Impact of monocular vs. binocular contrast and blur on the range of functional stereopsis.

Stereopsis depends on the smallest stereo threshold (lower limit) and the upper fusion limit. While studies have shown that the lower limit worsens with reduced contrast and blur, more strongly in monocular than in binocular conditions, the effect on the upper limit remains uncertain. Here, we assess the impact of contrast and blur on the range of the disparity sensitivity function (DSF) in a stereo letter recognition task. Subjects had to identify the stereo letters embedded in a random dot stereogram, and adaptive staircases were used to estimate the two limits. Five subjects performed the experiment at baseline contrast (100%), with different contrast (32% and 10%) and blur (+0.75DS and +1.25DS) in monocular and binocular degradation. We proposed three possible outcomes: 1) the range collapses in both directions 2) the lower limit threshold reduces, but the upper limit is not affected 3) the threshold for both limits increases and the range remains the same. We found that the curve for both limits was lowpass in shape, resulting in a smaller range at higher SFs. The results were similar to the first prediction, where the threshold for the lower limit increased while the upper limit was reduced at lower contrast and higher blur. The shrinkage of DSF is significant in monocular conditions. However, with blur, there was inter-subject variability. A simple cross-correlation stereo-matching algorithm was used to quantify the effect of contrast and blur. The results were consistent with the behavioral result that the range of DSF decreases with image degradation.

Case Report: When Two Is Worse Than One-Stereo Imbalance in a Case of Wavefront-guided Scleral Lenses.

SIGNIFICANCE: Wavefront-guided scleral lenses (WGSLs) reduce visually debilitating residual higher-order aberrations. Although reduced higher-order aberrations lead to improvement in monocular high-contrast visual acuity (VA), the success of the lenses in everyday life depends on additional factors such as retinal contrast, binocular balance, and stereoacuity. PURPOSE: This report describes a case where WGSLs provided improved monocular vision compared with scleral lenses (SLs) but reduced binocularity and stereoacuity. CASE REPORT: A 48-year-old woman with moderate keratoconus right eye (OD) and severe left eye (OS) was fitted with SLs and WGSLs. Visual acuity with best SLs was 20/20 -2 OD and 20/25 -2 OS. Residual higher-order root-mean-square (HORMS) wavefront error (6 mm pupil) was 0.56 µm OD and 1.38 µm OS. Visual acuity with WGSLs was 20/16 -2 OD and 20/25 +2 OS, and residual HORMS was 0.41 µm OD and 0.98 µm OS. Monocularly, WGSLs were reported to provide better VA. However, binocularly, the patient reported an "imbalanced feeling" and preferred the SLs over WGSLs. Binocular VA at distance was 20/25 with SLs and 20/25 -2 with WGSL. To investigate, the Worth Four-Dot test was performed, and the outcomes reported fusion with SLs but suppression OS at distance with WGSLs. Stereoacuity was 160 arc seconds at near and 120 arc seconds at distance with SLs and 400 arc seconds at near and >1200 arc seconds at distance with WGSLs. Dichoptic contrast balancing showed a balance point of 0.48 with SLs and 0.17 with WGSLs, indicating a strong preference toward OD. Simulation of the patient's retinal image revealed a greater difference in image contrast between the two eyes with WGSLs. CONCLUSIONS: Wavefront-guided scleral lenses reduced HORMS and improved VA compared with SLs. However, in this case, it inadvertently caused binocular imbalance. As WGSLs become more widely available, future work should include methods to optimize binocular balance to maximize overall patient satisfaction.

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.

Stimulus dependence of interocular suppression.

Interocular suppression is the phenomenon in which the signal from one eye inhibits the other eye in the presence of dissimilar images. Various clinical and laboratory-based tests have been used to assess suppression, which vary in color, contrast, and stimulus size. These stimulus variations may yield different spatial extents of suppression, which makes it difficult to compare the outcomes. To evaluate the role of stimulus characteristics, we measured the suppression zone using a binocular rivalry paradigm in normally-sighted observers by systematically varying the stimulus parameters. The stimuli consist of a constantly visible horizontal reference seen by one eye while two vertical suppressors were presented to the other eye. With a keypress, the suppressors appeared for 1 s, to induce a transient suppression zone in the middle part of the reference. Subjects adjusted the width between the suppressors to determine the zone. The zone decreased significantly with increasing spatial frequency and lower contrast. The width was 1.4 times larger than the height. The zone was smaller with negative compared to positive contrast polarity but independent of eye dominance, luminance, and colored filters. A departure from scale invariance was captured with a model suggesting a stimulus-dependent and a small fixed non-stimulus-dependent portion.