Differences in the pupillary responses to evening light between children and adolescents

Abstract

Background: In the mammalian retina, intrinsically-photosensitive retinal ganglion cells (ipRGC) detect light and integrate signals from rods and cones to drive multiple non-visual functions including circadian entrainment and the pupillary light response (PLR). Non-visual photoreception and consequently non-visual sensitivity to light may change across child development. The PLR represents a quick and reliable method for examining non-visual responses to light in children. The purpose of this study was to assess differences in the PLRs to blue and red stimuli, measured one hour prior to bedtime, between children and adolescents.

Methods: Forty healthy participants (8-9 years, n = 21; 15-16 years, n = 19) completed a PLR assessment 1 h before their habitual bedtime. After a 1 h dim-light adaptation period (< 1 lx), baseline pupil diameter was measured in darkness for 30 s, followed by a 10 s exposure to 3.0 × 10¹³ photons/cm²/s of either red (627 nm) or blue (459 nm) light, and a 40 s recovery in darkness to assess pupillary re-dilation. Subsequently, participants underwent 7 min of dim-light re-adaptation followed by an exposure to the other light condition. Lights were counterbalanced across participants.

Results: Across both age groups, maximum pupil constriction was significantly greater (p < 0.001, η_p² = 0.48) and more sustained (p < 0.001, η_p² = 0.41) during exposure to blue compared to red light. For adolescents, the post-illumination pupillary response (PIPR), a hallmark of melanopsin function, was larger after blue compared with red light (p = 0.02, d = 0.60). This difference was not observed in children. Across light exposures, children had larger phasic (p < 0.01, η_p² = 0.20) and maximal (p < 0.01, η_p² = 0.22) pupil constrictions compared to adolescents.

Conclusions: Blue light elicited a greater and more sustained pupillary response than red light in children and adolescents. However, the overall amplitude of the rod/cone-driven phasic response was greater in children than in adolescents. Our findings using the PLR highlight a higher sensitivity to evening light in children compared to adolescents, and continued maturation of the human non-visual photoreception/system throughout development.

Keywords: Adolescents; Children; Intrinsically photosensitive retinal ganglion cells; Light exposure; Pupillary light reflex.

Fig. 1

Pupillary assessment protocol. On the day of the pupillary assessment, participants remained indoors wearing a pair of dark-tinted glasses from their habitual wake time until the start of the dim-light adaptation at the laboratory (2 h before scheduled bedtime). Following a 1 h dim-light adaptation, pupil diameter was measured during a 30 s baseline, 10 s light exposure, and 40 s recovery. After a 7 min dim-light re-adaptation, the measurement was repeated for the remaining light condition, counterbalanced across participants.

Fig. 2

Average pupil constriction adjusted to baseline pupil diameter across the 80 s measurement. Average pupil constriction is displayed at each time point for (A) children and (B) adolescents for red and blue light separately. The shaded red and blue areas around each average line represent the standard error of the mean. The shaded yellow area denotes the timing of the 10 s light exposure

Fig. 3

PLR metrics during light onset. Results depicting (A) phasic constriction; (B) constriction latency; (C) maximum pupil constriction; and (D) slope during light exposure broken down by age group and experimental light condition. Asterisks denote statistical significance (p < 0.05)

Fig. 4

PLR metrics following light offset. Results depicting (A) area under the curve and (B) post-illumination pupil response (PIPR) broken down by age group and experimental light condition. Asterisks denote statistical significance (p < 0.05)