Breathing influences pupil size, with contraction during inhalation and dilation during exhalation. This effect, controlled by the brainstem, may enhance vision and has potential clinical applications, including neurological diagnostics.
Researchers at Karolinska Institutet in Sweden have identified a fundamental mechanism that influences pupil size: breathing. Their study, published in The Journal of Physiology, reveals that the pupil contracts during inhalation and dilates during exhalation, a discovery that could impact our understanding of vision.
Like a camera aperture, the pupil regulates how much light enters the eye, playing a crucial role in vision and perception. For over a century, scientists have recognized three primary factors that affect pupil size: light exposure, focal distance, and cognitive influences such as emotions or mental effort. Now, researchers have identified a fourth factor—breathing. Their findings show that the pupil is smallest at the start of inhalation and largest during exhalation.
“This mechanism is unique in that it is cyclical, ever-present, and requires no external stimulus,” explains Artin Arshamian, associate professor at the Department of Clinical Neuroscience, Karolinska Institutet, who led the research. “Since breathing affects brain activity and cognitive functions, the discovery may contribute to a better understanding of how our vision and attention are regulated.”
Extensive Testing Confirms the Effect
The researchers conducted five experiments with over 200 participants, examining how breathing affects pupil size under different conditions. The results showed that the effect persisted whether participants breathed quickly or slowly, through their nose or mouth, if lighting conditions or fixation distance varied, if they were resting or performing visual tasks. The difference in pupil size between inhalation and exhalation was large enough to theoretically affect vision.
The study also showed that the function is intact in people born without the olfactory bulb, a brain structure that is activated by nasal breathing. This suggests that the mechanism is controlled by the brainstem, a fundamental and evolutionarily conserved part of the brain.
The researchers are now investigating whether changes in pupil size during breathing also affect vision. Previous research shows that smaller pupils make it easier to see details, while larger pupils help us find hard-to-see 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 the same department at Karolinska Institutet and the study’s first author.
There may also be clinical applications, according to the researchers.
“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 Artin Arshamian. “This is something we want to explore in the future.”
Reference: “The pupillary respiratory-phase response: pupil size is smallest around inhalation onset and largest during exhalation” by Martin Schaefer, Sebastiaan Mathôt, Mikael Lundqvist, Johan N. Lundström and Artin Arshamian, 21 February 2025, The Journal of Physiology.
DOI: 10.1113/JP287205
The research was funded by the Swedish Research Council and the European Research Council (ERC). There are no reported conflicts of interest.
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