During sleep, the brain undergoes a set of processes to ensure we wake up feeling refreshed. Humans experience different phases of sleep, rapid eye movement (REM) and non-REM sleep, which cause distinct changes in physiology, brain activity, and cognition. During REM sleep, the brain is very active and we experience vivid, bizarre, and emotional dreams. During non-REM sleep, the brain is less active and clears out waste products by flushing cerebral spinal fluid through the brain’s interconnected chambers, known as ventricles, and then through the brain. This process supposedly helps the body to remove harmful protein deposits from the brain that are associated with the development of Alzheimer’s disease.

What happens in a pigeon’s brain when they sleep?

The question of whether similar processes also take place in birds has remained unresolved until now. Professor Onur Güntürkün, head of the Biopsychology Department at Ruhr University Bochum, said:

“The last common evolutionary ancestor of birds and mammals dates back about 315 million years, to the early days of land vertebrates. Yet the sleep patterns in birds are remarkably similar to those in mammals, including both REM and non-REM phases.”

To determine how birds sleep, researchers used infrared video cameras and functional magnetic resonance imaging (fMRI) to observe and record the sleeping and wakeful states of 15 pigeons that were specially trained to sleep under these experimental conditions.

The video recordings shed light on the sleep phases in the birds.

“We were able to observe whether one or both eyes were open or closed, and to track eye movements and changes in pupil size through the pigeons’ transparent eyelids during sleep.”

Simultaneously, the fMRI recordings provided information about brain activation and the flow of cerebral spinal fluid in the ventricles.

Dreams of flying

During REM sleep, the researchers found that brain regions responsible for visual processing, including those areas that analyze the movement of a pigeon’s surroundings during flight, were highly active. They also observed activity in areas that process signals from the body, particularly from the wings.

“Based on these observations, we think that birds, just like humans, dream during REM sleep, and might be experiencing flight in their dreams.”

Furthermore, the scientists observed that a specific part of the brain called the amygdala is activated during these stages. This indicates that if birds undergo a process similar to human dreaming, then emotions might also be present in pigeons’ dreams. This assumption is backed up by the fact that the birds’ pupils contract quickly during REM sleep, which is similar to what happens during courtship or aggressive behavior while they are awake.

Washing out the day’s dust

During non-REM sleep in pigeons, the flow of cerebral spinal fluid through ventricles increases, similar to humans. However, the team found that the flow decreased significantly during REM sleep, which was the first time it was observed in any animal.

Niels Rattenborg, head of the Avian Sleep Group, explained:

“We think that the increased flood of blood into the brain during REM sleep, which supports the elevated brain activity, might block the cerebral spinal fluid from moving from the ventricles into the brain. This suggests that REM sleep and its functions might come at the expense of waste removal from the brain.”

However, the scientists are also entertaining the possibility that REM sleep contributes to waste removal in unexpected ways. Gianina Ungurean, added:

“At the onset of REM sleep, the influx of blood increases vessel diameter. This might force cerebral spinal fluid that entered the space during non-REM sleep to flow into the brain tissue, and enhance the outflow of fluids carrying waste products.”

The researchers suggest that cleaning the brain during sleep may be particularly important for birds. This is because their brains have more neurons per unit of volume than mammals, which means that the removal of waste products may require more efficient or more frequent flushing cycles. Because birds experience more and shorter REM phases during sleep than mammals, the frequent surge of blood flow may help to keep their densely packed brains free of harmful waste products.

Image Credits

In-Article Image Credits

Feral Rock Dove bird flying via Natures Pics Online by Alan D. Wilson with usage type - Creative Commons License. 2006
Common pigeon bird closeup via Wikimedia Commons by Satdeep Gill with usage type - Creative Commons License. July 28, 2022