Dark vergence in relation to fixation disparity at different luminance and blur levels.

Fixation disparity was measured psychophysically when subjects viewed a central fusion stimulus at 50 cm distance. Reducing luminance and increasing target blur resulted in an exophoric shift of fixation disparity. Fixation disparity was significantly more exophoric in subjects with distant dark vergence, i.e. the resting position of vergence. This was true for the bright and sharp target, as well as for targets impaired by blurring or dimming. These results are interpreted as a bias of fixation disparity towards the individual resting position of vergence not only for degraded stimuli but also for strong central fusion stimuli.