Automatic Pupillary Light Reflex Detection in Eyewear Computing

There are many benefits to facilitating “always-on” pupillary light reflex (PLR)-aware pupil size measurement in eyewear, including improving the reliability of pupil-based cognitive and affective load monitoring and enabling PLR-based diagnosis of cognitive and eye-related diseases which have neurological symptoms manifested in the form of aberrant PLR responses. However, the detection of PLR responses for application in eyewear devices for everyday usage, beyond PLR measurement in confined clinical sessions, has not been investigated. To this end, a means of characterizing PLRs in less controlled environmental settings is investigated and subsequently a method of PLR detection is developed and evaluated. A low-cost head-mounted Web camera was used to record near-field eye video sequences which were processed with the self-tuning threshold algorithm for pupil diameter estimation and blink detection. PLR was induced by luminance change of a monitor and brightness change of the displayed image on a monitor. A transient model-based PLR detection algorithm which utilizes the general correlation of PLR amplitude and velocity was developed and evaluated on the data sets in terms of false alarm and false rejection rates. The findings from this research suggest that the PLR can be detected reliably using low-cost wearable pupil-measurement systems without using a separate sensor for detecting the luminance conditions. The correlation between pupil diameter amplitude and maximum velocity of PLR was shown to be sufficiently consistent for PLR detection.

I. Introduction

The pupillary light reflex (PLR) is a well-known mechanism of the eye’s light adaptation function and has been the subject of various optometric, ophthalmologic, and neuroscientific studies. It comprises constriction and dilation of the pupil to the full range of diameter sizes, which is typically 1.5–8 mm [1]. The dynamics of PLR have previously been modeled using both biological theory and empirical data [2]–[4] as seen in Fig. 1, PLR constriction and dilation are characterized by rapid decrease and increase in pupil size, respectively, followed by stabilization to a certain pupil size. This behavior allows the eye to quickly adjust the amount of retinal illumination necessary to restore optimal visual acuity [5]. Fig. 1.

Example trace of pupil diameter pd (solid blue) and pupil diameter velocity (dotted red) showing endpoints of constriction of PLR constriction ( to ) and dilation ( to ) marked in green (below). Selected video frames from low-cost eyewear with output pupil ellipses detected and fitted using the self-tuning algorithm (above). A property of the transient behavior of PLR constriction and dilations is a smooth increase and decrease to and from a point of maximum absolute pupil diameter velocity (V_maxcon and V_maxdil, respectively).