Fixational saccadic eye movements are altered in anisometropic amblyopia

Purpose: Amblyopia develops during a critical period in early visual development and is characterized by reduced visual sensory functions and structural reorganization of the brain. However, little is known about oculomotor functions in amblyopes despite the special role of eye movements in visual perception, task execution and fixation. Therefore, we studied the relationship of visual deficits in anisometropic amblyopia and fixational saccadic eye movements. Methods: We recruited twenty-eight anisometropic amblyopes and twenty-eight age-matched control subjects. Using a highspeed eye-tracker, fixational eye-movements of both eyes were recorded. A computerized fixational saccadic component analysis of eye-movement waveforms was developed to quantify the parameters of fixational saccades (FSs) and a simulation model was developed to help explain the FS performances. Results: Amblyopic eyes, but not control eyes, showed fewer FSs, but these had increased amplitudes, increased peak velocities, and longer inter-saccadic intervals. The reduced FSs occurred mainly in the 0- to 0.6-degree amplitude range, and the probability of FSs with larger amplitudes and longer inter-saccadic intervals was significantly higher than in controls. A new simulation model analysis suggests that an excitatory-inhibitory activity imbalance in superior colliculus may explain these FSs changes. Conclusions: We propose that the abnormal visual processing and circuitry reorganization in anisometropic amblyopia has an impact on the fixational saccade generation. We see two possible interpretations: (i) altered FSs may be an attempt of the visual system to adapt to the deficit, trying to capture more information from a broader spatial domain of the visual world so as to enhance the contrast sensitivity to low spatial frequencies viewed by the amblyopic eye, or (ii) it may be the cause of amblyopia or a contributing factor to the original deficit that aggravates the early deprivation.