Variation in stereoacuity: normative description, fixation disparity, and the roles of aging and gender.

PURPOSE Variation in stereoacuity was examined in a large group of observers with Snellen acuity of 20/30 or less. METHODS Threshold retinal disparity for 2.78 degrees x 2.28 degrees rectangular test stimuli was determined as a function of the retinal disparity (varied from 55 arcmin uncrossed to 55 arcmin crossed) of a 5.57 degrees x 4.8 degrees rectangular pedestal stimulus in 160 observers 15 to 79 years of age. In most cases, data were collected during viewing of random dot stereograms (RDSs) presented for 100-ms, which prevents involvement of vergence or monocular depth cues. RESULTS When plotted logarithmically, 100-ms thresholds in 106 observers less than 60 years of age approximated a normal distribution (mean, 1.57 +/- 0.227 [SD] log arcsec [37 linear arcsec]). Among these, one observer was supernormal, 88% were within the normal range (+/-2 SD of the log mean), 2% had elevated thresholds, and 8% failed testing with 100-ms stimuli but had residual binocular depth discrimination; 1 observer was stereoblind. In contrast, only 37% of the observers aged 60 to 69 and 25% of the observers aged 70 to 79 had stereoacuity within the normal range. Moreover, the extent of the stereo deficiencies became more pronounced with age. Fixation disparity was operationally defined as optimal stereoscopic threshold with a nonzero retinal disparity pedestal. Of the 151 normal observers tested, 89% were maximally sensitive to disparities within 11 arcmin of fixation: all males were maximally sensitive to pedestals within 22 arcmin of fixation, whereas 8% of females had fixation disparities of more than 22 arcmin. Males were more likely to be sensitive with uncrossed-disparity pedestals, whereas females were more likely to be sensitive with crossed disparity. CONCLUSIONS Age-related deterioration in stereoacuity is reflected not only by a linear correlation between age and threshold but also by a catastrophic factor that produces more marked deterioration after age 60. Both factors are probably cerebral and not specifically related to stereopsis. The prevalence of fixation disparity in the normal population is probably more common than previously reported.