Opponent motion interactions in the perception of structure from motion

Motion provides important cues for the perception of depth and object structure. The kinetic depth effect illustrates this phenomenon: dots moving in a two-dimensional plane can produce a vivid perception of a rotating three-dimensional object. We studied the origin of this depth percept in a psychophysical study employing inducing and test stimuli. The inducing stimulus, containing dots moving with simple harmonic motion in the fixation plane, was perceived as a rotating cylinder. The test stimulus had binocular disparity that placed it close to either the near or far surface of the cylinder. We found that sensitivity to the test was lower when it moved in the opposite direction to the adjacent surface of the inducing stimulus than when the two stimuli moved in the same direction. We also simplified the inducing stimulus by using two uniform arrays of dots translating in opposite directions. Subjects saw one array as being closer than the other, and test sensitivity was again reduced when the test was close to a surface moving in the opposite direction. These results support the idea that there are suppressive interactions between opposing motions at the same depth, leading to a single perceived direction of motion at each depth.