Elsevier

Behavioural Brain Research

Volume 226, Issue 2, 15 January 2012, Pages 592-596
Behavioural Brain Research

Review
How we remember the stuff that dreams are made of: Neurobiological approaches to the brain mechanisms of dream recall

https://doi.org/10.1016/j.bbr.2011.10.017Get rights and content

Abstract

Intrinsic and historical weaknesses delayed the spread of a sound neurobiological investigation on dreaming. Nevertheless, recent independent findings confirm the hypothesis that the neurophysiological mechanisms of encoding and recall of episodic memories are largely comparable across wakefulness and sleep. Brain lesion and neuroimaging studies converge in indicating that temporo-parieto-occipital junction and ventromesial prefrontal cortex play a crucial role in dream recall. Morphoanatomical measurements disclose some direct relations between volumetric and ultrastructural measures of the hippocampus–amygdala on the one hand, and some specific qualitative features of dreaming on the other. Intracranial recordings of epileptic patients also provide support for the notion that hippocampal nuclei mediate memory formation during sleep as well as in wakefulness. Finally, surface EEG studies showed that sleep cortical oscillations associated to a successful dream recall are the same involved in encoding and recall of episodic memories during wakefulness.

Although preliminary, these converging pieces of evidence strengthen the general view that the neurophysiological mechanisms underlying episodic/declarative memory formation may be the same across different states of consciousness.

Highlights

► Dreaming and episodic memory encoding may share similar brain mechanisms. ► Sleep EEG oscillations predict successful dream recall. ► Functional anatomy of hippocampus–amygdala correlates with dream features.

Section snippets

Introduction to the neurobiological investigation on dreams

The production of dreams during sleep is a largely unexplained phenomenon of human existence, and its underlying brain mechanisms are mostly unknown. This is mainly due to the unaccessibility of dreams to a direct study, but only by using dream recall after spontaneous or provoked awakening from sleep. Like the Schrodinger's Cat, we can investigate dreaming only interrupting dream experience and, in this way, altering the physiological scenario in which dreams are produced (i.e., the electrical

Dreaming in brain damaged patients

The so-called “neuropsychology of dreaming” has evaluated dream features in patients with selective brain lesions and provided univocal evidences that confirmed the assumption that phenomenology of dreams is closely related to the brain activity and organization. Preliminary neuropsychological observations were addressed to the brainstem, in keeping with the view that the high co-occurrence of REM sleep and dream recall implies that brainstem mechanisms are also responsible of dream generation.

Neuroimaging studies: functional measures

Neuroimaging techniques like Positron Emission Tomography (PET), using H215O measurements of regional cerebral blood flow (rCBF), have allowed to describe a functional neuroanatomy of human sleep and the possible correlates of dream features. Although consistent evidence has been provided on dream recall upon awakenings from any stage of NREM sleep [2], [4], neuroimaging studies mostly focused their attention on the peculiar association between REM sleep and dreaming, in order to identify which

Neuroimaging studies: morphoanatomical measures of subcortical nuclei

Neuroimaging measures and intracranial recordings delineate a fairly coherent picture of how two deep grey matter structures may play a pivotal role in dreaming: the hippocampus and the amygdala.

The specific activity of the hippocampus should mediate the partial reproduction of memories of events occurred during wakefulness into dream contents. The first evidence in favour of this hypothesis was provided by studies using intracranial recordings in epileptic patients, which showed that

Local (cortical) electrophysiology of sleep and dream recall

In the last decades, a large body of evidence corroborated the hypothesis that sleep is not a spatially global and uniform state. Indeed, the application of the technical advancements (i.e., multichannel sleep EEG and Fast Fourier Transform – FFT – analyses of EEG) allowed to evaluate the spatio-temporal dynamics EEG during sleep, showing that the homeostatic dynamics of EEG power in the recovery process are intrinsically local and did not involve the whole cerebral cortex to the same extent

Concluding remarks and future perspectives

Neurobiological evidences discussed in this review, by integrating findings from different approaches and methods, depicts a fairly coherent picture that emphasizes the continuity between cortical and subcortical mechanisms of waking and sleep cognitive activity [40], [41]. Regional EEG results suggest that the electrophysiological mechanisms involved in encoding and recall of episodic memories across wakefulness and sleep are the same [39]. Brain lesions and functional neuroanatomy of healthy

Acknowledgements

This work was partly supported by the Compagnia di San Paolo, Programma Neuroscienze 2008/09 (3889 SD/sd, 2008.1300 to L.D.G), in the form of a fellow grant to C.M., and by the Fondazione Cassa di Risparmio di Bologna (Project n. 2008.2443 to C.C.).

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