Contextual effects on perceived three-dimensional shape.

Binocular disparity is a powerful cue for the perception of depth. The accuracy with which observers can judge depth from disparity can, however, be very poor. This has been attributed to difficulties associated with the scaling of disparity to take account of distance (Johnston, 1991). We test potential strategies that could be used to improve this scaling. Using the depth-to-width ratio task introduced by Bradshaw, Parton, and Eagle (1998), observers adjusted a depth interval to match the vertical distance between two points. The first experiment examined the effect of placing additional visual stimuli between the observer and the target. Despite the potential of these stimuli to provide reliable distance information, the accuracy of depth settings did not change. The second experiment demonstrated that the degree of binocular correlation present in natural images provides useful distance information, and investigated whether this is used by observers in scaling disparity. To do this, we measured whether varying the magnitude of relative disparity presented in the surround of the target affected depth settings. No such effect was observed. We conclude that the effect of information presented in the surrounding context on settings of depth is limited to those situations in which it provides direct information about the distance to the target.