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Even During Deep Sleep, Mouse Pupils Filter the Outside World

(Credit: Shutterstock)

The eye may not be the window to the soul in the conventional sense, but it is a window into the intricate workings of the mind.

The pupil of the eye fluctuates and varies a lot in humans and many mammals. If tracked during the day, the pupil will not only respond to changes in external stimuli such as light, but also to internal conditions such as attention and emotional states. It is a signifier of what goes on in a person’s head and is linked to brain activity. Does this revelatory behavior continue even when we are no longer awake? Perhaps.

An eye-opening revelation

A simple experiment meant to train mice to sleep for a study took a turn when researchers found that many of their mice slept with their eyes open. Daniel Huber, along with his colleagues and students at the University of Geneva, paired this discovery with a new technique of photographing the pupil to see if there was a connection between pupil activity and the sleep cycle.

They found that pupil size fluctuated wildly, but predictably, during sleep.

“You can measure sleep by measuring the oscillation in the pupil,” says Huber.

There are two types of sleep: Rapid eye movement (REM) sleep and Non-rapid eye movement (NREM) sleep. Brain activity during REM sleep is similar to the wake state; this is when dreams occur. The NREM sleep is further broken up into four stages, with stage four, also known as slow-wave sleep, being the deepest stage of sleep. Throughout the night, the brain cycles between REM and various stages of NREM sleep.

During NREM sleep, researchers found, the pupil appears smaller during deeper sleep stages and widens during lighter sleep. It also moves left to right during periods of REM sleep.

Pupil size variation was due to active constriction by the parasympathetic pathway of the nervous system, which regulates homeostasis and the body at rest. Actively constricting the pupil requires a lot of energy, according to Huber, and they experimentally deduced that its utility is to prevent light-induced wake up.

“The pupil has a protective function to keep the mice sleeping during very deep sleep,” says Huber. “This might be important because we think that these periods of very deep sleep are somehow related to memory consolidations. If we wake up easily during these periods, our memories might also take a hit.”

Huber said it was interesting to see the pupil continue working and playing an active role even during sleep.

“It’s one of the first times we’ve seen the brain, by its activity, gate sensory information out at the periphery,” continued Huber.

From this experiment, the researchers also found that other bodily rhythms, such as the heartbeat, can be predicted from pupil size in sleep. There are also questions about whether other sensory are affected during sleep.

Looking forward, Huber said that there are many avenues to explore. But right now, most of it is speculation. The system could be complimentary in humans, but only testing will yield a definitive answer. Even then, outcomes are hard to predict due to the questions of evolving technology and best methodology.

Even so, while human sleep systems may be more complex than a mouse’s, there’s no denying certain unmistakable similarities. Huber even thinks that mice might dream.

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A Functioning Fake Womb

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Chemicals in Non-Stick Pans May Contribute to Weight Gain

Uhoh: Those non-stick pans you love cooking with are often made with a chemical that could contribute to weight gain. (Credit: Shutterstock)

More than 38 percent of American adults and 17 percent of American children are obese. And while there are numerous ways to shed pounds, it’s often difficult for many people to keep them off. It turns out some common items regularly used by people across the world could be the culprit.

A study released Tuesday in PLOS Medicine suggests that perfluoroalkyl substances (PFASs) could be contributing to weight gain and lead to obesity. Since the 1950s, these environmental chemicals have been used in food packaging, non-stick cookware, stain resistant fabrics and carpets, water-repellent clothing, and even some cosmetics. These manmade compounds’ effects on humans aren’t well known, but past studies on animals have shown they may disrupt the endocrine system, or the collection of glands that produce hormones. PFASs have also been linked to cancer, immune issues and high cholesterol.

Down and Up

Over the course of two years, researchers put 621 obese and overweight men and women on energy-restricted diets and tracked their weights. Measuring the plasma concentrations of PFASs, they were able to gather metabolic information including body weight and resting metabolic rate (RMR).

Researchers found that those with higher levels of PFASs at the beginning of the experiment were associated with regaining the pounds they lost, especially in women. Participants lost on average 14 pounds (6.4 kg) in the first six months, regaining almost half of the weight throughout the study. The weight gain could be due to a decline of RMR over the first six months.

“These chemicals may lead to more rapid weight gain after dieting,” Qi Sun, co-author of the study, told the Guardian. “It is very hard to avoid exposure to PFASs, but we should try to. It’s an increasing public health issue.”

With that said, the authors can’t definitively link PFAS chemicals to the weight regain. Some potentially important influences weren’t measured including socioeconomic and psychosocial factors and potential relapses to prior diets weren’t considered. Still, the authors hope this study will lead to further research of environmental chemicals and their possible impact on obesity.

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