Depth perception in strabismus.

Depth perception in strabismus SIR, Recently Henson and Williams published an article entitled 'Depth perception in strabismus." Regettably, the authors did not review the literature completely. If they had, they would have noted that Cooper and Warshowsky2 scientifically evaluated monocular cues in the Titmus stereo test, and that Cooper and Feldman36 looked at the availability of monocular cues in random dot stereograms. In those studies they found that monocular cues are used by patients when a line stereogram is made up of symmetrical shapes decentered in a small area, i.e., Titmus circle test, and that monocular cues are rarely used by subjects viewing random dot stereograms. Cooper and Feldman also found, as Henson and Williams reported, that small-angle strabismics have real stereopsis on line stereograms, while large-angle strabismics do not.6 This is not surprising in light of the fact that fusion is not needed for stereopsis, and that stereopsis has been reported with up to 14° of disparity in normals. Contrary to Henson and Williams, we have not found any constant strabismics who appreciate random dot stereograms.3 This is consistent with Fender's and Julesz's findings in normals that fusion is needed for appreciation of a random dot stereogram. Another way of saying the same thing is that the deviation must be smaller than Panum's area for appreciation of a random dot stereogram. The difference between our findings and Henson and Williams's may be explained if some or all of their subjects were intermittent strabismics. Three of the 7 subjects who had good stereopsis were defined as nonamblyopic, small-angle, constant exotropes. This is an extremely rare situationone which we have never seen. In addition the esotropes who demonstrated 'good' stereopsis had a mean angle of deviation, 3.3A with best optical correction and no real amblyopia. This is such a small angle of deviation that it is possible that all these patients were actually intermittent. (No cover testing was reported by Henson and Williams during actual testing of stereopsis.) It is our clinical and research experience that any patient with 20 seconds of stereoacuity must be bifoveal and cannot have a constant strabismus. Thus we suspect that most of the Henson and Williams subjects were intermittent strabismics, not constant strabismics, as they reported. In conclusion, we have found that small-angle strabismics have reduced stereoacuity on lined stereograms and no stereopsis on random dot stereograms. Large-angle strabismics possess no stereopsis on either line or random dot stereograms. State College of Optometry, JEFFREY COOPER State University of New York, JERRY FELDMAN 100 East 24th Street, New York, NY 10010, USA.