Memory's penumbra: episodic memory decisions induce lingering mnemonic biases

Abstract

How do we decide if the people we meet and the things we see are familiar or new? If something is new, we need to encode it as a memory distinct from already stored episodes, using a process known as pattern separation. If familiar, it can be used to reactivate a previously stored memory, by a process known as pattern completion. To orchestrate these conflicting processes, current models propose that the episodic memory system uses environmental cues to establish processing biases that favor either pattern separation during encoding or pattern completion during retrieval. To assess this theory, we measured how people's memory formation and decisions are influenced by their recent engagement in episodic encoding and retrieval. We found that the recent encoding of novel objects improved subsequent identification of subtle changes, a task thought to rely on pattern separation. Conversely, recent retrieval of old objects increased the subsequent integration of stored information into new memories, a process thought to rely on pattern completion. These experiments provide behavioral evidence that episodic encoding and retrieval evoke lingering biases that influence subsequent mnemonic processing.

Fig. 1

(A) Participants are presented with a series of objects and are asked to identify whether each item is new (first presentation), old (exact repetition), or similar (a modified object). The top sequence provides a preceding new example: the similar object follows a new object. The bottom sequence provides a preceding old example: the similar object follows an old object. (B) Results from Experiment 1. The left graph plots accuracy on similar trials as a function of the preceding response (“new in blue and “old” in red) (N=15). The bars on the right graph are further divided according to perceptual similarity ratings. (C) Results from Experiment 2. The left bar graph displays the preceding new benefit (preceding “new” – preceding “old”) for similar trial accuracy at 0.5, 1.5 and 2.5 s inter-stimulus interval (ISI) (N=52). The inset figure plots accuracy on similar trials that were preceded by new responses (blue) or old responses (red) at the three ISIs. The graph to the right plots the similarity rating interaction for Experiment 2 in the same way as it is presented above. Error bars mark the standard error of the difference between preceding new and old conditions. *p<.05 ** p<0.005 ***p<0.001

Fig. 2

Integrative encoding was tested using a multiphase design. (A) Participants first learned face-scene associations in which pairs of faces were associated with the same scene (A-X & B-X associations). (B) Next, they learned a new scene association for one face from each pair (A-Y association). These A-Y learning trials were interleaved with object recognition trials. Each A-Y association was preceded by either a novel object (preceding new condition) or one that had been studied at the beginning of the session (preceding old condition). Participants indicated whether each object was old or new. (C) The final phase tested whether they integrated the overlapping associations from the prior two phases by testing whether participants chose to pair the indirectly related faces and scenes (B-Y association). (D) The influence of preceding memory decisions on integration in Experiment 3. The graph compares how likely participants were to pair indirectly associated faces and scenes (B-Y associations) during the final test phase. Trials are divided according to whether their learned counterpart (A-Y) was encoded following new (blue) or old (red) trials during the prior phase. Error bars mark the standard error of the difference between preceding new and old conditions. *p<.05