03.03.2025.
Researchers at Karolinska Institutet in Sweden have uncovered a fundamental mechanism that influences pupil size—our breathing. Their study, published in The Journal of Physiology, demonstrates that the pupil is smallest during inhalation and largest during exhalation, a finding that may have implications for vision and cognitive function.
The role of the pupil in vision
Like the aperture in a camera, the pupil regulates how much light enters the eye, playing a crucial role in visual perception. For over a century, scientists have known three main factors that affect pupil size:
- Light exposure – pupils constrict in bright light and dilate in darkness.
- Focus distance – pupils adjust based on the distance of objects being viewed.
- Cognitive factors – emotions and mental effort can cause pupil dilation.
Now, researchers have identified a fourth factor: breathing.
How breathing affects pupil size
The study found that pupil size changes in sync with the respiratory cycle. Specifically, the pupil is smallest around the onset 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.”
Experimenting with pupil responses
To investigate this phenomenon, the researchers conducted five experiments with over 200 participants. The results were consistent across various conditions:
- Participants breathed quickly or slowly, through their nose or mouth.
- Different lighting conditions and fixation distances were tested.
- Pupil responses were observed during rest and during visual tasks.
The findings showed that the difference in pupil size between inhalation and exhalation was significant enough to potentially influence vision.
Investigating the impact on vision
The researchers are now exploring whether these changes in pupil size affect how we see. Prior studies indicate that smaller pupils enhance the ability to see fine details, whereas larger pupils improve the detection of faint 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.
Potential clinical applications
The discovery may also have clinical implications. Damage to pupil function is an early sign of neurological conditions such as Parkinson’s disease.
“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.”
