Sleep Spindles: Mechanisms and Functions

Abstract

Sleep spindles are burstlike signals in the electroencephalogram (EEG) of the sleeping mammalian brain and electrical surface correlates of neuronal oscillations in thalamus. As one of the most inheritable sleep EEG signatures, sleep spindles probably reflect the strength and malleability of thalamocortical circuits that underlie individual cognitive profiles. We review the characteristics, organization, regulation, and origins of sleep spindles and their implication in non-rapid-eye-movement sleep (NREMS) and its functions, focusing on human and rodent. Spatially, sleep spindle-related neuronal activity appears on scales ranging from small thalamic circuits to functional cortical areas, and generates a cortical state favoring intracortical plasticity while limiting cortical output. Temporally, sleep spindles are discrete events, part of a continuous power band, and elements grouped on an infraslow time scale over which NREMS alternates between continuity and fragility. We synthesize diverse and seemingly unlinked functions of sleep spindles for sleep architecture, sensory processing, synaptic plasticity, memory formation, and cognitive abilities into a unifying sleep spindle concept, according to which sleep spindles 1) generate neural conditions of large-scale functional connectivity and plasticity that outlast their appearance as discrete EEG events, 2) appear preferentially in thalamic circuits engaged in learning and attention-based experience during wakefulness, and 3) enable a selective reactivation and routing of wake-instated neuronal traces between brain areas such as hippocampus and cortex. Their fine spatiotemporal organization reflects NREMS as a physiological state coordinated over brain and body and may indicate, if not anticipate and ultimately differentiate, pathologies in sleep and neurodevelopmental, -degenerative, and -psychiatric conditions.

Sleep spindles preferentially consolidate weakly encoded memories

“…Spindles are involved in memory reactivation processes and are believed to function mechanistically by inducing plasticity in learning-related regions (Fogel and Smith, 2011;Fernandez and Luthi, 2019). The fact that spindles showed the largest association to weak item consolidation compared items with stronger encoding suggests that spindles are less effective in enhancing more strongly encoded information.…”

Sleep State Classification Using Power Spectral Density and Residual Neural Network with Multichannel EEG Signals

“…The Welch PSD determines the distribution of signal power over frequency [34], which reduces the estimated power spectra at the cost of reduced frequency resolution [35]. Human EEG studies have revealed that aging reduces the power of lower frequency bands, i.e., delta, theta, and sigma, while increasing that of the higher frequency bands, i.e., beta [36,37]. Moreover, it has also been noted that female human subjects have higher PSD than their male counterparts in the delta and theta range [36,37].…”

“…Human EEG studies have revealed that aging reduces the power of lower frequency bands, i.e., delta, theta, and sigma, while increasing that of the higher frequency bands, i.e., beta [36,37]. Moreover, it has also been noted that female human subjects have higher PSD than their male counterparts in the delta and theta range [36,37]. Similarly, in [38], Gabryelska et al validated that higher individual sleep quality is correlated with reduced NREM stage 2 (N2) sigma 2 spectral power in healthy individuals.…”

In humans, sleep spindles are generated by local thalamic pacemakers

“…The cellular and network properties leading to the emergence of spindles during NREM sleep are now well established (see Fernandez & Lüthi, 2020 for review). The pacemaker of this oscillation is the thalamus, a subcortical structure that is crucial for routing sensory information to the neocortex and for coordination of activity across different cortical areas.…”

“…One promising line of research is that the spatiotemporal distribution of spindles during NREM sleep is a potential biomarker of various neuronal disorders (Fernandez & Lüthi, 2020), reflecting a disorganization in the underlying brain networks at the neuronal and circuit levels. It is possible that overnight sleep EEG will become a standard exam in yearly check‐ups, especially since cheap and high‐quality EEG recording systems are becoming available.…”