The Respiratory-Pupillary Phase Effect: Pupils size is smallest around inhalation onset and largest during exhalation

Abstract

The prevailing view in the study of animal and human pupillary function has been that pupils dilate and are largest during inhalation and constrict and are smallest during exhalation. However, this notion has recently been challenged. Here, we systematically address this question by conducting a study encompassing three experiments (two resting tasks and one visual perception task), with the latter two being pre-registered. Collectively, across nasal and oral breathing, resting, and task conditions, our experiments consistently and robustly demonstrate that pupil size is smallest around inhalation onset and largest around peak exhalation. This phenomenon, which we term the Respiratory-Pupillary Phase Effect (RPPE), directly contradicts the long-held notion that pupils are largest during inhalation and smallest during exhalation. Notably, the dilation and constriction processes overlap with both phases. The observed consistency and significance of the RPPE across various conditions underscore the need for further investigation into its underlying mechanisms and potential impact on human behavior.