Monocular and binocular depth discrimination thresholds

Background. Measurement of stereoacuity at varying distances, by real or si mulated depth stereoacuity tests, is helpful in the evaluation of patients with binocular imbalance or strabismus. Although the cue of binocular dispa rity underpins stereoacuity tests, there may be variable amounts of other b inocular and monocular cues inherent in a stereoacuity test. In such circum stances, a combined monocular and binocular threshold of depth discriminati on may be measured-stereoacuity conventionally referring to the situation w here binocular disparity giving rise to retinal disparity is the only cue p resent. A child-friendly variable distance stereoacuity test (VDS) was deve loped, with a method for determining the binocular depth threshold from the combined monocular and binocular threshold of depth of discrimination (CT) . Methods. Subjects with normal binocular function, reduced binocular funct ion, and apparently absent binocularity were included. To measure the thres hold of depth discrimination, subjects were required by means of a hand con trol to align two electronically controlled spheres at viewing distances of 1, 3, and 6 m. Stereoacuity was also measured using the TNO, Frisby, and T itmus stereoacuity tests. BTs were calculated according to the function BT = arctan (1/tan alpha(C) - 1/tan alpha(M))(-1), where alpha(C) and alpha(M) are the angles subtended at the nodal points by objects situated at the mo nocular threshold (alpha(M)) and the combined monocular-binocular threshold (alpha(C)) of discrimination. Results. In subjects with good binocularity, BTs were similar to their combined thresholds, whereas subjects with reduc ed and apparently absent binocularity had binocular thresholds 4 and 10 tim es higher than their combined thresholds (CT). The VDS binocular thresholds showed significantly higher correlation and agreement with the TNO test an d the binocular thresholds of the Frisby and Titmus tests, than the corresp onding combined thresholds (p = 0.0019). Conclusion. The VDS was found to b e an easy to use real depth (distance) stereoacuity test. The method descri bed for calculating the BT provides one simple nonlinear solution for deter mining the respective contributions of binocular and monocular (MT) depth d iscrimination to the combined depth threshold.