The size of our pupils fluctuates as we breathe. They are at their smallest as we start to breathe in, increase steadily to reach their largest size when we are in the middle of breathing out, then shrink rapidly as we finish exhaling.
鈥淚 haven鈥檛 tried to see it with my naked eye,鈥� says at the Karolinska Institute in Stockholm, Sweden. 鈥淚 think it would be difficult to see.鈥�
The fact that breathing can affect pupil size has been known for decades, but all the scientific papers that Schaefer found mentioning this effect say the size is largest during inhalation and smallest when exhaling.
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, Schaefer found that they were based on studies of anaesthetised cats and don鈥檛 even hold true for conscious cats, so he decided to do a study in people.
His team used a specialised camera to measure the pupil size of 100 volunteers while monitoring their breathing with a sensor-equipped mask. When pupil size was analysed over many breath cycles, a clear pattern emerged: the size was smallest at the start of inhaling and largest in the middle of exhaling.
This relationship聽held true whether the volunteers breathed through their mouth or nose, and whether they just looked at one spot on a screen for 5 minutes or carried out a simple on-screen task.
However, the pattern often wasn鈥檛 clear over a single breath cycle. This is because pupil size is influenced by a lot of factors, which can mask the breathing effect, says Schaefer.
Our vision is sharpest when our pupils are smaller, but bigger pupils are more sensitive to faint stimuli, especially in our peripheral vision, he says. His team is now asking more volunteers to carry out various visual tasks to see if the changes in pupil size due to breathing have a measurable impact on people鈥檚 performance.
Schaefer is also investigating whether we can intentionally change the size of our pupils by controlling our breathing. If so, the findings could help people who rely heavily on vision, such as in sports like shooting.
Pupil size is also used to help diagnose some conditions, such as concussion, he says. 鈥淭herefore, just understanding what influences pupil size better is very helpful for scientists.鈥�
Schaefer thinks the breathing effect occurs because the part of the brain stem that controls inhaling and exhaling has links to the pupils. Whether or not this is an accidental result of brain evolution isn鈥檛 clear, he says. 鈥淚s this just an artefact, or is there a purpose behind it? We don鈥檛 really know yet.鈥�
bioRxiv
