To produce movement sequences is to interact meaningfully with the world around us. Large amounts of the human behavioural repertoire, such as typing on a keyboard, executing a dance routine, or playing a piece of music require the sequential execution of movements. Moreover, to execute sequences is to plan them. Without planning, execution is far more prone to failure. This thesis Investigates the cognitive and neural mechanisms which underly the planning and execution of movement sequences.
In Chapter 1, relevant research is described and summarised that elucidates how sequences are prepared, controlled, and implemented under a motor hierarchical framework. Evidence from neuroimaging and electrophysiology is integrated to provide an overarching account of how the brain plans movements and transitions into effective execution. However, how high- and low-level hierarchical sequence features map onto the planning and execution of movement sequences is unclear.
To investigate such mapping, Chapter 2 assesses the presence of high-level order and timing, and low-level integration, in motor cortical regions throughout sequence planning and execution using multivariate analysis of functional magnetic resonance imaging (fMRI). A shift from the control of order and timing during planning, to integration with online timing control during execution was identified.
With a hierarchical shift identified in the cortex, Chapter 3 investigates the presence of sequence features in subcortical regions which have been shown to play important roles in the generation of movement. The left hippocampus is shown to plan the order of movements in advance. Additionally, the basal ganglia, thalamus, and bilateral hippocampus show distinct activity patterns between planning and execution, supporting findings from electrophysiology which show that planning and execution occupy orthogonal subspaces.
Chapter 4 explores the role of inhibition of unused effectors in the resolution of competitive planning processes. Behavioural markers of competitive queueing were found in the hand to be used in the upcoming sequence; however, the contralateral hand displayed a mirrored gradient of inhibition which is thought to reflect interhemispheric transcallosal inhibition processes used to reduce the likelihood of incorrect movements.
The summary Chapter 5 formulates a systems-level framework for the planning and execution of skilled movement sequences, integrating findings from both the literature and the previous empirical chapters. The hippocampus and parietal cortex are thought to plan the order of upcoming movements in extrinsic and intrinsic space respectively, after which a signal to move ascends through the thalamus and causes reorganisation of cortical and subcortical neural patterns, with the former shifting to low-level sequential integration with elements of online timing. The implications and future directions of such a model are discussed with respect to both research and clinical significance.