How breathing affects pupil size

Researchers from the Karolinska Institutet in Sweden and the University of Groningen in the Netherlands have investigated the relationship between breathing and pupil size.

Writing in The Journal of Physiology, scientists describe a series of experiments that explore how pupil size changes across the breathing cycle.

The experiments included tracking pupil size and respiratory phase while at rest, completing a visual task, and undertaking controlled breathing. The researchers also explored the relationship between breathing cycle and pupil size among people born without olfactory bulbs – the part of the brain responsible for receiving information about smell.

The authors found that pupil size is smallest at inhalation onset and largest during exhalation. Pupil constriction occurred mostly during the final part of exhalation, while pupil dilatation occurred through inhalation and the early phase of exhalation.

“This pattern was consistent across all experimental conditions, demonstrating that it is robust and likely controlled by brainstem circuits,” the scientists explained.

The researchers have named this effect the pupillary respiratory-phase response ­– which is the fourth known mechanism influencing pupil size.

In terms of the size of the effect, the authors highlighted that the human pupil can vary up to 6mm from its smallest to its largest size under normal circumstances.

The authors found that there was a 0.11 to 0.23 mm change in pupil size on average across the respiratory cycle.

“This effect is too small to be readily apparent in individual breathing cycles, but it emerges when averaging pupil size over several breathing cycles,” the scientists shared.

In contrast, the pupillary light response can change the size of the pupil by several millimetres.

The size of the effect in response to respiratory phase is comparable to the pupillary psychosensory response ­­­– where pupil size changes in response to arousal or mental effort.