A Role for Pattern Completion in Lure Rejection Evinced in Subsequent Order Memory

Mnemonic discrimination tasks such as the Mnemonic Similarity Task (MST) are used to assess hippocampal pattern separation. However, lures designed to evoke pattern separation include features that trigger pattern completion. In two experiments, we modified the object-based MST by adding a final order memory test to evaluate whether lure rejections were memory-based and promoted integrative encoding with studied objects. Participants studied objects (Phase 1), classified repeated, similar, and new objects (Phase 2), and completed a two-alternative forced-choice order memory test with lures next to corresponding studied objects (Phase 3). Lure rejections in Phase 2 were associated with more accurate identification of lure objects as recent in Phase 3, especially when participants also recognized that the corresponding studied object had appeared in Phase 1. Experiment 2 further showed that lure rejections preceded by clear memories of studied objects were associated with memory benefits, suggesting that retrieval and comparison processes promoted memory integration. These findings demonstrate that lure rejections are not process-pure indicators of pattern separation but instead reflect an interplay of pattern completion and separation. The addition of a final order memory test offers a useful approach for identifying conditions under which integrative encoding supports subsequent memory for similar events.