Deficient stereopsis in the normal population revisited: why current clinical stereo tests may not be adequate

Significance statement Applied applications for occupational screening, clinical tests should assess sensitivity to the sign as well as the magnitude of disparity. Purpose To determine why the high incidence of stereo anomaly found using laboratory tests with polarity-based increment judgements (i.e., depth sign) is not reflected in clinical measurements that involve single-polarity incremental judgements (i.e., depth magnitude). Methods An iPod-based measurement that involved the detection of an oriented shape defined by a single polarity-depth increment within a random dot display was used. A staircase procedure was used to gather sufficient trials to derive a meaningful measure of variance for the measurement of stereopsis over a large disparity range. Forty-five adults with normal binocular vision (20 - 65 years old) and normal or corrected-to-normal (0 logMAR or better) monocular vision participated in this study. Results Observers’ stereo acuities ranged between 10 and 100 arc seconds, and were normally distributed on a log scale ( p = 0.90, 2-tailed Shapiro-Wilk test). The present results using a single polarity depth increment task (i.e., depth magnitude) show a similar distribution to those using a similar task using the Randot preschool stereo test on individuals between the ages of 19-35 using either the 4-book test (n = 33) or the 3-book test (n = 40), but very different results when the iPod test involved a polarity-based increment judgement (i.e., depth sign). Conclusions The present clinical stereo tests are based on magnitude judgements and are unable to detect the high percentage of stereo anomalous individuals in the normal population revealed using depth sign judgements.