The Strength of Target Representations Drives Mnemonic Discrimination: Evidence From Extended Delay and Hippocampal Damage

Mnemonic discrimination is critical for episodic memory and is dependent on the computational process of pattern separation, which allows the memory system to disambiguate prior memories from potentially overlapping new memories and avoid interference. Prior work with tasks such as the Mnemonic Similarity Task (MST) has demonstrated that discrimination performance is disproportionately affected by hippocampal insult. Further studies have demonstrated that the quality of initial encoding may affect subsequent discrimination performance. Taken together, these results imply that the strength of target memory representations determines discrimination performance in the MST. Here, we manipulated target memory strength by examining MST performance at delays ranging from a few minutes to seven days in three experiments with healthy young adults and a patient with hippocampal damage. Consistent with prior forgetting literature, both target recognition and lure discrimination declined with delay in controls. In the immediate delay condition, the similarity of lures to targets drove discrimination performance, with more similar lures eliciting false alarms and less similar lures eliciting correct rejections, as reflected in the lure discrimination index (LDI). This graded relationship was attenuated or absent at delays longer than one day. However, receiver operating characteristic (ROC) analyses revealed preserved discrimination across delays and target-lure similarity levels. Similarly, the hippocampal patient was impaired relative to controls when considering LDI, but ROC-based analysis revealed intact lure discrimination for the lowest-similarity lures at immediate delays. Finally, we observed an inversion of the typical confidence-accuracy relationship for lure trials at longer delays, with low-confidence responses more likely to be correct for high-similarity lures. Together these findings demonstrate that both the strength of target representations and the degree of lure similarity interact to drive discrimination performance.