Introduction

Though stereoscopic vision is often regarded as a means of gauging distance, human stereo depth judgements depart strikingly from the predictions of geometry. Not only are judgements of absolute distance highly inaccurate, as shown by Helmholtz's experiments using a single vertical thread against a featureless background (Helmholtz, 1909), but also the perceived distance of an object is strongly affected by other objects nearby (Gogel, 1963; Gogel & Mershon, 1977), with the result that relative distance is often incorrectly estimated.

In the case of horizontal rows of features, what stereo vision seems to deliver is a measure of local protrusion or curvature (Mitchison & Westheimer, 1984). For instance, a linear horizontal gradient of disparity generates no curvature and is therefore poorly perceived. However, the same is not true of a vertical gradient of (horizontal) disparity, and indeed there is known to be a marked horizontal/vertical anisotropy in stereo perception (Rogers & Graham, 1983). We suggest here a rationale for some of these phenomena. It turns out that oblique viewing introduces gradients of horizontal disparity which are largely eliminated by the curvature measure, thereby allowing a stable percept under changing viewing conditions. These disparity gradients are present only in the horizontal direction, and this may be the basis of the horizontal/vertical anisotropy.

Depth judgements in horizontal rows of features

We first review the experiments which show how stereo depth judgements are made in figures consisting of horizontal rows of lines or dotted lines (Mitchison & Westheimer, 1984).