On the trail of animal dreams: Researchers have looked into the brains of sleeping pigeons and thus gained information about processes during their sleep state. Certain activity patterns suggest that the birds dream of flight scenes during the so-called REM sleep phase. In addition, patterns of fluid movement in the brain are emerging that could be linked to cleansing functions during sleep, the scientists report.
It may seem like a natural part of our lives, but from a scientific point of view, sleep raises many questions: Why do we and other living beings have to sleep at all and what happens when we do it? One thing is clear: you cannot do without sleep and a lack of duration or quality can seriously impair your health. Basically, it is known that sleep in humans is characterized by two phases. In one, there can be no question of rest: During REM sleep (Rapid Eye Movement) our brain is particularly active and we can experience dreams. In the non-REM sleep phase, on the other hand, the metabolic activity of the nerve tissue drops sharply. Studies have shown that harmful protein deposits are removed from the nerve tissue during this deep sleep phase. This process is characterized by an increased flow of cerebrospinal fluid through the brain structures – apparently “rinsing” takes place during the non-REM sleep phase.
Looking into the brains of sleeping pigeons
There are already indications from earlier studies that the basic patterns of our sleep also occur in many animals and have functions similar to those in humans. It is also assumed that monkeys, dogs, cats and the like also experience “virtual” scenes in the REM sleep phase, and also that their brains are flushed in the non-REM sleep phase. In contrast to mammals, comparatively little is known about bird sleep. The researchers led by Gianina Ungurean from the Max Planck Institute for Biological Intelligence in Seewiesen (MPIO) have now looked more closely at this group of animals.
They used 15 pigeons as test animals. These birds were used to sleeping under experimental conditions. The scientists were able to observe them using infrared video cameras, while at the same time the brain activity of the pigeons was recorded using a magnetic resonance tomograph (fMRT). As the team explains, the video recordings gave information about the respective sleep phases of the birds. Because the infrared images made it possible to record the pupil movements of the pigeons even when their eyes were closed. At the same time, the fMRI recordings then provided information about active areas of the brain and about the movement of the cerebrospinal fluid through the cerebral chambers.
It turned out: “During the REM phase, brain areas responsible for processing visual stimuli were particularly active, including areas that analyze how the environment of a pigeon moves during flight,” reports co-author Mehdi Behroozi from the Ruhr University Bochum. Another clue came from activity in areas known to process signals from the wings, among other things, the team reports. “Based on these observations, we suspect that birds, like us humans, also dream in REM phases,” says Behroozi. What is happening in the mind’s eye of the pigeons can also be guessed from the clues: “Maybe they are going through flight sequences,” says Behroozi.
Clues to Dreams and “Flushing Functions”
Another clue to the pigeon dreams was the activation of the amygdala – a brain structure that plays an important role in emotional processes. “This suggests that birds also have feelings in their dreams when they experience something similar to our human dreams,” says Ungurean. According to her, this hypothesis also supports the observation that during REM sleep the pupils of birds contract rapidly, just as they do during certain behaviors during wakefulness.
As for the study of the physiological processes during the two sleep phases of the birds, the researchers found: As in humans, the flow of cerebrospinal fluid increases during non-REM sleep. According to the findings, however, the movement of the liquid during REM sleep in the pigeons surprisingly decreases. This could mean that REM sleep could come at the expense of removing waste products from the brain. On the other hand, the comparatively more frequent and shorter REM phases in birds could also have a beneficial effect, say the researchers: “At the beginning of REM sleep, the diameter of the blood vessels increases due to the increasing blood flow. This could allow cerebrospinal fluid that has flowed into the ventricles during non-REM sleep to flow into the brain tissue and allow the waste-laden fluids to drain out,” explains Ungurean.
As the team concludes, they will now pursue further investigations into the unclear role of REM sleep in waste clearance in bird brains. They also want to devote themselves to the aspect of dreams: it is possible that the birds can use certain behaviors to convey what they “experienced” before waking up, say the researchers.
Source: Max Planck Institute for Biological Intelligence, specialist article: Nature Communications, doi: 10.1038/s41467-023-38669-1