One of the strange defining factors of dreaming is how our brainwave activity during this important stage of sleep resembles that of a waking brain. Because of this it has been difficult for researchers to determine when a subject enters a REM sleep stage using electroencephalogram (EEG) data alone. Other physiological markers alongside EEG data have been necessary to confirm REM sleep, including, of course, rapid eye movement and a type of muscle paralysis called atonia.
Now a new study has, for the first time, homed in on a novel EEG signature that can identify when a subject is in a REM sleep stage. The international research, led by scientists from the University of California, Berkeley, initially stemmed from a team of anesthetists wanting to better understand what kind of unconscious state their patents were in during surgery.
"We often tell our patients that, 'You will go to sleep now,' and I was curious how much these two states actually overlap," says Janna Lendner, first author on the study. "Anesthesia can have some side effects. If we learn a little bit about how they overlap – maybe anesthesia hijacks some sleep pathways – we might be able to improve anesthesia in the long run."
For over a century scientists have studied electrical activity in the brain using EEG readings. Alongside aiding epilepsy diagnoses, EEG data offers useful insights into conscious and unconscious states, helping doctors, for example, understand brain activity in coma patients.
Generally, EEG measurements can determine whether a person is awake, in deep sleep, or anesthetized. However, when we enter a REM sleep stage and dream, EEG data gets noisy and resembles waking brain activity. Of course, other observational data points can easily indicate a person is obviously dreaming and not awake, but until now researchers have not been able to find a pattern in dreaming EEG data to offer an objective neurophysiological measure to differentiate REM sleep from wakeful arousal activity.
"There is this background activity, which is not rhythmic, and we have overlooked that for quite a long time," explains Lendner. "Sometimes, it has been called noise, but it is not noise; it carries a lot of information, also about the underlying arousal level. This measure makes it possible to distinguish REM sleep from wakefulness by looking only at the EEG."
Digging into this EEG “noise”, known as 1/f activity, the new study characterizes a novel pattern that can be directly associated with REM sleep. The EEG pattern is distinct from any activity seen during wakefulness and offers researchers the first independent neurophysiological marker to signify a person is dreaming.
"These new findings show that, buried in the electrical static of the human brain, there is something utterly unique – a simple signature," says co-author Matthew Walker. "And if we measure that simple electrical signature, for the first time, we can precisely determine exactly what state of consciousness someone is experiencing – dreaming, wide awake, anesthetized or in deep sleep."
Identifying this particular REM signature will help researchers conduct new investigations into brain activity during anesthesia, a brain state that is still quite a mystery to neuroscientists. Robert Knight, senior author on the new study, also points out how this novel EEG signature could help doctors better understand different degrees of consciousness in coma patients.
"More importantly, I think it is another metric for evaluating states of coma," says Knight. "1/f is very sensitive. It could resolve, for instance, if someone was in a minimally conscious state, and they are not moving, and whether they are more alert than you think they are."
The new study was published in the journal eLife.
Source: UC Berkeley
