The Effects of Deprivation Amblyopia on Fixation Stability and Optokinetic Nystagmus

Abstract

Vision and oculomotor control play an essential role in visual perception and motor coordination. Individuals born with unilateral cataracts ultimately develop deprivation amblyopia, which is associated with sensory deficits. By measuring two fundamental eye movements, fixation stability and optokinetic nystagmus, this study aims to understand how congenital cataracts impact oculomotor control. Fixation stability and OKN were evaluated using the Eyelink eye tracker during binocular and monocular viewing. An infrared filter was used in the monocular condition to block the visible light (i.e., open loop condition) but allowed recording of eye position. The fixation stability experiment utilized a 3° fixation crosshair, while the OKN test involved a black-and-white vertical square-wave grating moving at 10 deg/s. Eye dispersion during fixation was quantified using bivariate contour ellipse area (BCEA), as well as microsaccades rate, amplitude, and slow drifts. OKN response analysis involved calculating the slow-phase gain based on the velocity of the stimulus, and subsequently determining if the response in the fellow eye was symmetrical across nasalward and temporalward stimulus directions. The findings of 18 control participants showed that fixation stability was best during binocular viewing, as indicated by the lowest BCEA value. Fixation stability was poorer during monocular viewing, where the covered eye (open loop condition) exhibited the largest dispersion. Further data analysis revealed that the poorer fixation during monocular closed-loop viewing was explained by increased microsaccade rate and higher slow drift velocity, while the poorer fixation during the open-loop condition was explained by the increase in microsaccade amplitude. The patient group included 7 participants. The fellow eye had fixation that was similar to the control group across binocular and closed-loop monocular viewing, while it was poorer during monocular viewing for the open-loop condition. Amblyopic eye viewing was poorer compared to all viewing conditions in the control group. The OKN response was asymmetrical for 2 of the patients, with one patient showing no response in the temporalward direction, while all 18 control participants exhibited a symmetrical response that was similar for the nasalward and temporalward trials. This study has provided insight into oculomotor control in unilateral deprivation amblyopia. Further research that investigates the underlying neural mechanisms disrupted in oculomotor control in unilateral deprivation amblyopia can help to uncover more effective treatment options to improve the quality of life and sensorimotor deficits in individuals with deprivation amblyopia.