A Performance-Based Study Of Stereopsis And Fixation Disparity.

A need exists for suprathreshold tests of stereoscopic performance that have application to everyday situations. Such tests should emphasize the quality (strength and robustness of the depth sensation) rather than the quantity (threshold) of stereopsis. Tests commonly used today in the clinical setting test on the order of one or a few minutes of disparity angle, while everyday use of stereopsis requires a more suprathreshold disparity angle closer to 10 to 20 minutes of disparity angle in a normally functioning binocular system (e.g. Ogle, 1964). We also note the need to take account ofthe differences between exophores and esophores for performance on quantitative stereoscopic tests which rely on crossed or uncrossed disparity (Shippman and Cohen, 1983). The very tight relationship between stereoscopic threshold and fixation disparity is well known; less understood is that same relationship at the suprathreshold levels, in which we all live and work. Studies (Shippman and Cohen, 1983) indicate that esophores have better stereoacuity with uncrossed disparity and exophores have better stereoacuity with crossed disparity. The difference between esophoric and exophoric performance may be due to the asymmetric distribution ofPanum's areas about the fixation point. Exophores usually have exo fixation disparity and esophores usually have
eso fixation disparity. Exo fixation disparity moves the collective centers of the Panum's
areas behind the plane of regard and eso fixation disparity moves them in front of the
plane of regard (Mitchell, 1966). Stereopsis is at its best at points at or near the horopter.
It can be theorized that a subject with eso fixation disparity would have better stereoacuity if an entire scene was in uncrossed disparity thus causing most points of the scene to fall on or near that persons horopter. Likewise, the reverse can be expected with subjects having exo fixation disparity. Exophores should prefer most of the points to have crossed disparity in a scene. Stereoscopic performance also has a relationship to the amount of the
heterophoria present within a system (Saladin, 1995). Moderate amounts of exophoria ( 6
pd) and mild amounts of esophoria (less than 2 to 3 pd's) have lesser effect on stereopsis,
while the heterophoric values above these amounts do have a more profound effect.
Similarly, levels above mild amounts of fixation disparity contribute to decreased stereoscopic perception (Saladin, 1990). Symptomatic amounts of fixation disparity (greater than 6' exo and 4' eso) appear to correlate to exophoria above moderate amounts
and esophoria above mild amounts. We designed this study as a pilot to guide us in our study of stereopsis and fixation disparity at suprathreshold levels and to help us develop a suitable test and accompanying task. We ask: can a subject successfully arrange a series ofanaglyphs made with successive disparity gradients and can the same sense of depth quality difference be used to investigate the effect of fixation disparity on suprathreshold stereopsis.