Reinforcement learning over time: spaced versus massed training establishes stronger value associations
Over the past few decades, neuroscience research has illuminated the neural mechanisms supporting learning from reward feedback, demonstrating a critical role for the striatum and midbrain dopamine system. Learning paradigms are increasingly being extended to understand learning dysfunctions in mood and psychiatric disorders as well as addiction in the area of computational psychiatry. However, one potentially critical characteristic that this research ignores is the effect of time on learning: human feedback learning paradigms are conducted in a single rapidly paced session, while learning experiences in ecologically relevant circumstances and in animal research are almost always separated by longer periods of time. Event spacing is known to have strong positive effects on item memory across species and in reward learning in animals. Remarkably, the effect of spaced training on human reinforcement learning has not been investigated. In our experiments, we examined reward learning distributed across weeks vs. learning completed in a traditionally-paced or \"massed\" single session. Participants learned to make the best response for landscape stimuli that were either associated with a positive or negative value. In our first study, as expected, we found that after equal amounts of extensive training, accuracy was high and equivalent between the spaced and massed conditions. However, in a final online test 3 weeks later, we found that participants exhibited significantly greater memory for the value of spaced-trained stimuli. In our second study, our methods allowed for a direct comparison of maintenance of conditioning. We found that spaced training again had a beneficial effect: more than 87% of conditioning was maintained for spaced-trained stimuli, while only 30% was maintained for massed-trained stimuli. In addition, supporting a role for working memory in massed learning, across both studies we found a significant positive correlation between initial learning and working memory capacity. Our results indicate that single-session learning tasks may not lead to the kind of robust and lasting value associations that are characteristic of \"habitual\" value associations. Overall, these studies begin to address a large gap in our knowledge of fundamental processes of human reinforcement learning, with potentially broad implications for our understanding of learning in mood disorders and addiction.