Abstract
Purpose :
In a real-world scenario, disparity sensitivity depends on both the smallest stereo threshold (Dmin) and its upper limit to double vision (Dmax). Studies have shown that Dmin reduces with lower contrast and blur, more strongly in the monocular than in the binocular conditions. However, little is known about the effect of Dmax across different spatial frequencies in a complex task, especially with unequal image properties between the two eyes, such as contrast differences. Here, we assess the effect of contrast on the range of disparity sensitivity function (DSF) in a stereo letter recognition task.
Methods :
Previously, we developed a stereo test consists of Sloan letters with crossed disparity embedded in Random dot stereograms, viewed through active-shutter goggles and a 3D monitor. A letter, corresponding to 0.5-2cpd at 10ft, appeared in front of the background RDS (1sec) at different depth planes, and subjects identified the letter seen. Adaptive staircases were used to obtain Dmin and Dmax. Uncorrelated background RDS were used to measure Dmax. With correct responses, the letters in the Dmax staircase are presented closer to the observer until fusion breaks, making the letters unrecognizable. The range of the DSF was determined as the difference between Dmax and Dmin. Three subjects performed the experiment, at high contrast, and with different contrast combinations (100, 32, and 10%) in monocular and binocular conditions. One eye's contrast was fixed at 100% in the monocular condition.
Results :
With stereo letters varying in sizes, the Dmin and Dmax curves were lowpass in shape. The range of DSF was more extensive at low SF (~35 to ~4500 arcsec) and condensed at higher SF (~80 to ~1800 arcsec). With lower contrast, Dmin increased while Dmax reduced, resulting in a smaller DSF range (Kruskal-Wallis, p=0.005). Comparing between congruent and incongruent contrast conditions, the range was smaller across the SFs, but was statistically significant only at 10% contrast.
Conclusions :
Our result illustrates the shrinkage of the range of functional stereopsis, especially under unequal contrast. With lower contrast, maintaining fusion is more challenging, the range of DSF is smaller, and the probability of seeing double is higher, more so in the unequal condition. One should consider both Dmin and Dmax when the image quality between the two eyes is unequal, such as in the case of monovision or contrast differences.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.