Resting positions of vergence and their relation to asthenopic complaints

Dark vergence is a resting position of vergence, measured in a dark visual field to eliminate fusional, accommodative, and proximal stimuli. Dark vergence differs among subjects: the average subject converges to a viewing distance of about 1 m, while the inter-individual range is from infinity to about 40 cm. This paper reviews previous ergonomic studies on dark vergence in relation to asthenopic complaints that occur when working at near computer screens: subjects with distant dark vergence exert more convergence and experience stronger asthenopic complaints than subjects with near dark vergence. The dissociated phoria, a common clinical measure of resting vergence, is only weakly correlated with asthenopic complaints, probably because of accommodative influences that cause the difference between dark vergence and dissociated phoria.