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.
Highlights
Video eye trackers measure eye movements based on the position of the pupil: the pixel position of the centre of the pupil is converted to the angular rotation of the eye based on the calibration of the eye tracker (Yu, Lin, Tang, Xu, Schmidt, Wang, & Guo, 2015)
Concerning the pupil centre shift, it has been reported earlier that a reduction in pupil size typically shifts the centre of the pupil nasally (Walsh, 1988; Wilson, Campbell, & Simonet, 1992; Wyatt, 1995; Yang, Thompson, & Burns, 2002; Camellin, Gambino, & Casaro, 2005; Park, Kim, & Joo, 2009; Tabernero, Atchison, & Markwell, 2009; Wyatt, 2010; Ivanov & Blanche, 2011; Wyatt, 2012; Wildenmann & Schaeffel, 2013; Drewes, Zhu, Hu, & Hu, 2014; Fischinger, Seiler, Schmidinger, & Seiler, 2015; Choe, Blake, & Lee, 2016)
These earlier reports provide the average estimation that a 1 mm change in pupil size induces a horizontal artefact of a 14 min arc change in measured position of a single eye or a 28 min arc change in binocular vergence
Summary
Video eye trackers measure eye movements based on the position of the pupil: the pixel position of the centre of the pupil is converted to the angular rotation of the eye based on the calibration of the eye tracker (Yu, Lin, Tang, Xu, Schmidt, Wang, & Guo, 2015). Even if the eye does not rotate, the pupil center may shift horizontally and vertically as a function of pupil size for physiological reasons. If the pupil contracts (or dilates), the pupil centre shifts nasally (or temporally) in most observers
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