Motion parallax as a determinant of perceived depth.

Motion parallax is the optical change of the visual field of an observer which results from a change of his viewing position. It is often denned as the set of "apparent motions" of stationary objects which arise during locomotion. Psychologists assert that it is a "cue" for perceiving the depth of the objects, but the optical fact of motion parallax must be distinguished from its capacity to induce perceptions. It has not been experimentally demonstrated that motions in the field of view will actually yield corresponding judgments of depth. This is a purely psychological problem. The optics of motion parallax, on the other hand, is a problem for geometry and ecology. Recently, the suggestion has been made that a continuous gradient of motions in the field of view will induce the perception of slant-depth (J. J. Gibson, Olum, •& Rosenblatt, 1955) inasmuch as the perception of depth is intimately connected with the perception of surfaces (J. J. Gibson, ^950). This statement also needs experimental test. The purpose of the present study is to investigate what kinds of motion in the light entering an eye do in fact consistently arouse certain judgments of depth, and what do not. The experiments must be carried out with artificial motions in a field of view rather than those obtained 1 This work was supported by the Office of Naval Research under Contract NONR 401 (14) with Cornell University. Reproduction in whole or in part is permitted for any purpose of the U. S. Government. in a natural environment if we wish to study the effect of motion parallax in isolation from other cues or stimuli for depth. The variables of size, density, linear perspective, differential blur, and binocular parallax should be eliminated or so reduced as to be ineffective in the array of light entering O's eye. A method of achieving this result has been devised, and a suitable control employed. The experimental method should also preclude actual movement or locomotion of 0. If the cue of motion parallax is so defined as to require active head movement or locomotion, proprioceptive and vestibular stimulation is also present. This definition is unjustified, since passive locomotion in trains and airplanes should be admitted as circumstances when motion parallax occurs. Certain patterns of motion in the field of view of 0 do induce impressions of being moved through space if we accept as evidence the illusions of locomotion obtained in viewing a panoramic motion picture, or in a training device for simulating aerial flight.