Ebbinghaus Illusion Distortions in Amblyopia: Impairments of Visual Size Processing and Interocular Integration

The perceived size of an object can be manipulated by its surroundings. In the current study, we wanted to investigate whether amblyopic patients have normal object size perception and how their perception is affected by abnormal interocular interactions.

Sixteen adult amblyopes and 16 controls participated in this experiment. We measured the Ebbinghaus illusion magnitude with large and small inducers under one binocular, two monocular, and four dichoptic viewing configurations. Exploratory factor analysis was used to extract common structures and underlying mechanisms in different conditions.

The mean Ebbinghaus illusion magnitude in amblyopia was smaller with large inducers (F1,30 = 5.874, P = 0.021) and larger with small inducers (F1,30 = 5.814, P = 0.022) than in controls. For the small-inducer geometry, the interaction effect between groups and viewing configurations (F6,180 = 4.472, P < 0.001) was significant. Compared to the control population, the exploratory factor analysis in amblyopia extracted three more factors mainly accounting for dichoptic viewing configurations.

Amblyopes tended to overestimate the object size relative to controls in the Ebbinghaus illusion, thereby amplifying it in the small-inducer geometry and diminishing it in the large-inducer geometry. Our factor analysis revealed a limited interocular transfer of the Ebbinghaus illusion in amblyopia, suggesting a defective mechanism in local binocular integration circuitry.