Abnormal Fixational Eye Movements in Amblyopia

Abstract

Purpose: Fixational saccades shift the foveal image to counteract visual fading related to neural adaptation. Drifts are slow eye movements between two adjacent fixational saccades. We quantified fixational saccades and asked whether their changes could be attributed to pathologic drifts seen in amblyopia, one of the most common causes of blindness in childhood.

Methods: Thirty-six pediatric subjects with varying severity of amblyopia and eleven healthy age-matched controls held their gaze on a visual target. Eye movements were measured with high-resolution video-oculography during fellow eye-viewing and amblyopic eye-viewing conditions. Fixational saccades and drifts were analyzed in the amblyopic and fellow eye and compared with controls.

Results: We found an increase in the amplitude with decreased frequency of fixational saccades in children with amblyopia. These alterations in fixational eye movements correlated with the severity of their amblyopia. There was also an increase in eye position variance during drifts in amblyopes. There was no correlation between the eye position variance or the eye velocity during ocular drifts and the amplitude of subsequent fixational saccade. Our findings suggest that abnormalities in fixational saccades in amblyopia are independent of the ocular drift.

Discussion: This investigation of amblyopia in pediatric age group quantitatively characterizes the fixation instability. Impaired properties of fixational saccades could be the consequence of abnormal processing and reorganization of the visual system in amblyopia. Paucity in the visual feedback during amblyopic eye-viewing condition can attribute to the increased eye position variance and drift velocity.

Fig 1

An example of visual fixation during a 4-second epoch in a normal control (A) and a subject with severe amblyopia during fellow eye viewing condition (B) and amblyopic eye viewing condition (C). The top two traces plots horizontal and vertical eye position along the y-axis and time along the x-axis. The bottom trace depicts the composite vector of horizontal and vertical eye positions. The gray colored trace is the non-viewing eye whereas the blue, red and green trace represents the horizontal, vertical and composite vector of the viewing eye. Short arrows represent fixational saccades.

Fig 2

(A) Scatter plots of summary of the effects of severity of amblyopia on median amplitude of fixational saccades during fellow eye viewing condition (x-axis) and amblyopic eye viewing condition (y-axis). Each symbol depicts one subject whereas the symbol type represents severity of amblyopia. The dashed line is the equality line. (B, C) Summarizes the distribution of the fixational saccade amplitude during fellow eye viewing condition (B) and amblyopic eye viewing condition (C). A normalized number of events in a given bin is plotted along the y-axis while the x-axis represents the bins of fixational saccade amplitude of the viewing eye. The gray line illustrates the normalized frequency histogram of microsaccade amplitude in normal controls. The red, green and black lines represent mild, moderate and severe amblyopia subjects.

Fig 3

Scatter plots of summary of the effects of severity of amblyopia on median frequency of fixational saccades less than 1° (physiological microsaccades) (A) and fixational saccades of all amplitudes (B) during fellow eye viewing condition (x-axis) and amblyopic eye viewing condition (y-axis). Each symbol depicts one subject whereas the symbol type represents severity of amblyopia (cross symbol = mild amblyopia; square symbol = moderate amblyopia and triangle symbol = severe amblyopia. The dashed line is the equality line.

Fig 4. Comparison of amplitude to velocity relationship of physiological fixational saccades less than 1 degree amplitude (microsaccades, green symbols) and those with pathologically increased amplitudes in amblyopia (red symbols).

Both symbols fall on the same continuum suggesting a common neural mechanism generating both types of fixational eye movements. Summary of all fixational saccades from all subjects are illustrated. Panel A depicts mild amblyopia; B is moderate; and C is severe amblyopia.

Fig 5

The variance of composite eye position of the viewing and non-viewing eye during fellow eye (A) and amblyopic eye viewing (B) conditions. In both cases, each symbol is one drift (epoch of eye position between two consecutive fixational saccade). Each color represents the severity of amblyopia (gray: mild; blue: moderate; red: severe). The dashed line is the equality line. Panel C plots the median variance of the non-viewing eye during fellow eye viewing and amblyopic eye viewing conditions. Each symbol represents a subject where the symbol type indicates the severity of amblyopia. Box and whisker plots of the effects of severity of amblyopia compared to normal controls on the composite variance in eye position during fellow eye viewing condition (D) and amblyopic eye viewing condition (E). The length of each box depicts the interquartile interval; notch suggests 95% confidence interval around the median (if the two notches of a box-whisker plot do not overlap it offers evidence of a statistically significant difference between the medians) and the horizontal line in the center of the notch is the median value. Overlapping notches indicate a lack of difference in the median values of two populations.