Selective memory retrieval can revive forgotten memories

Humans remember less and less of what was encoded as more and more time passes. Selective retrieval can interrupt such time-dependent forgetting, enhancing recall not only of the retrieved but also of the nonretrieved information. The recall enhancement has been attributed to context retrieval and the idea that selective retrieval reactivates the retrieved item’s temporal context during study, which can facilitate recall of other items that had a similar context at study. However, it is unclear whether context retrieval induces a transient discontinuity in the stream of temporal context only, or a more permanent updating of context that would entail a lasting interruption of time-dependent forgetting. In three experiments, we analyzed time-dependent forgetting of encoded information right after study and after time-lagged selective retrieval. Selective retrieval boosted recall of the nonretrieved information up to the levels observed directly after study. Intriguingly, it also created a restart of time-dependent forgetting that made forgetting after retrieval indistinguishable from forgetting after study and thus induced a reset of the recall process. The results suggest that selective retrieval can revive forgotten memories and cause lasting recall enhancement, effects likely mediated by context retrieval and a permanent updating of temporal context.

People recall much more detail of an event shortly after they observed the event than a few hours or even days later. In fact, recall typically declines rapidly soon after encoding followed by a long, much slower decline in recall performance (1–4). It is important to understand if and how such time-dependent forgetting can be attenuated or even be interrupted. Recent research has demonstrated that memory retrieval can interrupt time-dependent forgetting.When people study a list of items or study some prose passage and, after a longer time interval, selectively retrieve some of the studied information, recall of the other nonretrieved information is often enhanced (5–8). This recall enhancement interrupts time-dependent forgetting of this information, creating a recall level that can even be similar to the recall level shortly after study (Fig. 1). However, it is unclear whether the interruption represents a short-lived or a lasting effect on recall performance. The interruption may be transient in character, with the recall level of the nonretrieved information returning to the original course of forgetting soon after the selective retrieval. But the interruption may also be more permanent in character and, for instance, be accompanied by a restart of time-dependent forgetting. Such restart would make the forgetting after retrieval identical to the original time-dependent forgetting after study. Selective retrieval would thus revive the forgotten memories, induce a reset of recall of these memory contents, and create lasting effects of recall enhancement. It is the primary goal here to examine the time-dependent forgetting of nonretrieved information after selective retrieval and compare it with the time-dependent forgetting after study.Open in a separate windowFig. 1.Hypothetical time-dependent forgetting of studied items is shown before and after intermediate selective retrieval. The left curve represents time-dependent forgetting directly after study before selective retrieval. Selective retrieval interrupts the nonretrieved items’ forgetting and boosts their recall right after the selective retrieval. The two right curves represent hypothetical time-dependent forgetting of the nonretrieved items as time since selective retrieval passes. One curve assumes that the recall boost reflects a transient effect and recall quickly returns to the original course of forgetting. The other curve assumes that selective retrieval creates a restart of time-dependent forgetting that makes the forgetting after retrieval identical to time-dependent forgetting after study.The observed recall enhancement of the nonretrieved information right after selective retrieval has been attributed to context retrieval (7, 8). Temporal context, which reflects external conditions but also an ever-changing internal context state, changes gradually over time (9, 10), and each studied item is associated with the temporal context in which it is shown (11–14). A temporal lag between study and retrieval thus induces context change and makes context during retrieval different from context during study, which can cause forgetting (15). However, retrieval of an item can reactivate the context that was present when that item was studied, and this retrieved context can then serve as a retrieval cue for other items with a similar context at study (16–18). Thus, if retrieved and nonretrieved items share contextual features encoded during study, retrieval can reactivate part of the study context of nonretrieved items and thus facilitate recall of these items.Context retrieval updates context by adding the retrieved study context to the current state of temporal context (13, 14, 19). Such updating effectively shifts the study context closer to the later time-of-test context (20). If lasting, such shift of study context could cause a restart of the forgetting process and make time-dependent forgetting after selective retrieval similar to time-dependent forgetting after study. However, it is unclear whether such context updating is lasting. Another possibility is that the updating reflects a transient effect and study context becomes available for a short time after retrieval only. In such case, the effect would reflect a transient discontinuity in the stream of temporal context only (21) and recall would quickly return to the information’s original course of forgetting after study.Here, results from three experiments are reported aimed at shedding light onto how selective retrieval influences time-dependent forgetting. In each experiment, we compared time-dependent forgetting of studied items when recall was tested after study in the absence of selective retrieval with time-dependent forgetting of retrieved and nonretrieved items when recall was tested after selective retrieval. During selective retrieval, participants retrieved some studied items, thus creating retrieved and nonretrieved items. Both when recall was tested after study and when it was tested after selective retrieval, recall was assessed at different delay intervals, which allowed a comparison of the time-dependent forgetting before and after selective retrieval. Retrieval has recently been found to attenuate time-dependent forgetting of the retrieved information and improve its recall performance (22–24). Our experiments provide a link to this research by permitting a comparison of the time-dependent forgetting of retrieved and nonretrieved information.