Tag: pupil dilation

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The Pupil as a Window into the Sleeping Brain

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The eye of the sleeping subject was kept open with a special fixation device to record the pupil movements for several hours.  (Image: Neural Control of Movement Lab / ETH Zurich)

For the first time, researchers have been able to observe how the pupils react during sleep over a period of several hours. A look under the eyelids showed them that more happens in the brain during sleep than was previously assumed.

While eyes are typically closed in sleep, there is a flurry of activity taking place beneath the eyelids: a team of researchers, led by principal investigators Caroline Lustenberger, Sarah Meissner and Nicole Wenderoth from the Neural Control of Movement Lab at ETH Zurich, have observed that the size of the pupil fluctuates constantly during sleep. As they report in Nature Communications, sometimes it increases in size, sometimes it decreases; sometimes these changes occur within seconds, other times over the course of several minutes.

“These dynamics reflect the state of arousal, or the level of brain activation in regions that are responsible for sleep-wake regulation,” says Lustenberger. “These observations contradict the previous assumption that, essentially, the level of arousal during sleep is low.”

Instead, these fluctuations in pupil size show that even during sleep, the brain is constantly switching between a higher and lower level of activation. These new findings also confirm for humans what other research groups have recently discovered in studies on rodents, who also exhibit slow fluctuations in the activation level (known in the field as arousal).

New method for an old mystery

The regions of the brain which control the activation level are situated deep within the brainstem, making it previously difficult to directly measure these processes in humans during sleep. Existing methods are technically demanding and have not yet been established in this context. The ETH researchers’ study therefore relies on pupil measurements. Pupils are known to indicate the activation level when a person is awake. They can therefore be used as markers for the activity in regions situated deeper within the brain.

The ETH researchers developed a new method for examining the changes in people’s pupils while asleep: using a special adhesive technique and a transparent plaster, they were able to keep the eyes of the test subjects open for several hours.

“Our main concern was that the test subjects would be unable to sleep with their eyes open. But in a dark room, most people forget that their eyes are still open and they are able to sleep,” explains the study’s lead author, Manuel Carro Domínguez, who developed the technique.

Analysis of the data showed that pupil dynamics is related not just to the different stages of sleep, but also to specific patterns of brain activity, such as sleep spindles and pronounced deep sleep waves – brain waves that are important for memory consolidation and sleep stability. The researchers also discovered that the brain reacts to sounds with varying degrees of intensity, depending on the level of activation, which is reflected in the size of the pupil.

A central regulator of the activation level is a small region in the brainstem, known as the locus coeruleus. In animals, scientists have been able to show that this is important for the regulation of sleep stages and waking. The ETH researchers were unable to prove in this study whether the locus coeruleus is indeed directly responsible for pupil changes. “We are simply observing pupil changes that are related to the level of brain activation and heart activity,” Lustenberger explains.

In a follow-up study, the researchers will attempt to influence the activity of the locus coeruleus using medication, so that they can investigate how this affects pupil dynamics. They hope to discover whether this region of the brain is in fact responsible for controlling the pupils during sleep, and how changes in the level of activation affect sleep and its functions.

Using pupillary dynamics to diagnose illnesses

Understanding pupil dynamics during sleep could also provide important insights for the diagnosis and treatment of sleep disorders and other illnesses. The researchers therefore want to investigate whether pupil changes during sleep can provide indications of dysfunctions of the arousal system. These include disorders such as insomnia, post-traumatic stress disorder and possibly Alzheimer’s. “These are just hypotheses that we want to investigate in the future,” says Lustenberger.

Another goal is to make the technology usable outside of sleep laboratories, such as in hospitals where it could help to monitor waking in coma patients or to diagnose sleep disorders more accurately. The pupil as a window onto the brain could thus pave the way for new opportunities in sleep medicine and neuroscience.

Source: ETH Zurich

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Illusion Tricks Pupils into Dilating

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Looking at this image, do you perceive that the central black hole is expanding, as if you’re moving into a dark environment, or falling into a hole? If so, you’re not alone: a new study published in Frontiers in Human Neuroscience shows that this ‘expanding hole’ illusion, which is new to science, is perceived by approximately 86% of people.

The study’s first author, Professor Bruno Laeng at the University of Oslo, explained: “The ‘expanding hole’ is a highly dynamic illusion: The circular smear or shadow gradient of the central black hole evokes a marked impression of optic flow, as if the observer were heading forward into a hole or tunnel.”

Optical illusions aren’t simple curiosities: researchers study them to better understand the complex processes our visual system uses to anticipate and make sense of the visual world.

In the new study, Prof Laeng and colleagues demonstrated that the ‘expanding hole’ illusion deceives the brain so well that it even prompts a dilation reflex of the pupils to let in more light, just as if the observer was entering a dark area.

“Here we show based on the new ‘expanding hole’ illusion that that the pupil reacts to how we perceive light – even if this ‘light’ is imaginary like in the illusion – and not just to the amount of light energy that actually enters the eye. The illusion of the expanding hole prompts a corresponding dilation of the pupil, as it would happen if darkness really increased,” said Prof Laeng.

Prof Laeng and colleagues explored how the colour of the hole (besides black: blue, cyan, green, magenta, red, yellow, or white) and of the surrounding dots affect how strongly we mentally and physiologically react to the illusion. On a screen they presented variations of the “expanding hole” image to 50 women and men with normal vision, asking them to rate subjectively how strongly they perceived the illusion. While participants gazed at the image, the researchers measured their eye movements and their pupils’ unconscious constrictions and dilations. As controls, the participants were shown “scrambled” versions of the expanding hole image, with equal luminance and colours, but without any pattern.

The illusion appeared most effective when the hole was black. Fourteen percent of participants didn’t perceive any illusory expansion when the hole was black, while 20% didn’t if the hole was in color. Among those who did perceive an expansion, the subjective strength of the illusion differed markedly.

The researchers also found that black holes promoted strong reflex dilations of the participants’ pupils, while coloured holes prompted pupil to constriction. For black holes, but not for coloured holes, the stronger participants rated their perception of the illusion, the more their pupil diameter tended to change.

Minority not susceptible

Just why a minority seem unsusceptible to the “expanding hole” illusion is still unclear. It is also not known whether other vertebrate species, or even nonvertebrate animals with camera eyes such as octopuses, might perceive the same illusion as we do.

“Our results show that pupils’ dilation or contraction reflex is not a closed-loop mechanism, like a photocell opening a door, impervious to any other information than the actual amount of light stimulating the photoreceptor. Rather, the eye adjusts to perceived and even imagined light, not simply to physical energy. Future studies could reveal other types of physiological or bodily changes that can ‘throw light’ onto how illusions work,” concluded Prof Laeng.

Source: Frontiers