Why are dreams rarely remembered?

Jürgen Fell

University of Bonn, Department of Epileptology, Bonn, Germany

Memory for episodic events, like dreams or waking experiences, crucially depends on structures within the medial temporal lobe, in particular the hippocampus. The rhinal cortex represent the bottleneck or gatekeeper to the hippocampus. We have demonstrated that phase-synchronization of EEG activity between the rhinal cortex and the hippocampus is a predictor of successful episodic memory formation, probably enabling information transfer between both structures (e.g. Fell et al. 2001). The observation that rhinal-hippocampal synchronization is diminished during sleep, therefore, provides a neurophysiological explanation for the fact that dreams are rarely remembered (Fell et al. 2003b). A study investigating dream memory in epilepsy patients and its dependence on rhinal-hippocampal coupling yielded direct evidence supporting this hypothesis. Compared to a group with poor dream memory those patients with good dream memory exhibited more than twice as large rhinal-hippocampal coupling (Fell et al. 2006). Today, it is well-known that sleep supports the replay and consolidation of previously acquired memories (e.g. Marshall and Born 2007). Thus, an evolutionary reason for the reduced ability to create memories during sleep may be that this fresh encoding would interfere with and disturb the replay of older memories. References: Fell J, Klaver P, Lehnertz K, Grunwald T, Schaller C, Elger CE, Fernández G (2001) Human memory formation is accompanied by rhinal-hippocampal coupling and decoupling. Nat Neurosci 4:1259-1264. Fell J, Staedtgen M, Burr W, Kockelmann E, Helmstaedter C, Schaller C, Elger CE, Fernández G (2003) Rhinal-hippocampal EEG coherence is reduced during human sleep. Eur J Neurosci 18:1711-1716. Fell J, Fernández G, Lutz MT, Kockelmann E, Burr W, Schaller C, Elger CE, Helmstaedter C (2006) Rhinal-hippocampal connectivity determines memory formation during sleep. Brain 129: 108-114. Marshall L, Born J (2007) The contribution of sleep to hippocampus-dependent memory consolidation. Trends Cogn Sci 11: 442-450.

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