Impact of crowding on visual appearance and performance in amblyopia

Individuals with amblyopia have been reported to perceive stimuli as distorted with the amblyopic eye, and to have stronger foveal crowding than controls without amblyopia. Previous studies on visual appearance in amblyopia often lacked a control group and did not adjust stimulus size to account for visual acuity deficits. Here, we investigated whether observers with amblyopia perceived stimuli as more distorted and whether foveal crowding influenced visual appearance more strongly than in controls, while controlling for visual acuity differences. Isolated and flanked target letters were presented foveally to the dominant/fellow and non-dominant/amblyopic eye. Stimuli were scaled relative to each eye's acuity threshold and presented at 1.0× and 1.5× threshold size. Observers replicated target appearance by selecting squares on a novel grid interface. Results showed that accuracy, structural similarity (SSIM) and perceived similarity (PSS) were lower for observers with amblyopia compared to controls (Group), for small compared to large stimuli (Size), and for flanked compared to isolated targets (Crowding). Interestingly, a significant interaction between Group and Crowding was observed only for accuracy and SSIM, but not for PSS. These results showed group differences despite controlling for acuity, suggesting that factors beyond visual acuity, such as stronger foveal crowding, contribute to visual appearance differences in amblyopia compared to controls.