Our eyes help us take in the world around us, and they can reveal much about our inner worlds, from what we might be imagining to early signs of hearing loss.
Mysteriously, our eyes can also change with our breath. In a new study, researchers in Sweden and the Netherlands found pupil sizes fluctuate with every inhale and exhale.
For over 100 years, we've understood that pupils respond to much more than light, but studies on whether these depthless black holes expand and contract with our breath have been contradictory.
Some research suggests pupils dilate as we breathe in, while a review in 2022 found there was "inconclusive evidence" for this phenomenon.
So neuroscientist Martin Schaefer from the Karolinska Institute in Sweden and colleagues set out to address limitations of the previous studies. In a series of experiments, the team used a special camera to measure pupil sizes of more than 100 volunteers while they were at rest and performing a visual task.
"Pupil size consistently reaches its minimum around inhalation onset and its maximum during exhalation," the researchers write in their report, which is currently awaiting peer review.
The same pattern was present when volunteers performed a simple visual task or merely stared at one spot, when they breathed through their nose, or mouth.
This is in contrast to previous publications claiming pupils are the smallest when exhaling. Pupil size has a number of influences though, so it took Schaefer and team several breath cycles to pick up on their subtle adherence to our breath cycle. This may explain some of the past contradictory results.
"Is this just an artifact?" Schaefer said to Michael Le Page at New Scientist. "Or is there a purpose behind it? We don't really know yet."
But the researchers do have some theories about these dancing pupil sizes. Previous studies suggest smaller pupils are better at distinguishing between details, while larger pupils are better at detecting faint stimuli.
"Our findings hint at the possibility that visual perception itself might cycle between optimizing for discrimination during inhalation and detection during exhalation within a single breath," write Schaefer and team.
Pupil sizes also change according to emotional states (like widening in fear), physical arousal, and in response to drugs (such as enlarging in response to antidepressants).
Doctors can use our pupils to gauge levels of consciousness and detect mental health conditions, so understanding more about their behavior would make them more powerful diagnostic indicators.
Scientists have recently identified the mechanism behind at least some of these changes, but just as with pupil-size adherence to our breathing cycle, why many of these changes occur remains a mystery.
This research has been uploaded to bioRxiv and is awaiting peer review.