Abstract
Abstract: :
Purpose: A shift from binocular summation to binocular inhibition has been observed when there is significant decrease in contrast sensitivity or reduced retinal illuminance in one eye compared to the other as in case of unilateral cataract and amblyopia. This study aims to determine how binocular function in post–LASIK subjects with unsatisfactory outcomes is influenced by differences in visual acuity and contrast sensitivity between the two eyes. Methods: 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 VSRC. Photopic VA measured with ETDRS chart and sine–wave grating CS measured with FACT chart were converted to logMAR and log CS scores respectively for purposes of analysis. Results: The difference between binocular and better monocular VA and CS was calculated for each subject and is referred to as binocular difference, BD. The interocular difference, IO, is calculated as worse eye–better eye for VA and CS. Significant positive correlations (P<0.01) were observed between BD and IO 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). This means that greater the IO, greater the binocular inhibition. Linear regression was used to generate equations that predict the amount of binocular summation or inhibition from IO in VA and CS (ANOVA, P<0.01). VA, CS at 1.5c/d and at 3c/d have the highest R2 values of approximately 70–80%. The IO, exceeding which binocular inhibition occurs, was calculated. Binocular inhibition occurs for CS at 1.5c/d and 3c/d, when difference in CS between the two eyes exceeds roughly 0.15 log units while a larger difference is required at the higher spatial frequencies. For VA, an IO greater than approximately 3 lines can result in a lower binocular acuity. Conclusions: Binocular inhibition in post–LASIK subjects increases as the difference between the two eyes increases. A difference of just over 0.15 log units between the two eyes for CS at 1.5 and 3c/d can result in binocular inhibition. 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.
Keywords: contrast sensitivity • refractive surgery: optical quality • refractive surgery: complications