Effect of reading in low illumination on fixation disparity.

Fixation disparity has been shown to be produced by prism stress, and it has therefore been proposed that its presence constitutes a sign that binocular vision is under stress (decompensated heterophoria). Fixation disparity can be present in the absence of any other indication of stress, and therefore may have a physiological basis. This study investigates the effect on fixation disparity and visual discomfort of placing the visual system under a different type of stress, that of requiring the subject to read under conditions of inadequate illumination. In 50 young adults, it was found that fixation disparity increased under inadequate illumination and that this was accompanied by symptoms in the form of visual discomfort. After a period of rest from any near-vision task, the fixation disparity decreased and the symptoms were reduced.