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.