Is accommodation a confounder in pupillometry research?

L Kooijman,D Dodou,S.T Jansen,T.S Themans,J.N.M Russell,S.M Petermeijer,J.R.C Doorman,J.H Hablé,D.S Neubert,M.J.C Vos,J.C.F De Winter

doi:10.1016/j.biopsycho.2021.108046

L Kooijman, D Dodou + Show 9 more

Open Access

https://doi.org/10.1016/j.biopsycho.2021.108046

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Abstract

Much psychological research uses pupil diameter measurements to investigate the cognitive and emotional effects of visual stimuli. A potential problem is that accommodating at a nearby point causes the pupil to constrict. This study examined to what extent accommodation is a confounder in pupillometry research. Participants solved multiplication problems at different distances (Experiment 1) and looked at line drawings with different monocular depth cues (Experiment 2) while their pupil diameter, refraction, and vergence angle were recorded using a photorefractor. Experiment 1 showed that the pupils dilated while performing the multiplications, for all presentation distances. Pupillary constriction due to accommodation was not strong enough to override pupil dilation due to cognitive load. Experiment 2 showed that monocular depth cues caused a small shift in refraction in the expected direction. We conclude that, for the young student sample we used, pupil diameter measurements are not substantially affected by accommodation.

Highlights

Pupillometry is an important research method in psychology
The results showed that accommodation and vergence changed as a function of the real presentation distance, but not for the corresponding photographs that were displayed at a constant distance, indicating that accommodation is affected by real depth, not apparent depth
Pairwise comparisons showed that these three pupil dilation measures increased significantly with multiplication difficulty for the halfway presentation distance only

Summary

Introduction

Pupillometry is an important research method in psychology. It became popular in the 1960s (e.g., Hess & Polt, 1960) and is currently widely used. It is well known that pupil diameter is affected by the amount of light falling onto the retina. It is imperative that light conditions are controlled in pupillometry research. Another potential confounder of pupil response is accommodation: as part of the pupillary near reflex, the pupil constricts when focusing on a nearby object (Alpern, Mason, & Jardinico, 1961; Marg & Morgan, 1950)

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