Methods for measuring the proximity-fixation-disparity curve

The reliability and validity of new tests of fixation disparity were investigated as a function of viewing distance (100, 60, 40, and 30 cm). The 'nonius offset card' presents a set of paired dichoptic nonius (vernier) lines with different offset simultaneously: the subject has to indicate the coinciding lines. The 'computer test' uses a psychophysical procedure that presents one pair of nonius lines sequentially with varying offset; the effect of the number of short-term nonius test trials is investigated. As a reference, the Mallett unit and the Sheedy disparometer were used. Despite different group means, the methods had significant test-retest correlations and inter-correlations, as tested at 40 cm. All methods showed an increase in exo fixation disparity as the viewing distance was shortened; the slope of this proximity-fixation-disparity curve, however, was different between the nonius offset card and the computer test.