The Strength of Target Representations Drives Mnemonic Discrimination at Extended Delays

Mnemonic discrimination, or the ability to distinguish between similar remembered experiences, is critical for episodic memory and is dependent on the computational process of pattern separation. Prior work with tasks that place high demands on mnemonic discrimination such as the Mnemonic Similarity Task (MST) has focused on short retention intervals and it remains unclear how mnemonic discrimination evolves over longer delays. To address this gap in understanding, we examined MST performance at delays ranging from a few minutes to seven days in two experiments. Consistent with prior forgetting literature, both recognition of repeated items and discrimination of similar lures declined with delay. In the immediate delay condition, the similarity of lure stimuli 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. Additionally, 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 especially for high-similarity lures. Together these findings reflect the influence of the strength of target representations on lure discrimination: strong target representations lead to high-confidence correct rejections of low-similarity lures but also to high-confidence false alarms to high-similarity lures. As target representations weaken over extended delays, false alarms to high-similarity lures become less likely while low-similarity lures were more likely to be called “new”. This is the first demonstration of how mnemonic discrimination in the MST changes over extended delays.