Top-down and bottom-up influences on pupil hippus

Abstract

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. Pupil has been shown to have a physiological, rhythmic oscillatory activity called hippus which is naturally occurring in children and adults and is still poorly characterised. Our aim is to better characterise the hippus by investigating how its parameters are influenced by different bottom-up (illumination) and top-down (cognitive load) conditions. Thirty adults were included in the study (19-35 years old). In order to disentangle the influence of illumination and cognitive load, we recorded pupil size variations during 5 randomly appearing blocks. For each block, the participant had to fixate a central cross. 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. We observed clear differences between the conditions. For bottom-up variations of illumination, pupil size and hippus amplitude decreased when illumination increased. For top-down conditions, a high cognitive load led to a greater dilation of the pupil and a greater hippus frequency. Overall, top-down and bottom-up processing influenced pupillary parameters differently, probably through shared integrative structures like the locus coeruleus and superior colliculus.