Depth and size scaling created by the differential perspective of ground plane surfaces
The horizontal gradient of differential vertical size in the two eyes (differential perspective) provides information about the absolute distance to a frontal surface which has been shown to be effective in both size and depth scaling (Rogers and Bradshaw, 1993, Nature, 361). In the real world, however, extensive frontal surfaces are rare and the more typical ground plane surface instead creates a complex, higher-order gradient of differential perspective. It is straightforward to show that these higher-order gradients provide sufficient information to scale the retinal sizes and horizontal disparities of all objects lying on the ground plane surface as well as providing a potential source of information to calibrate binocular vergence. The present experiments were designed to measure the effectiveness of ground plane differential perspective gradients for size and disparity scaling and for vergence calibration. Large visual field stimuli were created by projecting binocular images onto separate ground plane screens in a modified Wheatstone stereoscope configuration. The test objects were triangular wave corrugations in which observers had to judge the corrugation angle (shape task) and the separation between the peaks (size task). The ground plane gradient of differential perspective was manipulated independently of the vergence demands of the simulated surface. The results show that both size and depth constancy were high (70-80%) when the differential perspective gradient and vergence cues were consistent and appropriate. A substantial independent effect of the differential perspective gradient on size and depth constancy was also revealed when it was set in conflict with the vergence demands of the simulated surface.