Rizzuto, V.; Montanari, R.; Mesin, L.; Bortolozzo-Gleich, H.; Laurino, M.; Bonneh, Y.; Yellin, D.; Laganovska, G.; Vanags, J.; Gemignani, A.

The human diving reflex (DR), an innate defensive reflex triggered during periods of apnea, concurrently activates both the sympathetic and parasympathetic branches of the autonomic nervous system to regulate physiology against challenging demands. Despite pupil dilation and constriction being antagonistically controlled by the two autonomic branches, the effect of the diving reflex on pupil diameter fluctuations is still unknown. Thus, we compared participants\' pupil diameter fluctuations while breathing or performing apnea either with (Wet) or without (Dry) face immersion in cold water. We found that pupil diameter fluctuations in both apneic conditions are associated with lower power in the low-frequency band (< 0.25 Hz) while in the Wet condition, a reallocation of power towards higher frequencies (> 0.25 Hz) occurs, together with an increased entropy, contrary to the remaining conditions. This indicates a shift in autonomic balance and an increased complexity of pupil fluctuations during the diving reflex. Our findings present pupil dynamics as a valuable entry point to the exploration of autonomic adjustments for this reflex.