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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - May 2021)
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.
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.
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.
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.
No abstract available.
Master's Student Supervision (2010 - 2020)
When we move our ability to detect tactile events on the moving limb is reduced. This process, known as movement-related tactile suppression, prevents unimportant sensory information from bombarding our central nervous system. This thesis aimed to extend the tenets of movement-related suppression during goal-directed reaching and explore any modulation of this suppression according to task-relevance. In three experiments participants performed volitional self-driven (Experiment 1, 3) and motor-driven (Experiment 1, 2) reaching and grasping movements. Over the course of the movement, weak electrical stimulation was presented at task-relevant (i.e., index finger and thumb) and task-irrelevant sites on the moving limb. In Experiment 1, participants displayed reduced detectability during movement of a readily detectable tactile stimulus (90% resting detection). This was true for all locations on the moving limb irrespective of task-relevance and during both self and motor-driven movements. In Experiment 2 and 3 a range of stimulus amplitudes was presented to one task relevant location during both self and motor-driven movements (Experiment 2) and to a task relevant and irrelevant site during self-driven movements (Experiment 3). This slight change in methodology allowed us to get a direct estimate of perceptual thresholds and asses the magnitude of movement-related tactile suppression. During both self and motor-driven movement participants exhibited an increased perceptual threshold at the index finger (Experiment 2). The magnitude of suppression however, was greater at the forearm than at the index finger (Experiment 3). Collectively these experiments suggest that tactile suppression is a general consequence of movement. Although evident at all locations on the moving limb, we suggest that tactile suppression can be modulated in a relevance-dependent manner.
Using both hands at the same time is an important ability of the human action system. This is referred to as bimanual coordination, and complex cases of coordination are often tested to reveal its limitations. A common limitation is that the limbs cannot make independent movements but are drawn to follow the same spatial trajectories with similar temporal properties. These examples of bimanual interference are called spatial and temporal interference. Another type of interference is seen in the initiation of bimanual reaching movements. When a reaching movement is directly-cued by illuminating the targets, the reaction time is the same for symmetric or asymmetric movements. However, the reaction time is longer for asymmetric compared to symmetric movements if they are symbolically-cued. The leading hypothesis for this reaction time cost is that the increased processing demands on response selection for symbolically-cued asymmetric movements results in bimanual interference (Diedrichsen, Hazeltine, Kennerley, & Ivry, 2001). In two experiments, we investigated the effect of this interference when it occurred as the result of a perturbation during a movement that required an on-line correction. We sought to determine if there was larger spatial interference in one limb when the other limb responded to a symbolically-cued on-line correction compared to a directly-cued correction. Participants made bimanual reaches to targets that were occasionally perturbed at movement onset. These perturbations required on-line corrections with one limb to the new target location. The new target location was indicated by illuminating the new target as a direct cue (experiments 1 and 2) or symbolically cueing the target with a colour change (experiment 1) or displaying the letter L or S (experiment 2). We found larger spatial interference for symbolically-cued on-line corrections compared to directly-cued corrections. Although there was greater interference with symbolic cues, the interference was small and transient with direct and symbolic cues. It was also subtle in comparison to spatial interference during preplanned bimanual reaches. Since a correction in one limb can be accomplished without a large or lasting effect on the other limb, we conclude that on-line control of the limbs during bimanual reaching is largely independent.