Binocular Imbalance in Amblyopia Depends on Spatial Frequency in Binocular Combination

PURPOSE: To assess the role of spatial frequency on binocular imbalance in binocular combination in adults with amblyopia. METHODS: Ten amblyopes (23 ± 4.9 [SD] years old; one deprivation, two mixed, seven anisometropia patients) and 10 age-matched normal adults (23 ± 2.3 years old) participated. The interocular contrast ratio (fellow eye/amblyopic eye, i.e., the balance point [BP]) that resulted in an equal contribution of both eyes in binocular combination was measured using a binocular orientation combination task at 0.5, 1, 2, and 4 cycles per degree (c/d). The extent of binocular imbalance was quantified as the absolute value of the BP on log scale (i.e., |logBP|). RESULTS: When the base contrast of the amblyopic eye was set at 100% (Experiment 1), the |logBP| was found to be significantly affected by stimulus spatial frequency (F(1.44, 26.01) = 51.6, P < 0.001, [Formula: see text] = 0.40) and group (F(1, 18) = 66.97, P < 0.001, [Formula: see text] = 0.74), the interaction between spatial frequency and group was also significant (F(1.44, 26.01) = 38.12, P < 0.001, [Formula: see text] = 0.33). Such spatial frequency–dependent binocular imbalance remained present, even when the base contrast of the amblyopic eye was set at equal suprathreshold contrast levels across spatial frequencies (Experiment 2). CONCLUSIONS: Binocular balance was more disrupted at higher spatial frequencies in binocular combination in amblyopia. This imbalance might not originate solely from the amblyopic eye's deficit in contrast sensitivity but is likely to be related to the difference in contrast sensitivity between the eyes.