Binocular vision without visual stress

The use of changes in fixation disparity, the associated phoria, and the dissociated phoria in assessing the degree of visual stress requires that we know the effect of vision under close working conditions (40 cm) on these parameters. The authors have previously reported that reading under the stress of abnormally low illumination, or at too close a working distance (20 cm), increases fixation disparity and associated phoria and is associated with symptoms and visual discomfort. The current study shows that reading at photopic levels of illumination and at a working distance of 40 cm does not change the magnitudes of fixation disparity and associated phoria. Reading under these conditions is not accompanied by an onset of symptoms or visual discomfort. This result indicates that changes in fixation disparity and associated phoria are a product of visual stress and are not produced by reading itself.