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 characterized. Our aim was to better characterize the hippus by investigating how its parameters were influenced by different low- (illumination) and high-level (executive functioning) conditions. Pupil size variations of 30 adults (19-35 yr old) were recorded during five randomly appearing blocks. Although always requiring central fixation, the blocks varied according to gradients of illumination (11-lx, 19-lx, and 28-lx) and cognitive load (fixation, dot counting, and mental subtraction). Three main parameters were assessed: the median pupil diameter, 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.<b>NEW & NOTEWORTHY</b> We demonstrate, for the first time in a single within-subject protocol, how low-level (illumination) and high-level (cognitive load) factors differentially shape both baseline pupil size and the oscillatory hippus. Bright light drives tonic constriction and reduces hippus amplitude probably via iris biomechanics, while mental effort elicits dilation and increases hippus frequency through central arousal circuits. Individual resting-pupil profiles predict the amplitude of these responses.