While we sleep, the brain produces particular activation patterns. When two of these patterns – slow oscillations and sleep spindles – gear into each other, previous experiences are reactivated.
The stronger the reactivation, the clearer will be our recall of past events, a new study reveals.
Scientists have long known that slow oscillations (SOs) and sleep spindles – sudden half-second to two-second bursts of oscillatory brain activity – play an important role in the formation and retention of new memories.
But experts in the UK and Germany have discovered that the precise combination of SOs and sleep spindles is vital for opening windows during which memories are reactivated; helping to form and cement memories in the human brain.
Researchers at the University of Birmingham and Ludwig-Maximilians-University Munich today published their findings in Nature Communications.
Co-author Dr. Bernhard Staresina, from the University of Birmingham’s School of Psychology, commented: “Our main means of strengthening memories while we sleep is the reactivation of previously learnt information, which allows us to solidify memories in neocortical long-term stores.
“We have discovered an intricate interplay of brain activity – slow oscillations and sleep spindles – which create windows of opportunity enabling this reactivation.”
Co-author Dr. Thomas Schreiner, from Ludwig-Maximilians-University, Munich, commented: “Memory reactivation is specifically bound to the presence of SO-spindle complexes. These results shed new light on the memory function of sleep in humans and emphasise the importance of orchestrated sleep rhythms in strengthening our powers of recall and orchestrating the creation of memories.”
Before this study, evidence of the brain’s capacity to reactivate memories during sleep was scarce, but the team devised novel tests where participants were shown information before taking a nap and closely monitored brain activity during non-rapid eye movement (NREM) sleep using EEG recording.
Those taking part were then tested on their memory recall after waking up, allowing the researchers to link the extent of memory reactivation during sleep to memory performance.
The results revealed reactivation of learning material during SO-spindle complexes, with the precision of SO-spindle coupling predicting how strongly the memory would be reactivated by the brain. This in turn predicted the level of memory consolidation across participants and the subsequent clarity of recall.
Active system memory consolidation theory proposes that sleep-dependent memory consolidation is orchestrated by three cardinal sleep oscillations (Diekelmann and Born, 2010; Helfrich et al., 2019; Klinzing et al., 2019; Mölle et al., 2011; Piantoni et al., 2013; Rasch and Born, 2013; Staresina et al., 2015): (1) Hippocampal sharp-wave ripples represent the neuronal substrate of memory reactivation (Vaz et al., 2019; Wilson and McNaughton, 1994; Zhang et al., 2018), (2) thalamo-cortical sleep spindles are thought to promote long-term potentiation (Antony et al., 2018; De Gennaro and Ferrara, 2003; Rosanova and Ulrich, 2005; Schönauer, 2018; Schönauer and Pöhlchen, 2018), while (3) neocortical SOs provide temporal reference frames where memory can be replayed, potentiated and eventually transferred from the short-term storage in the hippocampus to the long-term storage in the neocortex, rendering memories increasingly more stable (Chauvette et al., 2012; Diekelmann and Born, 2010; Frankland and Bontempi, 2005; Rasch and Born, 2013).
Importantly, these three oscillations form a temporal hierarchy, where ripples and spindles are nested in SO peaks, with ripples also being locked to spindle troughs. This hierarchy likely constitutes an endogenous timing mechanism to ensure that the neocortical system is in an optimal state to consolidate new hippocampus-dependent memories (Chauvette et al., 2012; Clemens et al., 2011; Helfrich et al., 2019; Klinzing et al., 2016; Klinzing et al., 2019; Latchoumane et al., 2017; Niethard et al., 2018; Piantoni et al., 2013; Staresina et al., 2015).
Recent findings indicate that the precise temporal coordination of SO-spindle coupling is deteriorating over the lifespan, which contributes to age-related memory decline (Helfrich et al., 2018b; Muehlroth et al., 2019; Winer et al., 2019). It is currently unclear if similar principles apply to brain maturation and how the dynamic interplay of SOs and spindles is initiated.
Critically, the transition from childhood to adolescence is marked by considerable changes in sleep architecture and cognitive abilities similar to the transition from young adulthood to old age (Carskadon et al., 2004; Huber and Born, 2014; Iglowstein et al., 2003; Ohayon et al., 2004; Shaw, 2007; Shaw et al., 2006).
Previous research mainly focused on the individual development of SOs and sleep spindles across brain maturation, showing that these cardinal sleep oscillations undergo a substantial evolution in their defining features such as amplitude, frequency, distribution and occurrence (Campbell and Feinberg, 2009; Campbell and Feinberg, 2016; Goldstone et al., 2019; Hahn et al., 2019; Kurth et al., 2010; Nicolas et al., 2001; Purcell et al., 2017; Shinomiya et al., 1999; Tarokh and Carskadon, 2010).
Currently, two major obstacles hamper our understanding of how the precise temporal interplay between SOs and spindles predicts brain development and memory formation. First, pronounced changes in sleep oscillatory activity pose major methodological challenges for assessing and comparing SOs and sleep spindles across the age spectrum (Muehlroth and Werkle-Bergner, 2020). Second, memory performance was rarely tested in developmental sleep studies, thus, impeding our understanding of the functional significance of temporal SO-spindle coupling for memory formation.
Here, we leverage a unique longitudinal study design from childhood to adolescence to investigate how SO-spindle coupling emerges during development and infer its functional significance for developing memory networks. To account for the substantial morphological alterations of SO and spindle morphology across brain maturation, we developed a principled methodological approach to assess SO-spindle coupling.
We utilized individualized cross-frequency coupling analyses, which enable a clear demonstration of SO-sleep spindles coupling during both developmental stages. Critically, over the course of brain maturation from childhood to adolescence, more spindles are tightly coupled to SOs, which directly predicts improved memory formation.
reference link: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7314542/
More information: “Endogenous memory reactivation during sleep 1 in humans is clocked by slow oscillation-spindle complexes” Thomas Schreiner, Marit Petzka, Tobias Staudigl and Bernhard P. Staresina, Nature Communications, 2021.