Interaction between the perceived shape of two objects

The difference between the way in which binocular disparity scales with vie wing distance and the way in which motion parallax scales with viewing dist ance introduces a potential indirect cue for viewing distance: the viewing distance is the only distance at which disparity and motion specify the sam e depth. The present study examines whether this information is used. Two s imulated ellipsoids were presented on a computer screen in complete darknes s. The two ellipsoids were 60 to the left and right of straight ahead. Subj ects set the width and depth of each ellipsoid to match a tennis ball, and set the distance of the one on the right to half that of the one on the lef t. The distance of the left ellipsoid varied between trials. On half of the trials it was static. On the other half it was rotating up and down around its frontal horizontal axis. Rotating the left ellipsoid influenced its se t depth: rotating ellipsoids were set to be much more spherical. There was no influence on the set depth of the other ellipsoid, or on the set width o f either. The set distance of the right ellipsoid was also unaffected. We c onclude that subjects do not combine binocular disparity and motion paralla x to obtain more veridical information about viewing distance. (C) 1999 Els evier Science Ltd. All rights reserved.