Differentiating Reinforcement Learning and Episodic Memory in Value-Based Decisions in Parkinson's Disease

Patients with Parkinson's disease (PD) are impaired at incremental reward-based learning. It is typically assumed that this impairment reflects a loss of striatal dopamine. However, many open questions remain about the nature of reward-based learning deficits in PD. Recent studies have found that even simple reward-based learning tasks rely on a combination of cognitive and computational strategies, including one-shot episodic memory. These findings raise questions about how incremental learning and episodic memory contribute to decision-making in PD. We tested healthy participants (n = 26; 14 males and 12 females) and patients with PD (n = 26; 16 males and 10 females), both on- and off-dopamine replacement medication, on a task designed to differentiate between the contributions of incremental learning and episodic memory to reward-based learning and decision-making. We found that PD patients performed equally well as healthy controls when using episodic memory but were impaired at incremental reward-based learning. Dopamine replacement medication remediated this deficit and enhanced subsequent episodic memory for the value of motivationally relevant stimuli. These results demonstrate that while PD patients are impaired at learning about reward from trial-and-error, their ability to encode memories for the value of one-shot experiences is intact.