Binocular interaction in post-LASIK subjects with unsatisfactory outcome

A shift from binocular summation to binocular inhibition has been observed when there is a significant decrease in contrast sensitivity or reduced retinal illuminance in one eye compared to the other as in cases of unilateral cataract and amblyopia. This study aims to determine how binocular function in post-LASIK subjects with unsatisfactory outcomes is influenced by differences between the two eyes in visual acuity and contrast sensitivity.

A retrospective analysis of monocular and binocular visual acuity (VA) and contrast sensitivity (CS) results was performed for 23 LASIK subjects who previously underwent a Functional Vision Test battery at Vision Sciences Research Corp. (San Ramon, CA). Photopic VA was measured with ETDRS chart and sine-wave grating CS was measured with FACT chart.

The difference between binocular and better monocular VA and CS was calculated for each subject and referred to as binocular difference. The interocular difference was calculated as better eye-worse eye for VA and CS. Significant negative correlations (P < 0.01) were observed between binocular difference and interocular difference for VA (r = -0.84) and CS at each spatial frequency of 1.5 (r = -0.90), 3 (r = -0.85), 6 (r = -0.79), 12 (r = -0.68), and 18c/d (r = -0.72). The greater the difference between the eyes, the lesser the binocular summation. Linear regression predicted a 1-line decrease in binocular summation for a 4-line increase in interocular acuity difference and a 35% decrease in binocular summation for a 50% increase in interocular difference of contrast sensitivity at 3c/d (ANOVA, P < 0.01).

Binocular inhibition in post-LASIK subjects increases as the visual sensitivity difference between the two eyes increases. Since the peak of human contrast sensitivity function is at the middle spatial frequencies, the decrease in binocular CS at 3c/d due to reduced CS in one eye is functionally significant.