Pigeons, when asleep, may experience visions of flight. This is shown in a study by researchers at Ruhr University Bochum, Germany, and the Max Planck Institute for Biological Intelligence, published in the journal Nature Communication. The scholars studied brain activation patterns in sleeping pigeons using functional magnetic resonance imaging. The study revealed that, as in mammals, most of the brain is highly active during Rapid Eye Movement (REM) sleep, which is the time of deepest sleep when the eyes move very rapidly under the eyelids and is the only circumstance in which dreams occur, which are possible precisely because of the brain’s high activity. However, this waking state may come at the cost of reduced waste removal from the brain.

During non-REM sleep, the brain is metabolically less active and eliminates waste products by flushing cerebral spinal fluid through the cerebral ventricles-the interconnected chambers surrounding brain structures-and then through the brain.

The question of whether similar processes occur in birds has remained unresolved until now. “The last common evolutionary ancestor of birds and mammals dates back about 315 million years, to the dawn of terrestrial vertebrates,” explained Professor Onur Güntürkün, head of the Department of Biopsychology at Ruhr University Bochum. “Yet the sleep patterns of birds are remarkably similar to those of mammals, including REM and non-REM phases,” Güntürkün continued.

To find out exactly what happens when birds sleep, the researchers used infrared video cameras and functional magnetic resonance imaging (fMRI) to observe and record the sleep and wake states of 15 pigeons specially trained to sleep under these experimental conditions.

The video recordings shed light on the sleep stages in the birds. “We were able to observe whether one or both eyes were open or closed and to follow eye movements and changes in pupil size through the transparent eyelids of the pigeons during sleep,” said Mehdi Behroozi, a postdoctoral research fellow at the Department of Biopsychology at Ruhr University Bochum. At the same time, functional magnetic resonance imaging (fMRI) recordings provided information on brain activation and cerebrospinal fluid flow in the ventricles.

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