New research reveals a fundamental link between breathing and vision

For more than a century, scientists have understood that three factors control pupil size: light, focus distance, and cognitive processes like emotions or mental effort. Now, researchers at Karolinska Institutet in Sweden have identified a fourth: breathing.

This discovery, published in The Journal of Physiology, shows that pupils are smallest during inhalation and largest during exhalation, revealing a fundamental mechanism that could influence vision.

Much like a camera’s aperture, the pupil controls the amount of light that enters the eye. Changes in its size can affect how the world appears, sharpening details or enhancing the ability to detect faint objects. The newly discovered breathing-linked pupil fluctuation suggests that vision might subtly shift throughout each breath, alternating between clarity and sensitivity.

“This mechanism is unique in that it is cyclical, ever-present, and requires no external stimulus,” says Artin Arshamian, an associate professor at the Department of Clinical Neuroscience at Karolinska Institutet. “Since breathing affects brain activity and cognitive functions, the discovery may contribute to a better understanding of how our vision and attention are regulated.”

The research builds on growing evidence that respiration does more than just oxygenate the body. It generates neural oscillations—rhythmic electrical activity in the brain—that influence cognition and perception.

These oscillations stem from the preBötzinger complex, a cluster of neurons in the brainstem that regulates breathing. They also arise from the olfactory bulb when air moves through the nose.

Past studies in animals have shown that these respiration-linked oscillations help synchronize sensory and motor behaviors like sniffing, whisking, and head movement. Similarly, pupil size plays a vital role in vision, influencing how the brain processes visual information.

However, direct evidence linking respiration to pupil changes remained elusive, with past research yielding inconsistent results due to small sample sizes and methodological flaws.

To clarify this connection, researchers at Karolinska Institutet conducted five carefully controlled experiments with over 200 participants. Their findings confirmed that breathing consistently influenced pupil size across various conditions, whether participants were breathing quickly or slowly, through the nose or mouth, in bright or dim lighting, at rest, or engaged in visual tasks.

One of the most striking results was that the pupil fluctuations occurred even in individuals born without an olfactory bulb, a brain structure typically associated with nasal breathing. This suggests that the brainstem alone may regulate the effect, highlighting its deep evolutionary roots.

The study raises intriguing possibilities about how vision shifts in response to breathing. Pupil size directly affects how light enters the eye, influencing visual acuity. A smaller pupil, like a camera lens set to a narrow aperture, enhances sharpness and depth of field, making it easier to see fine details. A larger pupil, by contrast, allows more light to enter, improving sensitivity to dim or distant objects.

“Our results suggest that our vision may switch between optimizing for distinguishing small details when we inhale and detecting faint objects when we exhale, all within a single breathing cycle,” says Martin Schaefer, a postdoctoral researcher at Karolinska Institutet and the study’s first author.

If this effect is strong enough to impact perception, it could have significant implications for daily activities. Tasks requiring high visual precision, such as reading or threading a needle, might be easier when inhaling. Meanwhile, scanning for movement in low light, such as spotting a shadow at night, might be enhanced during exhalation.

Beyond its effects on vision, the link between respiration and pupil size may have medical applications. The autonomic nervous system, which controls involuntary bodily functions, regulates both breathing and pupil response. Abnormalities in this system can signal neurological disorders.

“One potential application is new methods to diagnose or treat neurological conditions such as Parkinson's disease, where damage to pupil function is an early sign of the disease,” says Arshamian. “This is something we want to explore in the future.”

Pupil measurements are already used in clinical settings to assess brain function, detect brain injuries, and monitor states of consciousness. If respiration-induced pupil changes serve as an additional diagnostic marker, they could offer a new, non-invasive way to detect early signs of neurological disease.

Future research will determine how this discovery might be applied in medical diagnostics and whether breathing techniques could be used to modulate attention and perception.

While much remains to be explored, this study offers a new perspective on how breath, brain, and vision are interconnected.

Note: Materials provided above by The Brighter Side of News. Content may be edited for style and length.

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