Humans are dependent on the integral functions performed by the hands. Hand selection is a prerequisite for the execution of any manual action. Ultimately, how hand selection unfolds, and the neural mechanisms underpinning how these actions are formulated, is not yet understood. This thesis employs a multi-method approach to investigate the cognitive and neural mechanisms of human hand selection.
At the outset of the empirical chapters, a new model of human hand selection is proposed: The Posterior Parietal Interhemispheric Competition (PPIC) model. The PPIC model posits that cell populations in bilateral posterior intraparietal and superior parietal cortex (pIP-SPC) encode multiple action plans in hand-specific terms which compete for selection. There is a dominant representation of the contralateral hand within each hemisphere. A hand is selected, and an action is executed, once a competing action plan reaches suprathreshold levels. Using a multi-method approach, the hypotheses of the PPIC model are tested throughout this thesis.
In Chapter 2, functional MRI was used to identify brain areas involved in hand selection. Participants performed a reaching task in free-choice and instructed hand use conditions. Consistent with the PPIC model, bilateral pIP-SPC was preferentially modulated in the free-choice condition, with specificity for the contralateral hand. Further, the pattern of fMRI responses within parietal areas and behavioural data are consistent with the notion that hand selection unfolds via a neural competition.
These areas were targeted using continuous theta-burst stimulation (cTBS) in Chapter 3. Participants performed a reaching task in three sessions, following cTBS to Left-pIP-SPC, Right-pIP-SPC, and Sham stimulation. Continuous TBS to pIP-SPC was expected to supress cortical excitability, and bias action competition in favour of ipsilateral hand choice. Contrary to these predictions, hand choice was comparable across sessions and largely insensitive to cTBS.
In a follow up experiment, outlined in Chapter 4, the efficacy of cTBS in inducing cortical inhibition is examined. The change in excitability of left primary motor cortex was compared after the application of active or sham cTBS. Results demonstrate high inter-participant variability, though a group-level facilitative effect on cortical excitability following active cTBS.
Overall, our results partly support the PPIC model of hand selection. The act of choosing a hand for action is shown to modulate bilateral pIP-SPC. The data are consistent with a competitive process underlying hand choice. Continuous TBS applied to pIP-SPC does not significantly alter hand choice behaviour, though the efficacy of induced cortical inhibition is uncertain. The implications of these results are discussed with reference to both the theoretical and clinical fields.