THE RELATIONSHIP BETWEEN STEREOACUITY AND STEREOMOTION THRESHOLDS

There are in principle at least two binocular sources of information t hat could be used to determine the motion of an object towards or away from an observer; such motion produces changes in binocular dispariti es over time and also generates different image velocities in the two eyes. It has been argued in the past that stereomotion is detected by a mechanism that is independent of that which detects static dispariti es. More recently this conclusion has been questioned. If stereomotion detection in fact depends upon detecting disparities, there should be a clear correlation between static stereo-detection thresholds and st ereomotion thresholds. If the systems are separate, there need be no s uch correlation. Four types of threshold measurement were performed by means of random-dot stereograms: (1)static stereo detection/discrimin ation; (2)stereomotion detection in dynamic random-dot stereograms (te mporally uncorrelated); (3)stereomotion detection in temporally correl ated random-dot stereograms; and (4)binocular detection of frontoparal lel motion, Three normal subjects and five subjects with unusually hig h stereoacuities were studied. In addition, two manipulations were per formed that altered stereomotion thresholds: changes in mean disparity , and image defocus produced by positive spectacle lenses. Across subj ects and conditions, stereomotion thresholds were well correlated with stereo-discrimination thresholds. Stereo-motion was poorly correlated with binocular frontoparallel-motion thresholds. These results sugges t that stereomotion is detected by means of registering changes in the output of the same disparity detectors that are used to detect static disparities.