Evidence That Pupil Size and Reactivity Are Determined More by Your Parents Than by Your Environment.

Abdus Samad Ansari,Fion D Bremner,Jelle Vehof,Christopher J Hammond,Katie M Williams

doi:10.3389/fneur.2021.651755

Abstract

Purpose: A classic twin study to evaluate the relative contributions of genetic and environmental factors to resting pupil size and reactivity.Methods: Pupillometry was performed on 326 female twins (mean age 64 years) from the TwinsUK Adult Twin Registry, assessing resting pupil diameter in darkness and increasing levels of ambient light, alongside dynamic pupillary characteristics. Maximum-likelihood structural equation models estimated the proportion of trait variance attributable to genetic factors.Results: Mean (SD) pupil diameter in darkness was 5.29 mm (0.81), decreasing to 3.24 mm (0.57) in bright light. Pupil light reaction (PLR) had a mean (SD) amplitude of 1.38 mm (0.27) and latency of 250.34 milliseconds (28.58). Pupil size and PLR were not associated with iris colour, intraocular pressure or refractive error, but were associated with age (diameter β = −0.02, p = 0.016, constriction amplitude β = −0.01, p < 0.001, velocity β = 0.03, p < 0.001, and latency β = 0.98, p < 0.001). In darkness the resting pupil size showed a MZ intraclass correlation coefficient of 0.85, almost double that of DZ (0.44), suggesting strong additive genetic effects, with the most parsimonious model estimating a heritability of 86% [95% confidence interval (CI) 79–90%] with 14% (95% CI 10–21%) explained by unique environmental factors. PLR amplitude, latency and constriction velocity had estimated heritabilities of 69% (95% CI 54–79%), 40% (95% CI 21–56%), and 64% (95% CI 48–75%), respectively.Conclusion: Genetic effects are key determinants of resting pupil size and reactivity. Future studies to identify these genetic factors could improve our understanding of variation in pupil size and pupillary reactions in health and disease.

Highlights

Pupil size is important because it controls the amount of light that is able to reach the retina and because it affects the levels of chromatic and spherical aberration, thereby optimising visual perception [1]
In any individual person pupil size is influenced by a number of factors including ambient light, retinal sensitivity, central cognitive processes [2, 3], alertness and emotional unrest [4, 5] with pupil measurements being used by researchers as a biomarker of activity in the autonomic nervous systemboth in health and in disease
Many studies have evaluated this variation in pupil size in different illumination levels, suggesting pupil size decreases in a linear manner for each luminance level [6]

Summary

Introduction

Pupil size is important because it controls the amount of light that is able to reach the retina and because it affects the levels of chromatic and spherical aberration, thereby optimising visual perception [1]. In the healthy population there is significant variation in the resting size of the pupil and in the reflex constriction of the pupil to light. At a population level it has been shown that pupils tend to be smaller with increasing age [7]. This inverse correlation between age and resting pupil size has been confirmed preoperatively in refractive surgery candidates across a range of different luminance levels [8, 9], but the effect seems to be most marked at low luminance suggesting that it is due to a progressive decline in sympathetic tone throughout adult life [7, 10]

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