Systematic shifts of fixation disparity accompanying brightness changes

Video-based gaze tracking is prone to brightness changes due to their effects on pupil size. Monocular observations indeed confirm variable fixation locations depending on brightness. In close viewing, pupil size is coupled with accommodation and vergence, the so-called near triad. Hence, systematic changes in fixation disparity might be expected to co-occur with varying pupil size. In the current experiment, fixation disparity was assessed. Calibration was conducted either on dark or on bright background, and text had to be read on both backgrounds, on a self-illuminating screen and on paper. When calibration background matches background during reading, mean fixation disparity did not differ from zero. In the non-calibrated conditions, however, a brighter stimulus went along with a dominance of crossed fixations and vice versa. The data demonstrate that systematic changes in fixation disparity occur as effect of brightness changes advising for careful setting calibration parameters.