Stereoscopic correspondence for ambiguous targets is affected by elevation and fixation distance

Observers show a small disparity preference when presented with ambiguous binocular stimuli (McKee and Mitchison, 1988 Vision Research 28 1001 - 1012). This can be understood if it is assumed that small disparities are more probable than large disparities (Prince and Eagle, 2000 Vision Research 40 1143 - 1155). A consideration of binocular viewing geometry and the structure of the natural environment suggests that the most likely disparity will not necessarily be zero, and will depend on both elevation and the fixation distance. Their effects on binocular correspondence were therefore investigated. From five different distances, observers fixated a central cross, and were presented with a dichoptic square-wave pattern above or below fixation that could be matched with either a crossed or uncrossed disparity. A clear bias was observed for the ambiguous stimuli, with those presented below fixation tending to appear closer than fixation. Observers tend to report stimuli as closer than fixation with a tendency increasing with increasing fixation distance. No effects of fixation distance were observed for stimuli presented above fixation. These results show a clear influence of elevation and fixation distance on binocular correspondence, consistent with the spatial distribution of disparities in natural images.