Combining cues while avoiding perceptual conflicts

Testing a counter-intuitive prediction of optimal cue combination

Weighted averaging is said to be optimal when the weights assigned to the cues minimize the variance of the final estimate. Since the variance of this optimal percept only depends on the variances of the individual cues, irrespective of their values, judgments about a cue conflict stimulus should have the same variance as ones about a cue consistent stimulus. We tested this counter-intuitive prediction with a slant matching experiment using monocular and binocular slant cues. We found that the slant was indeed matched with about the same variance when the cues indicated slants that differed by 15 degrees as when they indicated the same slant.

Judging surface slant for placing objects: a role for motion parallax

People have a variety of sources of information (cues) about surface slant at their disposal. We used a simple placing task to evaluate the relative importance of three such cues (motion parallax, binocular disparity and texture) within the space in which people normally manipulate objects. To do so, we projected a stimulus onto a rotatable screen. This allowed us to manipulate texture cues independently of binocular disparity and motion parallax. We asked people to stand in front of the screen and place a cylinder on the screen. We analysed the cylinder's orientation just before contact. Participants mainly relied on binocular cues (weight between 50 and 90%), in accordance with binocular cues being known to be reliable when the stimulus surface is nearby and almost frontal. Texture cues contributed between 2 and 18% to the estimated slant. Motion parallax was given a weight between 1 and 9%, despite the fact that it only provided information when the head began to move, which was just before the arm did. Thus motion parallax is used to judge surface slant, even when one is under the impression of standing still.