Assessment of depth perception with a comprehensive disparity defined letter test: A pilot study

Current clinical tests mostly assess stereopsis with crossed disparity at near. These tests are designed with fine targets (high spatial frequency) and may fail to capture the "functional stereopsis" in real-world scenes, which consist of a range of spatial frequencies (SFs). We developed a stereo letter test that can assess crossed and uncrossed stereoacuity at near and far, at different SFs defined by the letter size. The test consists of disparity-defined letters embedded in random-dot stereograms. At each letter size, the letters are arranged in sets of trigrams like in the Pelli-Robson chart. The letter sizes correspond to SFs ranging from 0.3 to 2cpd. Within each triplet, all letters have the same disparity and the amount of disparity decreases after each set. Subjects report the letters verbally to determine the smallest disparity at each letter size. Twenty-four subjects were tested with eight different charts: crossed vs. uncrossed disparity at far and near, with two versions (different letter sequences). The disparity sensitivity function (DSF) had an inverted U-shape, with decreasing sensitivity for smaller stereo letters. The subjects had better stereopsis at far than near. All the subjects had lower stereo thresholds with crossed disparity than uncrossed, consistently at both distances. We found no effect of age or heterophoria on the DSF. The charts have good test-retest reliability (Pearson's r = 0.89, p<0.001) and are easy to perform. Our results with stereo letters as stimuli are comparable to results from studies using depth corrugations. This stereo acuity letter test permits assessment of stereopsis at different testing distances, directionality of disparity, and across a range of SFs, which can help diagnose selective stereo losses in binocular vision anomalies and monovision. Assessment of stereopsis at different SFs may provide additional information for understanding daily stereovision demands than the conventional tests.