What are the Memory Sources of Dreaming?

What are the Memory Sources of Dreaming? Rebecca Dai Nicole Garces Joanna Wong

Overview • Examine the relationship between dreaming, memory, and the hippocampus • Combined research of how episodic memory is consolidated over time and how that may be modified during dreaming • Episodic memories: Memories of personally lived experiences

History • Freud and “day residues” • Elements of dreams connected to experiences of the previous day • Became central to psychoanalytic theory and therapy

Psychophysiological Methods introduced to study dreaming • REM and NREM sleep stages identified • Pre-sleep stimulation • Sensory stimulation during sleep • Subjects’ post-hoc identification of memory sources of dream elements

Recent Studies • Suggests complete episodic memories are shown in dreams only 1.4% • Isolated fragments of memories occur in 28-38% of reports • 65% of dream elements are linked to features of waking events • Very emotional dreams can become highly episodic (nightmares)

Dreams and Simulation of Reality • Dreams contain spatially-coherent enviroments Contradiction: • The lack of a fully episodic structure but the portrayal of coherent virtual worlds in dreaming

Hippocampus and Episodic Memories in Dreaming • Consensus: Hippocampal changes contribute to the unique characteristics of dream content -Increased activity in hippocampal region during REM -Rhythmic slow activity during REM in humans

Question? • How can this activity be linked specifically to changes in episodic memory organization?

First Line of Inquiry: The Here and Now of Dream Experience • Hypothesis: A type of spatial and temporal binding underlies dreaming that is analogous to the perceptual binding thought to underlie waking consciousness • Dream-related binding sustains illusion that dream takes place “here” and “now”

Hippocampus in temporal and spatial pattern processing • Animal Studies

Suggested research • Subject training in dream reporting • Evaluate contributions of other hippocampal-dependent cognitive systems • Hippocampal damaged patients • Dream characteristics • Implications

2nd Inquiry: Memory Source of Dreaming Governed by Temporal Mechanisms May Obscure their Epsiodic Orgins • Selection of memory sources are influenced by chornobiological factors • Chronobiological factors • 90 min REM/NREM sleep cycle -> selection of episodic memory • Circadian cycle -> selection of dream elements

Interval timers with a fixed duration • Influences selection of dream memory source • 12 hours before dream • A week before dream • Observation: longer interval delay exists but there is a consistent appearance of memory sources rise from age 10-19

Mechanism • Hippocampal model of time-related memory consolidation • Memories relocate over time from hippocampus to neocortex and are reiterated in dreams during this process • Qualitated difference in memory sources of dreams arise from • Recent residue (day-residue) time period • Delayed residue (1 week) time period • Graph next slide

Figure 1

Duration of this relocation process varies for different types/attributes of memories (may extend to years) • Animal study: major transition takes about 1 week • Evidence: • Hippocampal cell excitability increase after a learning task • But returns to baseline level on the 7th day post training • However, there is no evidence that this observation relates to delayed memory source in dreams

Possible connection: • Animal study shows 3 post-learning changes • REM sleep duration increase 5-7 days post learning, Ach level increase during REM • Hippocampal theta cells decrease in firing rate after a week • Partial REM deprivation disrupts performance

Proposed methological improvements: • More sophisticated methods of probing subject’s memories • Cued autobiographical recall • Facilitate self-reflection • Asses subject’s confidence in memory ability

Dreams Structured by Emotion • Dream emotional pattern preserved • Individual’s dominant, pre-sleep emotional concern • Emotional dream regulation probably controlled by amygdala • Amygdala activity higher during REM • Reciprocal dependence with hippocampus in encoding and storing memories • Amygdala capable of gating sensory information to entorhinal cortex to hippocampus • Hippocampal processing is essential for encoding fear memories in amygdala circuits

Episodic Origins of Emotional Structures • Emotional sources go unnoticed because they are expressed metaphorically • Possibly to build adaptive new contexts to assimilate current concern • Evidence: Assault victims who experience anxiety may not notice the emotional connection in dream content • Problem: Freudian idea so might be questionable • Emotional sources of dreaming may require development of methods that sensitize subjects to be aware of the emotional aspect of their dream while controlling for confounds may affect self observations

Conclusion • Episodic origins of dream content are opening new research ideas • Hypothesis • Memory elements in dreams facilitate learning simply by reactivating those elements in original perception-like state • Binding different elements around emotionally relevant themes strengthens and consolidates those elements

Conclusion (cont.) • Dream related memory consolidation is regulated by oscillatory or interval timers of different frequencies • analogous to hippocampal mediated waves (zif-268 gene) cause induction of LTP

Dreaming about new material enhances subsequent recall • Three studies show this: • Interrelated dream elements are better recalled in surprise morning task • Pre-sleep stories are better recalled in the morning when subjects dream about frequent about constituents of the story • Completion of mirror-tracing task leads to dreams that metaphorically represents the task • i.e. trying to stay on the road

What are the Memory Sources of Dreaming?