Pupil size influences the eye-tracker signal during saccades

•The gaze signal, but not the CR signal, is influenced by pupil size during saccades.•Smaller pupil sizes are associated with lower peak velocities in the gaze signal.•Pupil size should be controlled for when investigating saccade dynamics.

While it is known that scleral search coils-measuring the rotation of the eye globe-and modern, video based eye trackers-tracking the center of the pupil and the corneal reflection (CR)-produce signals with different properties, the mechanisms behind the differences are less investigated. We measure how the size of the pupil affects the eye-tracker signal recorded during saccades with a common pupil-CR eye-tracker. Eye movements were collected from four healthy participants and one person with an aphakic eye while performing self-paced, horizontal saccades at different levels of screen luminance and hence pupil size. Results show that pupil-, and gaze-signals, but not the CR-signal, are affected by the size of the pupil; changes in saccade peak velocities in the gaze signal of more than 30% were found. It is important to be aware of this pupil size dependent change when comparing fine grained oculomotor behavior across participants and conditions.