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.