SFB 654: Plasticity and Sleep
Social and Behavioural Sciences
Final Report Abstract
After two successful funding periods the SFB 654 "Plasticity and Sleep" was changed into a “Transregio-SFB”, which spanned the Universities of Lübeck, Kiel and Tübingen. The framing hypothesis of the SFB was that sleep serves the consolidation of biological memories. As a matter of principle, the consolidation of memories during sleep was assumed to represent an active process (termed “Active Systems Consolidation”). Sleep provides optimal conditions for this process because, during sleep, interference and potential disruptions of underlying plastic processes originating from the encoding of external stimuli is kept to a minimum. Importantly, it was assumed that memories are formed during sleep not only in the neurobehavioral system but also in other organismic systems processing environmental information, i.e., the metabolic system, and the immune system. Moreover, it was assumed that the consolidation process in these three systems shares common features. Based on this conceptual framework, the research of the SFB showed that sleep indeed supports the consolidation of memory in the three systems of interest, i.e., in the neurobehavioral system, in the metabolic system and in the immune system. For the neurobehavioral system, it was shown that sleep consolidates memories for episodes experienced during prior wakefulness, and for the immune system it was shown that sleep enhances the formation of memories for antigens encountered during prior wakefulness (e.g., by vaccination). For the metabolic system, research of the SFB showed that sleep stabilizes set points of the homeostatic regulation of metabolic parameters like blood glucose level and body weight. Set points indeed reflect the internal “memory” of homeostatically regulated feedback systems. Moreover, research of the SFB unravelled some of the mechanisms underlying the offline memory consolidation during sleep. To this end, the SFB which was initially focused on research in humans, relied more and more on research in animal models. The studies showed that in all three systems of interest, the stage of slow wave sleep is more important for consolidating memory than rapid eye movement (REM) sleep. EEG oscillations hallmarking slow wave sleep, like neocortical slow oscillations, were found to be critically involved in the consolidation of episodic memories in the neurobehavioral system and in the consolidation of antigenic T cell memory in the immune system. Slow wave sleep and slow oscillations were likewise found to be essential for stabilizing metabolic set points. Moreover, this research identified the suppression of the stress hormone cortisol induced by slow wave sleep as another mechanism contributing to memory consolidation in the three systems of interest. The research of the SFB led us to propose the “Active Systems Consolidation” theory of memory formation during sleep. This theory can be considered the most important scientific advancement made by the SFB since it provides an integrative concept of the biological function of sleep and of how sleep supports the formation memory for environmental stimuli in the neurobehavioral system and in the immune system. The theory does not account for the stabilizing effects of sleep on metabolic set points, since set points represent genetically preformed memories and their stabilization during sleep can be explained by a simple process of synaptic re-normalization. By contrast, the consolidation of newly formed memories for external events in the neurobehavioral and immunological domain, as proposed by the Active Systems Consolidation concept, requires a two-stage process in which newly encoded memories are a gradually redistributed from a cell network serving as initial store to a cell network serving as long-term store. The Active System Consolidation concept is currently the most influential theory on the function of sleep. As a systems approach to memory, it is also becoming increasingly attractive to the study of immunological memory formation. In addition to the conceptual advances, the research of the SFB led to the development of two novel techniques to selectively influence slow wave sleep and memory processing during sleep, respectively. It was shown that the EEG slow oscillations that hallmark slow wave sleep can be enhanced by so-called “auditory closed-loop stimulation”. Moreover, it was shown that the consolidation of neurobehavioral memories can be enhanced by a procedure termed “targeted memory reactivation”. Both techniques are now widely used scientifically in the field of sleep and memory. They are also about to be available for the broad society as means to improve sleep and to enhance memory functions, respectively. Overall, this SFB has made Germany to a stronghold of sleep research and, increasingly, also of memory research. It has essentially contributed not only to an increased awareness of the importance of sleep in the broad society, but has also fostered the development of new technologies to improve and exploit sleep for cognitive enhancement and health.
