Cerebellar Single‐Pulse <scp>TMS</scp> Differentially Affects Early and Late Error Processing in Reinforcement Learning

There is increasing evidence that the cerebellum contributes to feedback processing in reinforcement learning. As yet, it has not been investigated whether the cerebellum also contributes to error processing in reinforcement learning. Studies have shown, however, that the cerebellum is involved in the processing of response errors in non-reinforcement learning contexts, for example, in response conflict tasks. In the present study, we aimed to extend these findings to the processing of response errors, which slowly emerges as a result of reinforcement learning. To this end, we inhibited the cerebellum via single-pulse transcranial magnetic stimulation (spTMS) and recorded cerebral electroencephalography (EEG) measures associated with error processing. If input from the cerebellum is required for error processing, error-correct differentiation should be decreased for cerebellar compared to vertex (control) stimulation. Cerebellar spTMS was applied and EEG was recorded while healthy adults performed a probabilistic feedback learning task. The error-related negativity (ERN), a component in the response-locked event-related potential (ERP), was used as a measure of error processing. It reflects a rapidly detected mismatch between representations of the actual and the desired response and is typically larger for errors than correct responses. Error-correct differentiation in the ERN was diminished for cerebellar compared to control TMS. However, increased error-correct differentiation was found in a later ERP component, the error positivity (Pe), which is more strongly associated with error awareness. Cerebellar spTMS thus impaired fast error processing reflected in the ERN and facilitated later, conscious error processing reflected in the Pe. These findings provide causal evidence of cerebellar contributions to error processing within reinforcement learning.