When pigeons dream

Newswise — Dreams have long symbolized human slumber. Yet, recent discoveries propose that pigeons, when asleep, may encounter flight hallucinations. Scientists from Ruhr University Bochum, Germany, and the Max Planck Institute for Biological Intelligence examined brain activity in dozing pigeons utilizing functional magnetic resonance imaging. The investigation unveiled that, akin to mammals, a significant portion of the brain exhibits heightened activity during REM sleep. Nonetheless, this alert-like condition may hinder efficient brain waste disposal. The researchers disclosed their findings in the June 5, 2023, edition of the journal Nature Communications.

While slumbering, our brain executes an intricate sequence of mechanisms to ensure a rejuvenating wakefulness. In humans, the various sleep stages, namely rapid eye movement (REM) and non-REM sleep, coincide with distinct alterations in physiology, cerebral activity, and cognitive function. For instance, REM sleep prompts heightened brain activity and engenders the most vivid, peculiar, and emotive dreams. Conversely, non-REM sleep induces reduced metabolic brain activity and facilitates the elimination of waste substances by circulating cerebrospinal fluid through the brain's ventricles, which are interconnected chambers enveloping its structures. This process purportedly aids in the expulsion of detrimental protein deposits from the brain, such as those implicated in the development of Alzheimer's disease.

What happens in a pigeon’s brain during sleep?

The inquiry concerning whether comparable mechanisms occur in avian species has remained unresolved until the present moment. "The most recent shared evolutionary precursor of birds and mammals goes back approximately 315 million years to the early stages of terrestrial vertebrates," remarks Professor Onur Güntürkün, who leads the Biopsychology Department at Ruhr University Bochum. "Nevertheless, sleep patterns in birds bear a striking resemblance to those observed in mammals, encompassing both REM and non-REM stages."

To ascertain the precise occurrences during avian slumber, the scientists employed infrared video cameras and functional magnetic resonance imaging (fMRI) to monitor and document the sleeping and wakeful states of 15 pigeons that were specially trained to sleep within these experimental parameters.

The recorded videos provided valuable insights into the sleep stages of the avian subjects. "By scrutinizing the video footage, we could discern whether the pigeons had one or both eyes shut or open, and we tracked their eye movements and variations in pupil size through their translucent eyelids during slumber," elucidates Mehdi Behroozi, a member of the Bochum team. In parallel, the fMRI recordings yielded data on brain activation and the movement of cerebrospinal fluid within the ventricles.

Dreams of flying

"Different regions of the brain responsible for visual processing, including those involved in analyzing the movement of the bird's environment during flight, exhibited robust activity during REM sleep," reveals Mehdi Behroozi. The team further observed activity in regions involved in processing sensory information from the body, particularly from the wings. "Drawing from these findings, we hypothesize that birds, similar to humans, engage in dreaming during REM sleep and potentially experience flight within their dreams," Mehdi Behroozi concludes.

Furthermore, the researchers observed heightened activation in a specific region of the brain called the amygdala during these sleep phases. Gianina Ungurean, from the Avian Sleep Group at the Max Planck Institute for Biological Intelligence, suggests, "This implies that if birds undergo experiences akin to human dreams, pigeons' dreams may encompass emotions as well." This hypothesis gains support from the rapid contraction of the birds' pupils during REM sleep, mirroring the pupillary response observed during courtship or aggressive behaviors while awake, as recently demonstrated by Gianina Ungurean and her colleagues.

Washing out the day’s dust

Similar to humans, the circulation of cerebrospinal fluid through the ventricles of pigeons intensifies during non-REM sleep. However, the research team made a significant discovery—unique in any known animal—showing that the flow diminishes substantially during REM sleep. "We postulate that the heightened influx of blood into the brain during REM sleep, which supports increased brain activity, might impede the movement of cerebrospinal fluid from the ventricles into the brain," elucidates Niels Rattenborg, leader of the Avian Sleep Group. "This suggests that REM sleep and its associated functions might potentially compromise the removal of waste products from the brain."

Nevertheless, the researchers are also considering the possibility that REM sleep may contribute to waste removal in unforeseen ways. Gianina Ungurean proposes an alternative perspective, stating, "During the onset of REM sleep, the influx of blood leads to an expansion in vessel diameter. This phenomenon could potentially facilitate the movement of cerebrospinal fluid that entered the space during non-REM sleep into the brain tissue, thereby enhancing the outflow of fluid carrying waste products."

The researchers hypothesize that the brain-cleaning process during sleep may hold particular significance for birds. Due to their higher neuronal density compared to mammals, efficient or more frequent cycles of waste removal may be required. As birds undergo more frequent and shorter REM phases during sleep compared to mammals, the accompanying surges of blood flow might aid in keeping their densely packed brains clear of detrimental waste products.

Tell us about your dreams!

In the future, the research team intends to delve deeper into investigating the potential role of REM sleep in waste removal. Furthermore, they are contemplating methods to gain insights into the content of a pigeon's dream. Gianina Ungurean elucidates, "We aim to train birds to communicate whether they perceived something and provide details upon awakening from REM sleep. This would be a crucial step towards determining if birds indeed dream." Even in the absence of an intricate analysis of dreams, the recent discoveries already enhance our comprehension of the significance of sleep, not only in birds but also in humans. They underscore the vital role of sleep in sustaining a healthy brain, preventing cognitive decline, and they also imply that the phenomenon of dreaming has an extensive evolutionary history.

Cooperation partners

The study was a collaborative effort involving multiple research teams. It was carried out by the Bochum Biopsychology team, along with researchers from the Max Planck Institute for Biological Intelligence, the Max Planck Institute for Neurobiology of Behaviour, the Neurophysiology Department at Ruhr University Bochum, and the Université Claude Bernard Lyon. Their collective expertise and contributions greatly contributed to the comprehensive exploration of avian sleep and its implications.

Funding

The project was funded by the Max Planck Society and the German Research Foundation – project no. 316803389 – SFB 1280, and AVIAN MIND, ERC-2020-ADG, LS5, GA No. 101021354.