Abstract: PURPOSE: There are three perceptual states of binocular vision: fusion, suppression and double vision. Fusion and suppression facilitate the two eyes to combine their visual input into a single percept. If they fail, double vision can occur. The purpose of this study is to investigate whether reducing luminance in front of the fellow eye can facilitate fusion and reduce double vision in amblyopic observers and whether the effect of luminance reduction is related to the state of interocular interaction.
METHODS: Normal adults and Amblyopic adults with best-corrected vision participated in this study. A novel 4-AFC binocular rivalry paradigm was used to test their visual function. There were four possible responses: right-tilt, left-tilt, fusion and mixed perception (i.e., double vision). An ND filter was placed in front of the fellow eye of amblyopic observers and the dominant eye of normal observers. Subjects reported their responses via continuous keypress. The duration of the keypress for each response before and after applying ND filters was then compared to see how luminance reduction could affect four perceptual durations. Besides, we adjusted the level of interocular suppression in amblyopic observers by reducing the contrast of stimuli shown to the fellow eye. We then further measured the sensitivity of ocular dominance change to monocular luminance reduction when the visual input of two eyes was intact or imbalanced in amblyopia.
RESULTS: During the reduction of luminance in the fellow eye, the perceptual imbalance between the two eyes was relieved in the amblyopic group. And the duration of fusion, rather than mixed perception, significantly increased relative to that in the baseline. Moreover, patients with lower interocular suppression levels would show greater ocular dominance change to monocular luminance reduction.
CONCLUSIONS: Our findings demonstrate that the reduction of monocular luminance can not only bring interocular balance but also promote fusion in amblyopia. Meanwhile, the effect of monocular luminance attenuation on ocular dominance is related to observers’ state of interocular interaction.

Key words: Amblyopia, luminance, fusion, mixed perception, interocular suppression