New research on brainwaves in the thalamus shows patterns distinctly linked to conscious states.

• Researchers giving epileptic patients a brain stimulation treatment unexpectedly found that brainwave oscillations during wakefulness and REM sleep are highly similar.
• The rapid eye movements that occur during REM sleep turned out to be associated with these oscillations, while oscillation frequencies were much lower during non-REM sleep.
• The similarity of the brainwaves during REM sleep and wakefulness may reveal important physical features of consciousness.

As a human being, you know you’re conscious, but it’s much harder to define exactly what that consciousness is made of. The subjective experience of being you, which seems so intangible, hasn’t yet been traced to a specific structure or function in your physical body. But now, an accidental discovery made during medical research might have revealed that a particular type of brain activity is responsible for the enigma we call consciousness.

When cognitive neuroscientist Tobias Staudigl and neurologist Elisabeth Kaufmann of Ludwig Maximilian University in Munich, Germany, were recording brain activity from epilepsy patients who had been implanted with electrodes for deep-brain stimulation therapy, they weren’t necessarily hoping to resolve the age-old riddle of consciousness. The electrodes, placed inside the thalamus to eventually reduce epileptic seizures, allowed the researchers to directly record thalamic activity during a brief post-surgical period before the stimulator was switched on. Coincidentally, there’s been some speculation among scientists that the thalamus, which relays sensory signals to the cortex and is also involved in attention and perception, has something to do with how we transition between conscious states from wakefulness to sleep.

During their work with the epilepsy patients, Staudigl and Kaufmann observed previously unknown activity in their thalami. When the patients were either awake or had fallen into REM sleep, the thalamus produced bursts of fast oscillations (19–45 Hz) that were both higher in frequency and more frequent in occurrence than the slower sleep spindles seen during non-REM (NREM) sleep. REM sleep is thought to be more “conscious” than NREM sleep because of the eye movements and vivid dreams it’s associated with. The dominant oscillations during NREM sleep are known as sleep spindles—short bursts of brainwave activity in the 11–17 Hz range, well below the 19–45 Hz frequency of the fast oscillations seen during REM sleep and wakefulness. This lower-frequency activity, combined with the near-absence of the fast oscillation during NREM, might be related to the reduced consciousness during this phase.

“The discovery of a distinct oscillatory signature in the central thalamus that distinguishes conscious states opens up avenues to further investigate thalamic contributions to states of consciousness in humans and potentially to refine interventions to treat disorders of consciousness,” they said in a study recently published in Nature Human Behavior.

While sleep spindles are important for consolidating memories through communication between the thalamus, hippocampus and cortex, they arise from inhibitory neurons in the thalamus, which prevent other neurons from firing. Sleep spindles are generated by inhibitory neurons, which suppress rather than activate other brain areas, and serve primarily to consolidate memories. The fast 19–45 Hz oscillation associated with REM sleep and wakefulness, by contrast, appears to arise from a different mechanism entirely, one that may support the kind of active, conscious processing needed to produce vivid dreams. Dreams during REM sleep are thought to indicate consciousness, but direct recordings of thalamic activity across different conscious states have rarely been possible in humans. With their electrodes in place, Kaufmann, Staudigl, and their team were able to fill that gap. They discovered that bursts of the fast thalamic oscillation are tightly coupled to the rapid eye movements that define REM sleep.

Even more amazing was that most frequency bands of oscillations detected during REM sleep overlapped with those seen in wakefulness. Both REM sleep and wakefulness showed more oscillation activity than NREM sleep, with more brainwave activity bursts, while there were far fewer oscillations in the sleep spindle frequency range. Eye movements “have a bursty nature,” as the researchers said, which is why they can predict oscillations happening in the thalamus. Observations revealed more bursts during one of the two microstates of REM sleep. Periods of tonic REM sleep involve no eye movements, while the electrodes picked up on at least two eye movements in periods of phasic REM sleep and wakefulness.

The researchers found it especially interesting that oscillations are so similar between conscious states that are otherwise different. When the brain is awake, it’s also alert to external stimuli and other sensory information, but that information is processed in an alternative way during REM sleep. The research gives more backing to the hypothesis that REM sleep is the brain simulating experiences as they would happen in waking life. Eye movements in sleeping rodents have already been shown to influence head direction, just as the animal would turn its head when awake. So it’s possible REM sleep oscillations simulate waking-life actions.

“REM sleep, and particularly phasic REM, has been reported to produce vivid dreams that involve conscious experience,” Staudigl and Kaufmann said. “That the thalamic oscillation reported here is specific to phasic REM and wakefulness might thus speak in favour of the two states being similar with respect to conscious access.”