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To be able to observe and quantify this phenomenon, Wyatt ( 1995) built a setup consisting of a camera, a half-silvered mirror, and a photographic slit lamp to produce bright retinal reflections (the red eye effect). Wyatt ( 1995) predicted that if during fixation the pupil changes size and shape (relative to the size and the shape during calibration), the eye tracker will report a deviation from the computed gaze direction even if the eye did not actually rotate.
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Depending on the pupil-shape changes, the center of the pupil may shift. Wyatt ( 1995) investigated the form of the human pupil and showed that when the pupil changes size, the shape of the pupil also changes. Research showed that this assumption can be violated (Walsh, 1988). One hidden assumption underlying the pupil-minus-CR technique is that the pupil center location is fixed with respect to the eyeball. Mulligan, 1997) or by an ellipse fit (e.g., Kristek, 1965). The pupil and CR centers can be estimated by an algorithm calculating the center of mass (e.g. In the pupil-minus-CR eye tracker (Merchant et al., 1974), the pupil center and the corneal reflection (CR) are used to estimate gaze (see Morimoto et al., 2000). Nowadays, the majority of eye trackers are non-invasive and use high-speed cameras to film the eye(s) illuminated with infrared light. Based on a simulation, we cannot claim that the viewing direction-dependent component of the PSA is caused by the optics of the cornea.Įye tracking is used in many branches of psychology, cognitive science, and medicine to investigate e.g., attention, perception, development of the oculomotor system, usability and to diagnose diseases associated with neurological disorders. (Vision Research 118(6755):48–59, 2016), that the PSA can be described by an idiosyncratic and a viewing direction-dependent component. These findings corroborate the suggestion by Choe et al. The PSA was non-zero for viewing direction 0 ∘ and depended on the viewing direction. We then investigated the PSA as a function of the participants’ viewing direction. When comparing the magnitude of the PSA between eye trackers, we found the magnitude of the obtained PSA to be related to the direction of the eye-tracker-camera axis, suggesting that the angle between the participants’ viewing direction and the camera axis affects the PSA. In the present study, we ask three questions: 1) how stable is the PSA over time, 2) does the PSA depend on properties of the eye tracker set up, and 3) does the PSA depend on the participants’ viewing direction? We found that the PSA is very stable over time for periods as long as 1 year, but may differ between participants. The pupil size artefact (PSA) is the gaze deviation reported by an eye tracker during pupil size changes if the eye does not rotate.