The interaction of binocular disparity and motion parallax in determining perceived depth and perceived size

Although binocular disparity and motion parallax are powerful cues for dept h, neither, in isolation, can specify information about both object size an n depth. It has been shown that information from both cues can be combined to specify the size, depth, and distance of an object in a scene (Richards, 1985 Journal of the Optical Society of America A 2 343 - 349). Experiments are reported in which natural viewing and physical stimuli have been used to investigate the nature of size and depth perception on the basis of disp arity and parallax presented separately and together at a range of viewing distances. Observers adjusted the relative position of three bright LEDs, w hich were constrained to form a triangle in plan view with the apex pointin g toward the observer, so its dimensions matched that of a standard held by the subject. With static monocular viewing, depth settings were inaccurate and erratic. When both cues were present together accuracy increased and t he perceptual outcome was consistent with an averaging of the information p rovided by both cues. When an apparent bias evident in the observers' respo nses (the tendency to underestimate the size of the standard) was taken int o account, accuracy was high and size and depth constancy were close to 100 %. In addition, given this assumption, the same estimate of viewing distanc e was used to scale size and depth estimates.