Sustained activity of human substantia nigra neurons reflect prior rewards

Dopamine (DA) signals from substantia nigra (SN) neurons encode reward prediction errors (RPEs) and have been implicated in motor control, reward processing, and motivational vigor. However, how recent reward history, related to reward expectations, is represented within the dopaminergic system remains poorly understood, particularly in humans, due to the difficulty of recording DA neuron activity directly. To address this, we performed single-unit recordings from the SN of patients with Parkinson's disease undergoing neurosurgery while they played a two-armed bandit decision-making task. We found that the firing rates (FRs) of putative DA neurons during reward expectation were modulated by previous trial outcomes, with higher FRs following positive outcomes. This increase in FRs was associated with faster subsequent reaction times (RTs), suggesting a link between neural signals reflecting prior reward and behavioral response vigor. These results provide a potential physiological substrate for how reward history influences behavior through the modulation of human dopaminergic activity.