There was a modest but statistically significant negative correlation between interocular contrast balance and stereoacuity across all participants in this study.

There was a modest but statistically significant negative correlation between interocular contrast balance and stereoacuity across all participants in this study. Click image to enlarge.

Researchers at the Brien Holden Vision Institute recently tested two hypotheses: (1) that stereoacuity is inversely correlated with the extent of contrast difference between both eyes in keratoconus and (2) a purposeful bias in the interocular stimulus contrast ratio either in favor of the stronger eye or in favor of the weaker eye will cause symmetric loss of stereoacuity in keratoconus. They found that partial improvement in stereoacuity may be achieved by balancing the contrast input to the two eyes of patients with bilaterally asymmetric keratoconus, independent of their baseline contrast imbalance level.

The study tested interocular contrast imbalance and stereoacuity of 43 participants (16 to 33 years old) with bilaterally asymmetric keratoconus with spectacle correction as baseline using a binocular rivalry paradigm and random-dot stereograms, respectively. Stereoacuity measurements were repeated in a subset of 33 participants at their contrast balance point (i.e., contrast level in stronger eye allowing balanced rivalry with 100% contrast in weaker eye) and with contrast levels biased in favor of the stronger or weaker eye, all conditions in randomized order.

Contrast imbalance level was significantly correlated with the subject’s stereoacuity at baseline. The median stereoacuity improved by 34.6% from baseline (748.8arc sec) to the contrast balanced condition (419.0 arc sec), independent of baseline stereoacuity or contrast imbalance levels. Contrast bias in favor of the weaker eye (881.3arc sec) worsened stereoacuity more than a bias toward the stronger eye (502.6arc sec), both relative to the contrast balanced condition.

“Stereoacuity may be partially restored in these patients by minimizing the contrast difference between the two eyes,” the researchers concluded in their paper. “Inherent biases may also exist in the way the monocular inputs from the two eyes are processed in keratoconus for cyclopean viewing that may determine the status of binocularity in these patients.”

Marella BL, Vaddavalli PK, Reddy JC, et al. Interocular contrast balancing partially improves stereoacuity in keratoconus. Optom Vis Sci. February 28, 2023. [Epub ahead of print].