Visual and vestibular sources of fixation instability in amblyopia.

Extraretinal and visual afferent sources of ocular fixation instability were investigated in a group of strabismic amblyopes. Extraretinal drift-bias was revealed by fixational eye movements in darkness. The resulting dark drift-bias was highly correlated with an imbalance of the vestibulo-ocular reflex (VOR). No significant difference in dark drift-bias or VOR imbalance was found between normal and amblyopic observers. Visual afferent sources of fixation instability were revealed by after-effects of nasalward and temporalward retinal image motion upon ocular drifts in the dark (motion after nystagmus) (MAN). Nasalward biases of MAN were significantly greater in amblyopic than nonamblyopic subjects. Directional biases of optokinetic nystagmus in amblyopia were accounted for by normal unindirectional extraretinal drift sources and by an abnormal visual afferent nasal drift-bias of the fixating eye coupled with reduced sensitivity for detecting errors of retinal slip.