Digital Stereo Test (DST): Static stereopsis assessment in simulated and real depth-deficit patients

TNO is a popular clinical test to assess static stereopsis and is useful in diagnosing various eye disorders like strabismus. However, it has certain shortcomings: 1) stereoacuity values differ with manufacturer changes 2) the book is expensive, difficult to procure in low-resource countries and fades over time, resulting in recurrent costs 3) research-grade electronic versions of random dot stereograms (e-RDS) are available; however, they are not directly comparable with current clinical stereotests due to differences in measured thresholds 4) these e-RDS tests are primarily tested on healthy controls, resulting in assessment biases. Consequently, we devised - Digital Stereo Test (DST) to overcome the aforementioned limitations. Here, participants performed a 3D object identification task. The stimuli consisted of an RDS square of size 8.6 cm subtending 2.891° when viewed at 40 cm (matching TNO) and was created in Psychtoolbox v3.0.17 (MATLAB-R2020a). This square had a hidden depth object – of shapes ‘⊔’, ‘⊓’, ‘⊏’ and ‘⊐’ and stereoacuities (480, 240, 120 and 60 arc-sec) – seen only with anaglyph glasses. A wide range of participants (n=150; mean-age: 24.9 ± 5.27 years) having stereo-impairments: Intermittent Divergent Squint (n=30), induced-anisometropia (n=66), stereoblindness (n=24) and controls (n=30) were included to validate DST against TNO. Across all participants, we found: 1) stereoacuities reported by DST are significantly positively correlated (Spearman r = 0.9528; p<0.05) with TNO, 2) Bland-Altman plot shows agreement, with DST overestimating stereoacuity values by only 0.0038 (Limits of Agreement: [-0.2605, 0.2350]) log arc-sec as compared to TNO. 3) Finally, no significant correlation (r=0.0209, p=0.9126; r=-0.0914, p=0.6310) of age with stereoacuities was reported by TNO & DST, respectively. These results show the test is clinically comparable to TNO even in different stereo-impaired conditions. We conclude that DST could be a potential low-cost digital tool to complement current static stereopsis tests in ophthalmic settings.