Organizational factors and the perception of motion in depth.

When two stationary, stereoscopically separated targets are viewed in a completely dark surround, and no cues concerning their egocentric distances from the observer are salient, the farther target tends to be seen at the same distance it would have assumed if it were by itself. The nearer target is seen as being closer than it would have been if seen alone. The present studies extend this previous finding (now termed the far-anchor effect) into the domain of targets that move in stereoscopic space. Observers viewed two small illuminated targets, which began at either the same or different stereoscopic distances. One of the targets was moved in depth and the observers identified the target that appeared to move. Conditions varied according to the initial depth location of the moving target. Significantly more correct responses were reported when the nearer target moved than when the farther one moved, consistent with the hypothesis that the perception of motion in depth is affected by the aforementioned perceptual anchoring effect of the farther target.