. As for the movement related potentials, significant effects of delay were observed for component MP in the randomized protocol (i.e., larger MP in the long vs. the short delay), but not the blocked protocol. The analysis of N4 for the randomized protocol yielded a main effect of vision, as reaches with vision of the target attained greater negative amplitudes as compared with reaches without vision of the target. In the blocked protocol the analyses of N4 yielded a main effect of delay period as movements following long delay periods resulted in larger negative amplitudes. In study 3, the effects of real-time vision of the hand and target during a reaching movement on target encoding and movement execution were investigated. The findings revealed that the P3 component was modulated by the visibility of target. For the movement related cortical potentials, larger amplitudes for N4 were yielded for reaches without vision of the hand vs. reaches with vision of the hand. The three studies taken together provide insight into the neural events associated with goal-directed manual aiming under various reaching conditions.

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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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The information processing of bimanual reaching movements was investigated in this thesis. All of the studies tested symmetric and asymmetric bimanual reaching movements that were made to targets as quickly and accurately as possible. The duration of movement preparation was measured by reaction time (RT). Study one found that bimanual asymmetric movements had longer preparation than bimanual symmetric movements. Donders’ subtraction method was used to isolate this bimanual asymmetric cost to a stage, or stages, of movement preparation that are unique to choice RT tasks; these included target discrimination, response selection, and response programming. Many different movement parameters could cause bimanual asymmetric costs. The results from study two suggested that the relative contribution of three parameters to the asymmetric cost, from most to least important, was movement amplitudes, target locations, and then startling locations. The relationship between unimanual and bimanual movements was tested in the third study by precuing the target for the left arm of a bimanual movement. RT and the start-react effect were used to determine how movement preparation changed. These measures suggested: 1) that the precued movement was not fully programmed but partially programmed before the imperative stimulus, and 2) that the asymmetric cost was caused by increased processing demands on response programming. Overall, the results supported that bimanual movements are not the sum of two unimanual movements; instead; the two arms of a bimanual movement are unified into a functional unit. When one target is precued, this critical unification likely occurs during response programming. Study four used the additive factors method to determine which stages of movement preparation contributed to the asymmetric cost when both targets were cued by the imperative stimulus. The results supported that the asymmetric cost was caused by increased processing demands on response selection. Target discrimination and response programming – contrary to previous hypotheses – did not contribute to the asymmetric cost. The critical process of bimanual unification likely depends on how the task is presented and conceptualised. It occurs during response selection when both targets are cued by the imperative stimulus, and it is deferred to response programming when one target is precued.

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The aim of this dissertation was twofold: (1) to examine task co-representation and joint action in efforts to identify necessary preconditions under which shared representations are formed and (2) to determine whether alternative explanations can account for the social Simon effect (SE). Using joint Simon effect protocols (e.g., Sebanz & Knoblich 2003), we began (Study 1) by showing that when paired participants responded to the same stimulus-response alternative, the joint SE was absent. When participants performed under a competitive context (Study 2), the joint SE was elicited, even though co-representation would have been disadvantageous with respect to the task goal. Next, we examined the influence of spatial and response discrimination factors on the joint action correspondence effect. Our first investigation (Study 3) did not support the assumption that the co-actor may be providing a reference for the spatial coding of alternative responses. Using Ansorge and Wühr’s (2004) response discrimination hypothesis as a framework, we showed in subsequent studies (Study 4 & 6) that a SE could be elicited in a Go/No-Go task when spatial codes were used to discriminate between alternative responses. This was demonstrated when a standard 2-choice task preceded a Go/No-Go task and when participants performed two independent tasks alongside each other. Examination of event-related potentials pertaining to action inhibition suggested reduced action suppression on no-go trials when performing with a co-actor compared to performing with alone under these independent task conditions. In a final study (Study 7), we explored task co-representation using a different experimental paradigm—the response-precuing task. Our results did not provide clear evidence for task co-representation. In cases where the ‘social’ SE was not observed, we propose that a form of ‘social loafing’ or an individualistic mindset approach to the joint action task may have been in operation. Our overall findings encourage further investigation of how task context can modulate the joint SE and highlights how an individualistic mindset can potentially preclude co-representation.

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No abstract available.