Vergence anomalies are associated with impaired stereopsis in amblyopia

We examined the relationship between stereopsis and fusional vergence in groups of amblyopic and stereo-normal control observers. As absolute disparity is thought to be the basis for relative disparity and for disparity-driven vergence, we hypothesized that vergence anomalies would be accompanied by impaired stereopsis. Specifically, we examined whether patterns of impaired stereopsis across the central 20° of the visual field were accompanied by impaired fusional vergence for stimuli confined to these regions. Stereopsis was measured locally across the visual field with disparity steps of 5 to 20 arcmin. Fusional vergence to large disparity steps (2 to 3°) was measured with binocular eye tracking. The vergence stimuli were random dot stereograms, in one of 3 spatial configurations: a large disc 16° in diameter, a small disc 4° in diameter, and an annulus with outer and inner diameters corresponding to the large and small discs. Of the controls (n = 25) with no history of abnormal visual development, 12 individuals exhibited normal stereopsis across the visual field and normal vergence gains for all configurations. Thirteen individuals with weak stereopsis in the central field tended to have anomalous vergence for small stimuli, but normal vergence for larger stimuli. Amblyopic/strabismic individuals (n = 12) had poor stereopsis and poor vergence for small stimuli. We report a strong correlation between vergence, coarse and fine stereopsis, with no double dissociation (no cases of impaired vergence with normal stereopsis). Taken together, the results suggest that compromised binocular interaction is the cause of both stereopsis and vergence deficits.