Sleep Spindles and Intellectual Ability: Epiphenomenon or Directly Related?

Abstract

Abstract■ Sleep spindles-short, phasic, oscillatory bursts of activity that characterize non-rapid eye movement sleep-are one of the only electrophysiological oscillations identified as a biological marker of human intelligence (e.g., cognitive abilities commonly assessed using intelligence quotient tests). However, spindles are also important for sleep maintenance and are modulated by circadian factors. Thus, the possibility remains that the relationship between spindles and intelligence quotient may be an epiphenomenon of a putative relationship between good quality sleep and cognitive ability or perhaps modulated by circadian factors such as morningness-eveningness tendencies. We sought to ascertain whether spindles are directly or indirectly related to cognitive abilities using mediation analysis. Here, we show that fast (13.5-16 Hz) parietal but not slow (11-13.5 Hz) frontal spindles in both non-rapid eye movement stage 2 sleep and slow wave sleep are directly related to reasoning abilities (i.e., cognitive abilities that support "fluid intelligence," such as the capacity to identify complex patterns and relationships and the use of logic to solve novel problems) but not verbal abilities (i.e., cognitive abilities that support "crystalized intelligence"; accumulated knowledge and experience) or cognitive abilities that support STM (i.e., the capacity to briefly maintain information in an available state). The relationship between fast spindles and reasoning abilities is independent of the indicators of sleep maintenance and circadian chronotype, thus suggesting that spindles are indeed a biological marker of cognitive abilities and can serve as a window to further explore the physiological and biological substrates that give rise to human intelligence. ■

Neural Correlates of human cognitive abilities during sleep

“…Standard multiple linear regression revealed that, taken together, Reasoning, Short Term Memory and Verbal abilities significantly accounted for variability in spindle amplitude (F(3, 25) = 4.884, r 2 = 0.370, p = 0.008), but not duration (F(3, 25) = 0.531, r 2 = 0.060, p = 0.665) or density (F(3, 25) = 2.522, r 2 = 0.232 p = 0.081) at Cz during NREM sleep ( Table 3 ). Similar to previous studies (Fang et al, 2017; Fogel et al, 2007), inspection of the semipartial coefficients revealed that Reasoning ability (t(25) = 2.191, r = 0.401, p = 0.038; Figure 1 ) uniquely accounted for variability in spindle amplitude over and above STM (t(25) = 0.314, r = 0.063, p = 0.756) and Verbal (t(25) = 0.972, r = 0.191, p = 0.341) abilities. The same regression analyses were conducted for slow spindles at Fz and fast spindles at Pz, however, we did not observe any significant relationship between spindles and cognitive abilities.…”

“…This demonstrates that a subset of spindle-related activations was specifically related to Reasoning abilities, but not to Short Term Memory or Verbal abilities. This is consistent with, and provides physiological support for the current and previous studies (Bódizs et al, 2005; Fang et al, 2017; Fogel et al, 2007; Schabus et al, 2006; Ujma et al, 2014, 2015), demonstrating that Reasoning abilities are uniquely correlated to sleep spindles. The same whole-brain spatial correlation analyses were conducted for fast spindle and slow spindle activation maps, however, we did not observe significant correlations between any cognitive ability and the activation maps for each individual spindle type.…”

“…Previous studies have revealed that interindividual differences in spindle characteristics are related to the capacity for reasoning (i.e., the ability to identify complex patterns and relationships, the use of logic, existing knowledge, skills, and experience to solve novel problems (Fogel & Smith, 2007; Fogel & Smith, 2006; Nader & Smith, 2001, 2003). Moreover, the relationship between spindles and cognitive abilities is specific to the capacity for Reasoning, over-and-above Verbal abilities and short-term memory(Fang et al, 2017; Fogel et al, 2007). These studies have provided insight into the electrophysiological correlates of Reasoning abilities, insofar as to suggest that efficient functioning of the neural substrates that support spindle generation (e.g., thalamocortical circuitry) may be related to the capacity for these cognitive skills.…”

“…Although, a relationship in men was subsequently identified by the same group in daytime sleep (Ujma et al, 2015). Most recently, Fang and colleagues (Fang et al, 2017) used the Cambridge Brain Sciences ( CBS ) test battery (Hampshire et al, 2012) to explore if the relationship between sleep spindles and intellectual ability was a direct relationship, or whether this could be partially (or fully explained) by other spindle-related factors such as sleep quality or circadian chronotype. They found that, indeed, the relationship between spindles and Reasoning abilities was independent of sleep quality and circadian chronotype.…”

Individual differences in frequency and topography of slow and fast sleep spindles

“…Despite decades of progress, the functional role of sleep spindles in general, and of slow and fast spindles in particular, is still unclear. While some recent evidence suggests fast spindles are more strongly implicated in cognitive processes [17][18][19][20][21][22] , only few studies finding such links target the purported brain rhythms in an individualized manner 26 . But if slow and fast spindles do indeed turn out to serve different functions, to serve the same function differently, or to be differentially affected in neuropsychiatric disorders, it becomes critical that these spindle types be adequately separated.…”