Measurement of light-mediated changes in pupil size under real-world conditions

Pupil size is sensitive to a variety of higher-level processes including attentional shifts, target detection and cognitive load. At a lower level, pupil size is a non-invasive behavioural readout of visual and non-visual processing in humans, determined largely by the melanopsin-mediated stimulus on the retina.

Here, we demonstrate a method to assess the light inputs regulating pupil size under dynamic real-world conditions.

A wearable infrared video-based eye tracker (Pupil Labs GmbH) was integrated with a small-scale spectroradiometer (Ocean Insight Inc.) and attached to a bespoke 3D-printed adjustable head mount. Both devices were connected to a miniature, battery-driven control computer (Raspberry Pi), enabling simultaneous sampling of pupil size and spectral irradiance at 10-sec intervals. We measured natural variation in pupil size across two protocols, in which healthy, young participants (n=7; age: 20-30 years) moved in and between indoor and outdoor environments varying in light conditions and engaged in a range of everyday tasks.

We accurately predict variation in pupil size as a function of near-corneal melanopic irradiance in the real world, yielding distinct dose-response curves for each participant. Under these uncontrolled conditions, data retention was reasonably high (~65% data retained). In summary, we demonstrate a robust paradigm for ambulatory research on pupil variation in humans outside of the laboratory. Controlling for the major effect of light, future studies can build on this approach to study cognitive factors affecting pupil size in the real world.