Sleep appears in the brain as slow waves that ripple across the surface at a rate of about one every tenth of a second—or so we thought.
A new study in mice suggests that there are patterns of sleep-related brain activity that we’ve been overlooking—that reflect the state of individual brain cells rather than the collective activity of millions or billions of neurons.
What’s more, by measuring these hyperlocal, submillimeter brain signals with single-wire electrodes, the researchers found that parts of the mammalian brain can doze off into a brief sleep while other areas remain fully awake.
“It was surprising to us as scientists to find that different parts of our brain actually take a little nap when the rest of the brain is awake,” says David Haussler, a bioinformatician at the University of California (UC) Santa Cruz and lead author. studies.
For about a hundred years, patterns of electrical activity throughout the brain have been used to define the difference between sleep and wakefulness in a quantitative sense. These brain waves are most often detected using an electroencephalogram (EEG), through electrodes placed on the scalp.
However, Haussler and his team questioned how we measured sleep and distinguished it from wakefulness—when it’s clear that there is some transition in the animals’ brains that keeps them awake while they sleep, a skill known as unihemispheric slow-wave sleep.
In the 1960s, scientists first suspected and then discovered how dolphins and other cetaceans could rest half of their brains while remaining active, sometimes keeping one eye open to monitor predators and maintain contact with others in their pod.
Seals and birds also exhibit variations of this part-sleep and part-awake rest—a clever trade-off between sleep and survival.
Humans can also temporarily exhibit asymmetric sleep patterns that resemble but are not the same as those seen in animals.
In 2016, researchers from Brown University in the US found that on the first night people slept in an unfamiliar place, the left side of the brain was more alert to deviant sounds than the right. Once we get used to the sleeping environment, this difference will disappear.
“The human brain, it turns out, is endowed with a less dramatic form of unihemispheric sleep than is found in birds and some mammals,” wrote neurologist Christof Koch. Scientific American when those results were published.
As for the mouse brain, the blurring of wake and sleep states in humans may be a neurological trait we share with other animals.
Haussler and team collected weeks of data from nine mice that had electrodes with thin wires implanted in 10 different areas of their brains, and fed that data into an artificial neural network that learned to distinguish between sleep and wake states.
Recordings were taken from 100 micrometers (one-tenth of a millimeter) of brain tissue, and the algorithm was able to reliably identify sleep-wake cycles based on brief “blinks” in brain cell activity lasting just 10 to 100 milliseconds.
These “hyperlocal” signals indicated that part of the animals’ brains fell asleep while other areas remained active and awake. Coincidentally, the researchers noticed that this happened at the very moment when the mouse could stop moving for a split second, almost as if it “shut down”.
“We could look at the individual time points when these neurons fired, and it was pretty clear [the neurons] they were transitioning into a different state,” explains Aiden Schneider, a computational biologist at Washington University in St. Louis, who led the study with David Parks, a computer science graduate student at UC Santa Cruz.
“In some cases, these flashes can be limited to just a single area of ​​the brain, maybe even smaller.”
The team think their new method of measuring sleep-wake states could reveal new secrets about how we sleep if these “flickers” can be observed by other research groups.
“They [the flickers] break the rules you’d expect based on a century of literature,” says neuroscientist Keith Hengen of Washington University in St.
The study was published in Nature Neuroscience.