Fixation disparity

Fixation disparity may be a symptom of binocular stress or a purposeful error signal to drive vergence eye movements. Two similar units for detecting fixation disparity (Mallett and Sheedy units) are compared and evaluated. The Mallett unit was found to be more reliable, with similar readings in symptom-free subjects, whereas the Sheedy unit gave a larger spread of fixation disparity measurements and appears to be less useful than the Mallett unit for routine clinical use. The exact role of fixation disparity remains unresolved. Fixation disparity has two components in symptomatic subjects; the vergence signal component and an oculomotor imbalance component.