From low-level to high-level factors’ influence on pupil hippus

Pupil diameter, controlled by the autonomic nervous system (ANS), reflects numerous processes, from arousal to cognitive functions. In particular, the characteristics of the pupil at rest, its size and dynamics, reflect the tonic activity of the ANS. However, which factor can influence these pupillary characteristics at rest remains unclear. The pupil exhibits a physiological rhythmic oscillation, called hippus, which remains poorly characterised. Our aim was to better characterise the hippus by investigating how its parameters were influenced by different low (illumination) and high-level (executive functioning) conditions. Pupil size variations of thirty adults (19-35 years old) were recorded during five randomly appearing blocks. While always requiring central fixation, the blocks varied according to gradients of illumination (11-lux, 19-lux and 28-lux) and cognitive load (fixation, dot counting and mental subtraction). Three main parameters were assessed: the median pupil diameter, and the frequency and amplitude of the hippus. Increasing illumination yielded smaller pupils and reduced hippus amplitude, while hippus frequency remained stable. In contrast, distinct effects were observed for the two high-level conditions. High cognitive load produced sustained pupil dilation and heightened hippus frequency, while intermediate counting showed minimal change except a decrease in hippus amplitude. Individual baseline pupil size was predictive of the amplitude of changes induced by the parameters. Our findings suggest that resting-state pupil oscillations are modulated differentially by low and high-level influences, likely via integrative structures such as the locus coeruleus and superior colliculus, but also by biomechanical constraints.