Procedural and declarative knowledge simultaneously contribute to category response selection.

Skilled behaviour in real-world contexts often relies on a combination of both declarative and procedural learning. However, precisely how declarative and procedural knowledge interact is not yet fully understood. Previous findings have shown that procedural and declarative learning may interact or compete at the systems level during encoding, consolidation, and retrieval, but beyond this, it is not known whether declarative and procedural representations themselves interact. The goal of the current study is to investigate whether procedural and declarative knowledge can contribute simultaneously to categorization response selection behavior. We designed a stimulus set in which information learned by each system sometimes supports different responses, and created trials in the test phase that are designed to maximize such divergence. Participants were instructed to use a completely diagnostic, verbalizable, shape-based rule to categorize exemplars, receiving feedback after each trial. However, unbeknownst to participants, the categories also differed probabilistically in their color distributions. Participants used both color (learned procedurally) and shape (learned declaratively) to categorize exemplars, responding more quickly when both sources indicated the same category judgement, and more slowly when they conflicted. Debriefing confirmed that most participants were unaware of the color distributions. These results show simultaneous trial-level contributions from both declarative and procedural memory systems. Our findings represent a novel form of interaction between the two systems and have implications for domains beyond the laboratory, such as decision-making and classroom instruction.