Depth constancy and the absolute vergence anomaly

Binocular disparity provides information about the depth structure of objects and surfaces in our environment. Since disparity depends on the distance to objects as well as the depth separation of points, information about distance is required to estimate depth from disparity. Our perception of size and shape is biased, such that far objects appear too small and flattened in depth, and near objects too big and stretched in depth. The current study assessed the extent to which the failure of depth constancy can be accounted for by the uncertainty of distance information provided by vergence. We measured individual differences in vergence noise using a nonius line task, and the degree of depth constancy using a task in which observers judged the magnitude of a depth interval relative to the vertical distance between two targets in the image plane. We found no correlation between the two measures, and show that depth constancy was much poorer than would be expected from vergence noise measured in this way. This limited ability to take account of vergence in the perception of depth is, however, consistent with our poor sensitivity to absolute disparity differences. This absolute disparity anomaly thus also applies to our poor ability to make use of vergence information for absolute distance judgements.