Evaluating The Mechanism By Which The TNO Stereo Test Overestimates Stereo Thresholds

Objective:The TNO stereotest is a commonly used clinical test.Several studies have revealed that results of the TNO stereotest may overestimate the stereoacuity value（the less the better）compared with other testing measurements.This study aimed to explore the possible reasons which may affect stereopsis measurements of TNO stereotest.Methods:（1）Red-green anaglyphs test system,active shutter 3D system,polarized 3D system and auto-stereoscopic system were established with the aid of different computer system.Participants were recruited and evaluated with those computer systems.Shapiro-Wilk test was used to explore the distribution of data.Then parametric tests or non-parametric tests to analyze the data based on the normality test result.（2）A stereopsis measurement system using a phoropter and two Sony smartphones was established.Four types of test patterns,including the original TNO stereo test pictures,isoluminant red–green pictures,grayscale pictures,and black and white dots pictures,were designed to evaluate stereopsis.Results:（1）The data of all of the three groups were not satisfied with a normal distribution according to the Shapiro-Wilk test（P all <0.001）.Friedman Test was used to test the data.A significant difference was found among the three groups（Chi-square=48.713,P<0.001）.Using the Wilcoxon signed-rank test to detect differences between the paired groups:shutter 3D test versus polarized 3D test,Z=-1.134,P=0.257;shutter 3D test versus red-green glasses test,Z=-4.553,P<0.001;polarized 3D test versus red-green glasses test,Z=-4.618,P<0.001.The stereoacuity of the red-green glasses test was significantly worse than those of the other two groups,while no significant difference was found between the shutter 3D test and polarized 3D test.（2）A significant difference was found among the four groups（Friedman test,Chi-square=50.985,P<0.001）.The Wilcoxon signedrank test was used to detect differences between the groups: original group vs.black–white group,Z=-4.378,P<0.001;original group vs.isoluminant group,Z=-2.828,P=0.005;original group vs.grayscale group,Z=-3.251,P=0.001;isoluminant group vs.grayscale group,Z=-2.140,P=0.032.The stereoacuity of the original TNO group was significantly worse than those of the isoluminant,grayscale,and black-white groups.However,no significant difference was found between the isoluminant and grayscale groups.Conclusions:Anaglyph glasses played an important role in determining the stereoacuity values with the TNO stereo test and the results were overestimated when compared with that of the other testing methods.The imbalance of chroma and luminance between the two eyes caused by the anaglyph glasses was indicated as one of the reasons for the overestimation of stereo thresholds.