Does binocularly perceived depth correlate with reduced stereoacuity?

Aim: To assess perceived depth in the Wirt Fly Stereotest for assessing stereoacuity.

Methods: Somers and Hamilton [Ophthalmic Physiol. Opt. 4 (1984) 245] reported a strong correlation for four observers with normal vision, between perceived depth in the Wirt Fly stereotest and degradation of Keystone stereothresholds from induced anisometropia created using a range of monocular plus lenses. Depth was measured with a pointer placed by subjects alongside the tip of the Fly's wing. Our study replicated that of Somers and Hamilton, but with several methodological refinements (mainly, use of a headrest to control head position) and more subjects.

Results: We confirm Somers and Hamilton's results for normal observers by finding (1) induced anisometropia causes systematic depth reduction for the Fly and (2) significant correlations between fly depth and stereothresholds (as measured with the Frisby Near and the Randot Circles stereotests). However, the mean values mask considerable between subject variation in responses.

Conclusion: The substantial individual and between-subject variation in Wirt Fly perceived depth causes us to doubt its value as a proxy for stereoacuity except as a rough estimate.