![]() But perhaps it's also a sign that brain regions activated during aroused states, are active again during REM sleep, possibly when memories are replayed." In this regard, pupil constrictions during REM sleep could help to determine when and how the sleeping brain processes memories. "The constriction could exercise the muscle's fine motor skills. "Pupil constrictions in sleeping birds probably fulfill important functions," Gianina Ungurean explains. The researchers were able to confirm this structural aspect as a possible cause for the different pupil behavior: If they used a drug to silence receptors that activate striated muscle cells, the pigeons no longer constricted their pupils during sleep. Birds, on the other hand, regulate the pupil by striated muscles, like those that move our arms or legs, for example. Mammals use smooth muscles, which cannot be controlled voluntarily. In birds and mammals, pupil size is regulated by different types of muscles in the iris. Video recording of a guira cuckoo transitioning from a period with dilated pupils associated with behavioural quiescence (presumed non-REM sleep to a period with rapid iris movements (RIMs) associated with acitve behaviours (presumed REM sleep). Again, the birds’ pupils did the opposite to mammals: they dilated during non-REM sleep, but during REM sleep, the pupils rapidly constricted over 1,000 times a night, a phenomenon the researchers called rapid iris movements. The transparent eyelids of pigeons and a special camera system allowed the scientists to record pupil sizes even with the birds’ eyes closed and in the dark. The team therefore next examined the pupil behavior of sleeping birds. Recently, it was shown in mice that pupil size also changes during sleep: In deep non-REM sleep with quiet behavior, pupils constrict in more active REM sleep, they can slowly dilate. Surprisingly, the pupils of male pigeons became smaller during courtship – in contrast to mammals, in which the pupils dilate during arousal. ![]() A team led by Gianina Ungurean from Niels Rattenborg's research group at the Max Planck Institute for Ornithology in Seewiesen now studied pupil behavior in pigeons. Yet, rapid pupil constrictions are a well-known feature of bird body language among parrot owners. In birds, pupil sizes have never been systematically studied. If we are excited or concentrated, for example, the pupils enlarge – without us being able to influence it. However, it not only regulates the incidence of light, but also reflects emotions and the state of the brain. If it is dark, the pupil becomes large and lets a lot of light through – if it is bright, it becomes small and prevents us from being blinded. The pupil literally plays a central role in vision: it is the opening in the middle of the eye that regulates how much light falls on the retina. ![]()
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