Memory updating in dreams

Abstract

Robert Stickgold's research was among the earliest to rigorously quantify the effect of learning on dream content. As a result, we learned that dreaming is influenced by the activation of newly formed memory traces in the sleeping brain. Exactly how this happens is an ongoing area of investigation. Here, we test the hypothesis that participants are especially likely to dream of recent experiences, which overlap with well-established semantic networks. We created an artificial situation in which participants encountered new information about a person with which they have extensive past experience-a favorite celebrity. We tracked the effect of novel information about a favorite celebrity on participants' dream content across 3 consecutive nights and queried participants about other recent and remote memory sources of their dreams. While the celebrity manipulation failed to affect dream content, this dataset provides rich descriptive information about how recent and remote memory fragments are incorporated into dreams, and how multiple memory sources combine to create bizarre, imaginative scenarios. We discuss these observations in light of the proposed "memory updating" function of sleep-dependent memory consolidation, as well as Stickgold and Zadra's NEXTUP (Network Exploration to Understand Possibilities) model of dreaming. This paper is part of the Festschrift in honor of Dr Robert Stickgold.

Keywords: dreaming; dreams; hippocampus; memory; memory consolidation; memory updating; sleep.

Figure 1.

Reports submitted by night and time of night. Number of reports successfully submitted across each of the 3 nights of the study, by the time of night.

Figure 2.

Effect of condition on incorporation of entertainment news articles into dreaming. Experimental condition had no significant effect on dreaming about celebrities (either those specifically represented in the news articles or other celebrities) and no effect on dream content related to other aspects of the entertainment news articles. Dreams in the nonexposure group contained more content judged to be indirectly related to celebrities. Dream content related to the study itself also did not differ by experimental condition.

Figure 3.

Temporal origin of participant-identified dream sources. Participants were asked to identify the probable episodic memory sources of each dream. For each source, participants indicated when the episode had occurred. Recent episodic sources were more commonly identified than remote episodic sources.

Figure 4.

Co-occurrence of multiple memory sources within single dreams. (A) While dreams were most commonly traced to a single episodic memory source, 39% incorporated multiple past episodes. (B) When multiple past episodes appeared together in a dream, participants most often perceived these experiences as unrelated to each other. However, there were notable exceptions, as discussed below. (C) Raw number of dreams referencing multiple episodic memory sources originating from different time points. (D) Mean semantic relatedness of memory source combinations by temporal origin. Numbers are the average participant rating of semantic relatedness for all possible pairs of memory sources appearing together in a single dream. yearplus = memory sources from more than 1 year ago.

Figure 5.

Temporal origin of dream sources by the time of night. The temporal origin of dream sources did not vary by the time of night from which the dream was reported. Each point represents an individual memory source within a dream report.

Figure 6.

Semantic relatedness of dream sources by time of night. There was a trend for sources co-occurring within the same dream to be more strongly semantically related to each other later in the night.