![]() ![]() Stimulation of the autonomic nervous system's sympathetic branch, known for triggering "fight or flight" responses when the body is under stress, induces pupil dilation. The iris is made of two types of muscle: a ring of sphincter muscles that encircle and constrict the pupil down to a couple of millimeters across to prevent too much light from entering and a set of dilator muscles laid out like bicycle spokes that can expand the pupil up to eight millimeters-approximately the diameter of a chickpea-in low light. Specifically, it dictates the movement of the iris to regulate the amount of light that enters the eye, similar to a camera aperture. But a different, older part of the nervous system-the autonomic-manages the continuous tuning of pupil size (along with other involuntary functions such as heart rate and perspiration). The visual cortex in the back of the brain assembles the actual images we see. ![]() He views the dilations as a by-product of the nervous system processing important information. "Nobody really knows for sure what these changes do," says Stuart Steinhauer, director of the Biometrics Research Lab at the University of Pittsburgh School of Medicine. And they do this without knowing exactly why our eyes behave this way. In fact, pupil dilation correlates with arousal so consistently that researchers use pupil size, or pupillometry, to investigate a wide range of psychological phenomena. They also betray mental and emotional commotion. The fact that birds regulate their pupil size by striated muscles, and that these are usually controlled voluntarily, opens up an exciting question: Can birds change the size of their pupils intentionally? Future studies will now investigate this question and the pupils’ role in communication between birds.What do an orgasm, a multiplication problem and a photo of a dead body have in common? Each induces a slight, irrepressible expansion of the pupils in our eyes.įor more than a century scientists have known that our eyes' pupils respond to more than changes in light. 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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