Causal effects of prefrontal transcranial magnetic stimulation on dopamine-mediated reinforcement learning in healthy adults

Background10-Hz repetitive transcranial magnetic stimulation (rTMS) to the left dorsal lateral prefrontal cortex (DLPFC) has been shown to increase dopaminergic activity in the dorsal striatum, a region strongly implicated in reinforcement learning. However, the behavioural influence of this effect remains largely unknown. ObjectiveHere, we tested the causal effects of rTMS on behavioral and computational characteristics of reinforcement learning. Methods40 healthy individuals were randomized into Active and Sham rTMS groups. Each participant underwent one 10-Hz rTMS session (1500 pulses) in which stimulation was applied over the left DLPFC using a robotic arm. Participants then completed a reinforcement learning task sensitive to striatal dopamine functioning. Participants trial-to-trial training choices were modelled using a reinforcement learning model (Q-learning) that calculates separate learning rates associated with positive and negative reward prediction errors. ResultsSubjects receiving Active TMS exhibited an increased reward rate (number of correct responses per second of task activity) compared to the Sham rTMS group. Computationally, the Active rTMS group displayed a higher learning rate for correct trials (G) compared to incorrect trials (L). Finally, when tested with novel pairs of stimuli, the Active group displayed extremely fast reaction times, and a trend towards a higher reward rate. ConclusionsThe present study provided specific behavioral and computational accounts of altered striatal-mediated reinforcement learning induced by a proposed increase of dopamine activity by 10-Hz rTMS to the left DLPFC. Together, these findings bolster the use of TMS to target neurocognitive disturbances attributed to the dysregulation of dopaminergic-striatal circuits.