Not a day goes by, it seems, without some reminder of how important sleep is for our brain health – whether a headline about the dangers of cell phone use before bed or the latest start-up encouraging its workers to nap during the day. While we are all increasingly aware of the necessity of sleep for healthy cognition, perhaps less understood is how sleep itself can be tapped for understanding cognitive performance. A new study is laying a foundation for just that: isolating that the electrical signals from sleep are biological markers of cognitive abilities and could offer a window into human intelligence.
The study focused on sleep spindles – high-frequency bursts of electrical activity that appear on EEG recordings during non-rapid eye movement (non-REM) sleep. Many scientists think sleep spindles reflect plasticity in the brain, including during memory formation. Previous research has suggested that the spindles are electrophysiological markers of cognitive abilities, particularly the ability to reason and solve new problems.
However, the past work did not examine whether the other factors could be influencing the relationship between sleep spindles and cognition. In particular, sleep spindles are thought to contribute to good sleep quality and are influenced by the circadian rhythms that govern our sleep. So the question was whether the role of sleep spindles in cognition is simply a result of the relationship between good sleep and cognitive ability, or whether sleep spindles themselves are markers for cognitive abilities?
Zhuo Fang of Western University in Ontario, Canada, with principle investigator Stuart Fogel and colleagues, used “mediation analysis,” a statistical method for teasing out different factors to better establish cause and effect. They found that fast sleep spindles in non-REM sleep and slow-wave sleep (aka deep sleep) are directly related to reasoning abilities, such as the capacity to identify complex patterns and the use of logic in solving new problems. These sleep spindles are not, however, directly related to verbal abilities or the capacity to briefly maintain information in memory, according to the new study, published in the Journal of Cognitive Neuroscience.
CNS spoke with Fang about these results and what they could mean for the study of cognition, including for people with learning disabilities.
CNS: Why are you personally interested in this research area?
Fang: Sleep plays a crucial role in physical and mental health as well being important for daytime functioning, learning and memory. Given my long-term interests in cognitive neuroscience research, I became interested in how sleep modulates our brain functions. We are becoming an increasingly sleep-deprived society, and sleep problems are very common, which results in cognitive deficits (i.e., memory, attention, reasoning etc.). Therefore, I am interested in investigating the relationship between sleep and cognitive performance, and the neural correlates of this relationship.
CNS: What’s your favorite way to explain sleep spindles?
Fang: Spindles are bursts of relatively fast brain waves that we measure from the scalp, that are short in duration, but large in amplitude and characterize non-rapid eye movement sleep. They are associated with good sleep quality and memory functions.
CNS: What have we known previously the relationship between sleep spindles and intelligence?
Fang: Our work has shown that spindles are important for the process of strengthening long-term memory. We have also demonstrated previously that spindles are highly related to trait-like cognitive abilities, specifically, with reasoning and problem solving skills (e.g., the ability to employ logic, and identify complex patterns).
CNS: What new insight were you seeking with this study?
Fang: Given that spindles are also related to other aspects of sleep, such as sleep quality, and are influenced by our daily biological (circadian) rhythms, the possibility remained that our previous finding was explained by these other related factors. We sought to investigate if the relationship between spindles and the capacity for reasoning could be accounted for by these possible confounds, or if they were directly related to reasoning ability when taking these alternative explanations into account.
CNS: What were you most excited to find?
Fang: It was surprising and exciting to find that the relationship between spindles and reasoning abilities was not at least partially explained by sleep quality or circadian rhythmicity.
CNS: What would you most like people to understand about sleep and cognition?
Fang: That sleep can serve as a window into understanding the neural activity that reflects individual cognitive strengths and weaknesses. Naturally occurring neural oscillations such as spindles are a potential means to better understanding the brain basis of waking cognitive performance.
CNS: What’s next for this line of work?
Fang: It will be important to investigate the neural correlates of this relationship and to expand this investigation to include other features of sleep. This work also has important implications for understanding academic performance, and perhaps may even lead to insight into learning disabilities or developmental disorders. Most importantly, this research highlights the importance of sleep for cognition.
CNS: Is there anything I didn’t ask you about that you’d like to add?
Fang: One of the advantages of this study was that we used the Cambridge Brain Science (CBS) test battery. This is a freely available platform for running cognitive tests over the Web, which was developed by our collaborator Dr. Adrian Owen. It assesses a broad range of cognitive abilities including reasoning, problem solving, planning, attention and memory, which can be widely used for cognitive tests.
-Lisa M.P. Munoz
