Respiratory fluctuations of the human pupil

We simultaneously recorded respiration and pupil size in six normal subjects with an age range of 23-40 years (mean = 30.1), to study the relationship between the respiratory rhythm and changes in pupil size. In all subjects, respiratory fluctuations of the pupil (inspiratory mydriasis and expiratory miosis) were observed during normal spontaneous respirations. The amplitude of these pupillary changes was closely proportional to the tidal volume.