Should a central fusion stimulus be used on the disparometer?

Clinical measurements of fixation disparity made with the disparometer have used a 1.5 degree diameter peripheral fusion stimulus. The addition of a central fusion stimulus decreases the variability of test measurements and more closely simulates everyday visual tasks. However, this also changes the fixation disparity measurement, associated phoria, slope of the curve, and possibly the curve type. Normative data and correlation data to other clinical tests are not available for the central fusional measurements as they are for the peripheral fusional measurements. More importantly, the peripheral fusional measurements have been shown to correlate with patient symptoms, whereas such studies have yet to be performed with the central stimulus. Until normative and discriminative information data are available for the central fusional stimulus, the peripheral stimulus is more useful clinically.