In a final analysis, we examined how perceptual instability experienced by our observers when viewing natural depth edges relates to stimulus features that are known to exacerbate binocular rivalry in simple stimuli. Previous studies on binocular rivalry that used synthetically mismatched stimuli (e.g., sinusoidal gratings with different orientations in the two eyes), have reported that low-contrast stimuli tend to be more perceptually stable than their high-contrast counterparts. Sometimes, stimuli that cause unstable percepts at high contrast are fused into stable unified percepts at low contrast. For example, when orthogonal gratings are dichoptically presented at high contrast, they cause strikingly unstable percepts that alternate between the two grating orientations. When the same orthogonal gratings are presented at low contrast, they are perceived as a stable plaid (
Liu et al., 1992). Does the relationship between contrast and perceptual instability hold in natural images when monocular regions are caused by depth edges? First, we examined the correlation between the instability ratings and the image contrast in the binocular background, monocular, and binocular foreground regions in Experiment 2. As expected, binocular instability tended to be higher when contrast was higher. Higher instability ratings were associated with higher contrast in the monocular region (
r = 0.20; 95% CI, 0.07–0.33;
p = 0.02) and with higher contrast in the adjacent binocular foreground (
r = 0.31; 95% CI, 0.15–0.45;
p ≪ 0.001). This relationship was weaker and not statistically significant in the binocular background (
r = 0.17; 95% CI, 0.04–0.30;
p = 0.06). Next, we examined the correlation between instability ratings and contrast in the transition regions. Instability ratings were higher with higher contrast in the background–monocular transition region (
r = 0.19; 95% CI, 0.05–0.32;
p = 0.03), but the same was not true for the foreground–monocular transition region (
r = –0.097; 95% CI, –0.25 to 0.09;
p = 0.28). The absence of a correlation between perceptual instability and contrast in the foreground–monocular transition region may be connected to the natural scene statistics. Because
dissimilarity between the monocular region and the binocular foreground is highly likely, a high contrast in this transition region provides information about the existence of an edge in the natural environment, which may facilitate rather than impede fusion.