Pupil size affects measures of eye position in video eye tracking: implications for recording vergence accuracy

Abstract

Video eye trackers rely on the position of the pupil centre. However, the pupil centre can shift when the pupil size changes. This pupillary artifact is investigated for binocular vergence accuracy (i.e. fixation disparity) in near vision where the pupil is smaller in the binocular test phase than in the monocular calibration. A regression between recordings of pupil size and fixation disparity allows correcting the pupillary artefact. This corrected fixation disparity appeared to be favourable with respect to reliability and validity, i. e. the correlation of fixation disparity versus heterophoria. The findings provide a quantitative estimation of the pupillary artefact on measured eye position as function of viewing distance and luminance, both for measures of monocular and binocular eye position.

Do pupil-based binocular video eye trackers reliably measure vergence?

“…We report a value of −0.40°/mm (EyeLink) and −0.72°/mm (SMI). These values resemble the quantitative conclusion of several earlier studies summarized in Jaschinski (2016).…”

“…We will not measure at a smaller binocular viewing distance. A recent study (Jaschinski, 2016) has measured in this range and shows that for smaller fixation distances, due to the accommodation-convergence reflex, the pupil size also depends on the vergence angle.…”

“…Choe et al (2016) offers a regression-based solution for errors caused by pupil-size changes over time that does not require additional calibration. Jaschinski (2016) is the first to offer a binocular regression-based correction method in order to directly correct the vergence angle for the pupil-size artefact. 2.…”

“…From the five suggested solutions presented above, two (solutions 1 and 5) could be applied in eye trackers. Manufacturers could implement correction methods as suggested by Choe et al (2016), Drewes et al (2012Drewes et al ( , 2014 and Jaschinski (2016). We are in favour of the other solution, however.…”

Pupillary response to moving stimuli of different speeds

“…Previous research has indicated pupillary responses to sensory and nonsensory stimulations, including color perception, cognitive load, and music prception,. ( 5 ; 52 ; 28 ; 17 ; 34 ; 18 ; 29 ). A pupillary response was also demonstrated during the transition from incoherent to coherent motion ( 43 ).…”