Reduced amblyopic eye fixation stability cannot be simulated using retinal-defocus-induced reductions in visual acuity

Amblyopia is associated with impaired visual acuity (VA) and reduced fixation stability (FS). To assess whether impaired VA may cause reduced FS, the effects of retinal-defocus-induced visual acuity reductions on FS were measured in observers with amblyopia and controls. Fixational eye movements were measured in 8 patients with amblyopia and 12 controls. Monocular near VA of a subset of controls (n = 5) was then varied from 20/20 to 20/100 using convex lenses. The amblyopia group completed three monocular conditions; 1) amblyopic eye fixating, 2) fellow eye fixating and 3) fellow eye fixating with VA reduced to match the amblyopic eye. Fixational eye movements were quantified using bivariate contour ellipse area (BCEA) and microsaccadic amplitude. Amblyopic eye BCEA was significantly larger with increased microsaccadic amplitude compared to the fellow eye and control eyes. BCEA and VA were positively correlated for amblyopic eyes. VA impairments induced by retinal defocus did not reduce FS in controls or the fellow eye of observers with amblyopia, even when fellow eye VA was matched to that of the amblyopic eye. This suggests that reduced FS in amblyopic eyes cannot be simulated by acute VA reductions. Therefore, reduced amblyopic eye FS may not be a direct consequence of the VA loss alone. As in previous studies, a correlation between BCEA and VA for amblyopic eyes was observed. This relationship could be due to a third, mediating variable or an effect of fixational eye movements on VA.