Effects of anisometropic amblyopia on visuomotor behavior, I: saccadic eye movements

Abstract

Purpose: Impairment of spatiotemporal visual processing is the hallmark of amblyopia, but its effects on eye movements during visuomotor tasks have rarely been studied. Here the authors investigate how visual deficits in anisometropic amblyopia affect saccadic eye movements.

Methods: Thirteen patients with anisometropic amblyopia and 13 control subjects participated. Participants executed saccades and manual reaching movements to a target presented randomly 5° or 10° to the left or right of fixation in three viewing conditions: binocular, amblyopic, and fellow eye viewing. Latency, amplitude, and peak velocity of primary and corrective saccades were measured.

Results: Initiation of primary saccades was delayed and more variable when patients viewed monocularly with their amblyopic eye. During binocular viewing, saccadic latency exhibited increased variability and no binocular advantage in patients (i.e., mean latency was similar to that during fellow eye viewing). Mean amplitude and peak velocity of primary saccades were comparable between patients and control subjects; however, patients exhibited greater variability in saccade amplitude. The frequency of corrective saccades was greater when patients viewed with their fellow eye than it was with binocular or amblyopic eye viewing. Latency, amplitude, and peak velocity of corrective saccades in patients were normal in all viewing conditions.

Conclusions: Saccades had longer latency and decreased precision in amblyopia. Once saccades were initiated, however, the dynamics of saccades were not altered. These findings suggest that amblyopia is associated with slower visual processing in the afferent (sensory) pathway rather than a deficit in the efferent (motor) pathway of the saccadic system.

Figure 1

Representative eye velocity profiles from individual trials from a control subject and a patient with anisometropic amblyopia when they made saccades to a 10° target to the right during binocular viewing (a) and monocular viewing (b) with the left eye in the control subject and with the amblyopic eye in the patient. The target was displayed on the monitor throughout the trial (a) or disappeared at the onset of hand movement (b). Visual feedback of the target had no significant effect on any saccade outcome measures. Saccade latency was increased and more variable in the patient during both binocular and monocular (amblyopic eye) viewing conditions than in the control subject.

Figure 2

Mean latency of primary saccades for control subjects and patients in each viewing condition: binocular, monocular fellow eye, and monocular amblyopic eye viewing. For control subjects, viewing was binocular, monocular left eye, and monocular right eye. Saccade initiation was significantly delayed when patients viewed with their amblyopic eye (P = 0.0015). No binocular advantage was found when patients viewed binocularly compared with control subjects. Error bars, ±1 SEM.

Figure 3

Mean variability in the latency of primary saccades for control subjects and patients in each viewing condition: binocular, monocular fellow eye, and monocular amblyopic eye viewing. For control subjects, viewing was binocular, monocular left eye, and monocular right eye. Variability in latency increased significantly (P = 0.0070) for patients in all viewing conditions compared with controls. Error bars, ±1 SEM.

Figure 4

Variability in amplitude of primary saccades for control subjects and patients in each viewing condition: binocular, monocular fellow eye, and monocular amblyopic eye viewing. For control subjects, viewing was binocular, monocular left eye, and monocular right eye. Variability in saccade amplitude was significantly greater (P = 0.0379) in the patient group in all viewing conditions. Error bars, ±1 SEM.