Use of a new composite index to demonstrate improved stereoacuity after training on stimuli with dichoptically asymmetric contrast

Many people with stereo-deficiency cannot perform stereoacuity tasks at any binocular disparity, so their threshold cannot be measured. We developed a generally useful new composite stereodepth or CSD score, with range 0 to 20, that combines stereoacuity (from 0 to 13.3) and percent correct (13.3 to 20) into a single index. CSD allows differences in stereodepth performance to be measured across groups when thresholds cannot be reliably measured, and across time when individuals gain stereodepth perception during training. We used CSD scores to assess the benefit of unequal (mixed) left- and right-eye contrast during dichoptic stereoacuity training in people with amblyopia. Nineteen adult participants with anisometropic amblyopia were assigned randomly to 10 sessions of either mixed-contrast or fixed-contrast training. Stimuli were rectangles, one above the other, viewed through a four-mirror stereoscope. Participants indicated which rectangle appeared closer. A staircase controlled disparity within the stimulus. CSD improvement was better by 1.6 CSD units for participants who received mixed-contrast training. Thus, mixed-contrast stereo training was more effective than fixed-contrast training in adults with anisometropic amblyopia.