Fixational Eye Movement Waveforms in Amblyopia: Characteristics of Fast and Slow Eye Movements

Abstract

Fixational eye movements comprise of fast microsaccades alternating with slow intersaccadic drifts. These physiologic eye movements play an important role in visual perception. Amblyopic patients are known to have fixation instability, particularly of the amblyopic eye. We examined eye movement abnormalities that contribute to this instability. We found that fixation stability is affected by the presence of fusion maldevelopment nystagmus (FMN). However, some amblyopes can have nystagmus without nasally directed slow phases and reversal in direction of the quick phase on ocular occlusion, features seen in FMN. In patients without nystagmus, we found increased amplitude of fixational saccades and inter-saccadic drifts. We categorized amblyopia patients by type (anisometropic, strabismic, or mixed) and eye movement waveform (no nystagmus, nystagmus without FMN, and FMN). We found specific fast and slow eye movement abnormalities of the fellow and amblyopic eye during fellow, amblyopic and both eyes viewing conditions across eye movement waveforms and types of amblyopia. These eye movement abnormalities can serve as biomarkers that can predict the impact of amblyopia as measured by visual acuity and stereopsis. Evaluation of fixational eye movements in amblyopia could be important to diagnose these common eye diseases and predict treatment effectiveness.

Keywords: Eye movement; binocular viewing; fixational stability; microsaccades; saccades.

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Figure 1.

Representative raw data tracings of fixational eye movements under different viewing conditions in a healthy control (N14) (A), severely amblyopic patient without nystagmus (S23) (B), amblyopic patient with nystagmus but no FMN (S30) (C) and a patient with FMN (S44) (D). In the healthy control (A) the horizontal (red and blue) and vertical eye (magenta and cyan) positions of the right and left eye are plotted on the y-axis while x-axis depicts corresponding time. In patients without nystagmus (B) the blue and cyan traces are horizontal and vertical eye positions of the fellow eye and the red and magenta traces are horizontal and vertical eye position of the amblyopic eye respectively. The black arrows represent fixational saccades whereas the red arrows represent the inter-saccadic drifts. In the patient with nystagmus without FMN (C), the red and magenta traces represent horizontal and vertical eye positions of the fellow eye and the blue and cyan represents horizontal and vertical eye positions of the amblyopic eye. The black arrows depict quick phases of the nystagmus and the red arrows depict the slow phases. Fig (D) represents horizontal and vertical eye positions of the fellow (red and magenta) and amblyopic eye (blue and cyan) of patients with fusion maldevelopment nystagmus. The black arrows depict the quick phases of nystagmus. The positive excursion on horizontal eye position = rightward movement and positive excursion on vertical eye position = downward movement.

Figure 2.

Representative horizontal and vertical eye position data in healthy controls (A) and subjects with amblyopia and no nystagmus (B), nystagmus but no FMN (C), and FMN (D) under fellow, amblyopic, and both eyes viewing conditions. BCEA values (in parentheses) provide a measure of fixational stability, or scatter. The greatest BCEA values are noted under amblyopic eye viewing conditions in both the viewing and non-viewing eye. Interestingly, greater fixational instability is also noted in the amblyopic eye of subjects under binocular viewing. This suggests that fixational stability is affected in amblyopia even when subjects are using both eyes.

Figure 3.

Mean and standard deviation of BCEA values in subjects categorized by fixation eye movement waveforms. Under both eyes viewing condition (A), the amblyopic eye had less stable fixation (positive values) compared to controls (negative values) irrespective of the waveform type. Under fellow eye viewing condition (B), no differences were seen in the viewing and non-viewing eye between controls and amblyopic patients. Under amblyopic eye viewing condition (C), the BCEA values were greater in both the viewing and non-viewing eye. *=p<0.05, one way ANOVA

Figure 4.

Mean and standard deviation of eye movement parameters of fixational saccade amplitude, position variance, and drift velocity in subjects across eye movement waveforms under both eyes viewing condition. Composite amplitude (A) of fixational saccades in controls and patients without nystagmus and quick phases of patients with nystagmus shows increased amplitude in both the fellow and amblyopic eye of amblyopes. Similarly, position variance (B) of drifts in controls and patients without nystagmus and slow phases of patients with nystagmus is also increased in both the fellow and amblyopic eye, although the increase is more pronounced in the amblyopic eye. The drift velocity (C) in controls and patients without nystagmus and slow phase velocities in patients with nystagmus is also increased in amblyopes compared to controls. Those with and without nystagmus have greater velocity compared to controls. *=p<0.05, one way ANOVA

Figure 5.

Mean and standard deviation of eye movement parameters of fixational saccade amplitude, position variance, and drift velocity in subjects across eye movement waveforms under fellow and amblyopic eye viewing conditions. Error bars represent one standard deviation. Composite amplitude (A) of fixational saccades in controls and patients without nystagmus and quick phases of patients with nystagmus shows increased amplitude in amblyopes under both fellow and amblyopic eye viewing. Similarly, position variance (B) of drifts in controls and patients without nystagmus and slow phases of patients with nystagmus is also increased. The drift velocity (C) in controls and patients without nystagmus and slow phase velocities in patients with nystagmus is also increased in amblyopes compared to controls. *=p<0.05, one way ANOVA

Figure 6.

Mean and standard deviation of disconjugacy of composite amplitude of fixational saccades in controls and patients without nystagmus, and quick phases in patients with nystagmus across eye movement waveforms during both eyes viewing (A), fellow eye viewing (B) and amblyopic eye viewing (C) conditions. Amblyopic patients have increased disconjugacy under all viewing conditions. *=p<0.05, one way ANOVA