Ocular prevalence: difference between crossed and uncrossed disparities of stereo objects.

BACKGROUND

Many observers perceive a horizontal offset between a stereoscopically viewed object and a reference object in the fixation plane, although both objects are presented along a single line that projects through the midpoint between both eyes. These observers weigh one eye more than the other when determining the visual directions of stereo objects. This unequal weighting has been called ocular prevalence. We considered that ocular prevalence might be asymmetric, that is, different for crossed and uncrossed stereo disparities.

METHODS

We examined 11 subjects with normal eyes and a stereo resolution of less than 100 sec arc. A vertical line was dichoptically presented for 100 ms at crossed or uncrossed stereo disparities of 1, 2, 3, 4, 5, 6, or 7 min arc; this stereo object was judged in relation to a reference line in the fixation plane.

RESULTS

Ten of the 11 subjects had ocular prevalence, but only 3 of the 10 showed a significant difference between crossed and uncrossed disparities.

CONCLUSIONS

Ocular prevalence occurs frequently among observers with normal binocular vision, but only a minority of these observers show a difference between crossed and uncrossed disparities of the stereo object. It is unclear whether this difference bears clinical importance.