Neural substrates of reward, error, and effort processing underlying adaptive motor behaviour

Human motor control is highly adaptive to new tasks and changing environments. Motor adaptation relies on multiple dissociable processes that function to increase attainment of reward and to reduce sensory error and physical effort as costs. This thesis tests the hypothesis that fronto-striatal and dopaminergic neural systems contribute to specific aspects of motor adaptation that occur through reinforcement of rewarding actions. Behavioral tasks were designed to isolate learning in response to feedback conveying information about reward, error, and physical effort. We also measured behavioral effects of savings and anterograde interference, by which memories from previous motor learning can facilitate or impair subsequent learning. Electroencephalography (EEG) was used to record neural event-related potentials (ERPs) elicited by task-related feedback. We measured the feedback-related negativity/ reward positivity (FRN/RP), a midfrontal component of ERP responses to feedback stimuli that correlates with neural activity throughout fronto-striatal circuits. Levodopa, a dopamine precursor, was used to manipulate dopamine release in healthy volunteers, as it has been shown to impair reward-based learning in various cognitive tasks. We first determined that medial frontal feedback processing indexed by the FRN/RP is a specific neural correlate of reward prediction error during motor adaptation, and that the FRN/RP is not elicited by sensory error. Next, we found that levodopa did not affect either the FRN/RP, reward-based motor adaptation, savings, or anterograde interference. Finally, we determined that medial frontal activity indexed by the FRN/RP does not respond to physical effort as a cost that discounts the value of reward. However, effort increased neural sensitivity to reinforcement outcomes in activity measured by a midfrontal ERP component that was spatially and temporally distinct from the FRN/RP. These findings suggest that mid-frontal feedback processing measured by the FRN/RP may play a specific role in reward-based motor learning that is distinct from error- and effort-based learning processes. Our findings also indicate that reinforcement learning mechanisms that contribute to motor adaptation do not depend on the same dopaminergic processes that are impaired by levodopa in cognitive learning tasks