Sleeping Participants Show Evidence of High-Level Cognitive Processing
Post by Lani Cupo
The takeaway
Participants respond to external stimuli during sleep, and responses are associated with cognitive activity, suggesting sleep is not a state of complete disconnection from the external world as previously believed.
What's the science?
Scientists have classically considered sleep a state with no reactivity to external stimuli, however, recent evidence suggests that stimuli can be processed on different cognitive levels during sleep, even to the point of learning new material. Nevertheless, few sleep studies attempt to elicit behavioral responses, as researchers consider them uniquely associated with wakefulness. This week in Nature Neuroscience, Türker and colleagues examined responsiveness to external stimuli during different sleep states, as well as lucid compared to non-lucid dreaming with an auditory decision task.
How did they do it?
The authors recruited two groups of participants: participants with narcolepsy who often lucid dream (N = 27), and healthy controls (N = 21). Participants napped in the laboratory during daytime hours while the authors spoke a combination of words and pseudowords. Participants were instructed to frown briefly three times in a row if they heard a pseudoword or smile briefly three times if they heard a real word. Facial muscle contractions were monitored with electromyography (EMG), and sleep/wake stage was assessed with a combination of electroencephalography (EEG) and electrooculography (EOG) which respectively measure electrical impulses from the brain and eye movements. After each nap, the participants reported if they dreamed, whether the dream was lucid, and whether they recalled performing the task.
In addition to the frequency of responses, response accuracy, and response rate on the task, the authors examined local brain activity associated with responses from EEG, and how well cognitive activity, measured with EEG, predicts responsiveness.
What did they find?
In terms of responsiveness, the authors found that participants could perform the task across almost any sleep state (except N3, deep sleep, in the healthy participants), meaning regardless of whether they were awake, in the lighter stages of sleep, or dreaming, they would respond to stimuli with smiles or frowns when they were presented and would not respond when no stimulus was presented. Interestingly, participants with narcolepsy responded more to stimuli across all sleep stages than healthy participants, including N3, and they reported lucid dreaming. As sleep deepened, the response rate decreased, although it increased slightly during rapid-eye movement (REM, dreaming sleep) in healthy participants. In participants with narcolepsy, the response rate increased greatly in non-lucid REM sleep and even further increased in lucid REM sleep. Examining accuracy, the authors found that all participants were more accurate than chance in responses, but healthy controls were more accurate than participants with narcolepsy, and increased depth of sleep was correlated with decreased performance. Regarding response time, as in wakefulness, during sleep participants were slower to respond to pseudowords than words, and participants were slower to respond overall while asleep than awake. Lucid dreaming was associated with significantly slower response times than non-lucid dreaming.
Using the EEG data, the authors found a signature of brain activity localized in frontal sites associated with responsiveness to stimuli across participants and sleep states. The authors computed markers of cognitive activity from all the electrophysiological data and compared these markers between responsive and nonresponsive trials, finding trials in which participants responded to the stimulus were associated with increased cognitive activity. The predictions were more accurate when tested on correct-only responses than incorrect-only responses, which provides evidence that the markers truly indicate cognitive activity, rather than merely motor activity. In contrast, lucid dreaming was associated with higher cognitive activity regardless of the response to stimuli.
What's the impact?
The results of this study demonstrate that humans maintain sensory connections to external stimuli while asleep and that they can process these external stimuli at a high cognitive level and physically respond to them. These results could precede further investigations of sleepers’ cognitive capacity and abilities, including the sleeping brain’s ability to learn new information.