Rapid-choice motor actions : the influence of age, brain connectivity, and cognitive processing

Movement is inarguably the most fundamental way in which humans interact with the complex and constantly changing world around them. Two key characteristics of our highly adaptable motor system are a) the ability to make fast and accurate decisions when faced with situations requiring a choice between two or more alternatives, and b) actively cancelling planned actions in light of changing relevant stimuli. However, little is known, in healthy young and older adults, of the influence of upstream cognitive processes on both these characteristics and the corresponding underlying neurophysiology. Thus, the overall aim of this research was to investigate, from young to older adulthood, how cognitive processing in the form of prior information influenced movement preparation, execution, and cancellation. To this end, sophisticated tools, from a behavioural (computational modelling to provide mechanistic insights), neurophysiological (temporally- and spatially-precise dual coil transcranial magnetic stimulation), and statistical (Bayesian generalized linear mixed models) standpoint were used.
This thesis consists of five chapters, namely three empirical studies bookended by an introductory chapter providing a rationale for the proposed research as well as focussed reviews of the relevant literature, and a concluding chapter providing a general discussion of the results.
Chapter 2 describes an empirical study that investigated the behavioural effects of prior information (i.e., ‘bias’) on rapid-choice decision-making from young to older adulthood. Biased responses were quicker and more accurate, and computational modelling provided a deeper understanding into latent cognitive processes. Besides the canonical effect of older adults being more cautious and biased decisions requiring lesser evidence, a robust effect on the nondecision-based process of response execution time was also observed. Specifically, biased decisions had a faster response execution time with this effect observed to a lesser degree in older adults, especially when provision of bias information was more cognitively demanding. These results provided evidence, from young to older adulthood, against a selective influence of bias on decision-based processes by demonstrating an influence on response execution time.
Chapter 3 describes an empirical study that provides insights into the neurophysiological effects of bias on movement preparation and execution using transcranial magnetic stimulation. Corticospinal excitability during movement preparation was lower in both hands – that which was biased towards and away from a response. Novel evidence for the role of direct (mediated via transcallosal pathways) and indirect (mediated via prefrontal regions) interhemispheric inhibitory interactions was observed. Specifically, greater direct inhibition was observed not only during movement preparation in the hand biased away from a potential response than towards, but also during movement execution in the biased hand when it was not responding than when it was. In addition, during movement preparation, indirect inhibition was more prominent in the hand biased away from a response when the provision of bias information was more cognitively demanding. Thus, results from the study elucidated the role of different neurophysiological mechanisms; hand-specific modulation of interhemispheric inhibitory interactions provided evidence for the ‘inhibition for competition resolution’ hypothesis, whereas the generic reduction in corticospinal excitability provided evidence for the ‘inhibition for impulse control’ hypothesis.
Chapter 4 describes an empirical study that again utilized a bias manipulation and transcranial magnetic stimulation, but in the context of movement cancellation, and more specifically, response-selective movement cancellation where cancellation of only one component of a bimanual movement was required. Behaviourally, provision of cues informative of stopping demands resulted in reduced interference delays in the continuing response than when uninformative cues were provided. Neurophysiologically, during movement cancellation, a consistent reduction in indirect inhibition was observed in the continuing hand compared to not only the stopping hand, but also to both hands when a bimanual response was made. Results from the study when interpreted in light of the ‘activation threshold’ framework – which suggests that the activation threshold of the continuing response is raised – provides evidence for the role of indirect inhibitory interactions, mediated via prefrontal regions, in reaching this elevated threshold and facilitating the response of the continuing hand.
Based on the research in this thesis, other tasks offering insights into movement control such as the anticipated response inhibition task were explored. In the spirit of open science adopted throughout the thesis with openly available data, work was undertaken to develop open-source software for utilisation of such a task as well as openly available models to analyse the data. Overall, the conducted research provides novel insights – using computational modelling, transcranial magnetic stimulation, and Bayesian statistics – into the role of cognitive processing and brain connectivity in rapid-choice motor actions from young to older adulthood.