Gaze-angle dependency of pupil-size measurements in head-mounted eye tracking

Abstract

Pupillometry - the study of temporal changes in pupil diameter as a function of external light stimuli or cognitive processing - requires the accurate and gaze-angle independent measurement of pupil dilation. Expected response amplitudes often are only a few percent relative to a pre-stimulus baseline, thus demanding for sub-millimeter accuracy. Video-based approaches to pupil-size measurement aim at inferring pupil dilation from eye images alone. Eyeball rotation in relation to the recording camera as well as optical effects due to refraction at corneal interfaces can, however, induce so-called pupil foreshortening errors (PFE), i.e. systematic gaze-angle dependent changes of apparent pupil size that are on a par with typical response amplitudes. While PFE and options for its correction have been discussed for remote eye trackers, for head-mounted eye trackers such an assessment is still lacking. In this work, we therefore gauge the extent of PFE in three measurement techniques, all based on eye images recorded with a single near-eye camera. We present both real world experimental data as well as results obtained on synthetically generated eye images. We discuss PFE effects at three different levels of data aggregation: the sample, subject, and population level. In particular, we show that a recently proposed refraction-aware approach employing a mathematical 3D eye model is successful in providing pupil-size measurements which are gaze-angle independent at the population level.

Neurophysiological measures and correlates of cognitive load in attention‐deficit/hyperactivity disorder (ADHD), autism spectrum disorder (ASD) and dyslexia: A scoping review and research recommendations

“…In addition, the gaze position can distort the pupil's apparent size, as rotations of the eyes change the angle at which the camera records the pupil, which is known as the ‘pupil foreshortening error’ (PFE) (Gagl et al, 2011; Hayes & Petrov, 2016). There have been some attempts at building eye models to correct this error, which can be larger than many cognitive pupillometric effects, but these models have only been tested on limited types of tasks (Brisson et al, 2013; Hayes & Petrov, 2016; Petersch & Dierkes, 2022). For these reasons, eye‐tracking manufacturers often recommend only measuring pupil dilation during tasks with a constant luminance and a constant fixation location (Hayes & Petrov, 2016).…”

Methods in cognitive pupillometry: Design, preprocessing, and statistical analysis

“…This eye-positionbased distortion of pupil size is often called the pupil-foreshortening error (PFE), and directly results from the fact that the surface of the pupil appears smaller in the camera image when it is recorded from the side, and thus looks like an ellipse, as compared to when it is recorded from the front, and thus looks like a circle. Some eye trackers are more susceptible to the PFE than others (Petersch & Dierkes, 2021), and there are ways to compensate for the PFE algorithmically during data analysis (Brisson et al, 2013;Gagl et al, 2011;Hayes & Petrov, 2016); however, although the PFE can certainly be minimized, you can never assume that pupil-size recordings are completely unaffected by the angle from which the eye tracker records the pupil.…”

Methods in Cognitive Pupillometry: Design, Preprocessing, and Statistical Analysis

“…This eye-position-based distortion of pupil size is often called the pupil-foreshortening error (PFE), and directly results from the fact that the surface of the pupil appears smaller in the camera image when it is recorded from the side, and thus looks like an ellipse, as compared to when it is recorded from the front, and thus looks like a circle. Some eye trackers are more susceptible to the PFE than others (Petersch & Dierkes, 2021), and there are ways to compensate for the PFE algorithmically during data analysis (Brisson et al, 2013; Gagl et al, 2011; Hayes & Petrov, 2016); however, although the PFE can certainly be minimized, you can never assume that pupil-size recordings are completely unaffected by the angle from which the eye tracker records the pupil.…”