The Initiation of Smooth Pursuit is Delayed in Anisometropic Amblyopia

Abstract

Purpose: Several behavioral studies have shown that the reaction times of visually guided movements are slower in people with amblyopia, particularly during amblyopic eye viewing. Here, we tested the hypothesis that the initiation of smooth pursuit eye movements, which are responsible for accurately keeping moving objects on the fovea, is delayed in people with anisometropic amblyopia.

Methods: Eleven participants with anisometropic amblyopia and 14 visually normal observers were asked to track a step-ramp target moving at ±15°/s horizontally as quickly and as accurately as possible. The experiment was conducted under three viewing conditions: amblyopic/nondominant eye, binocular, and fellow/dominant eye viewing. Outcome measures were smooth pursuit latency, open-loop gain, steady state gain, and catch-up saccade frequency.

Results: Participants with anisometropic amblyopia initiated smooth pursuit significantly slower during amblyopic eye viewing (206 ± 20 ms) than visually normal observers viewing with their nondominant eye (183 ± 17 ms, P = 0.002). However, mean pursuit latency in the anisometropic amblyopia group during binocular and monocular fellow eye viewing was comparable to the visually normal group. Mean open-loop gain, steady state gain, and catch-up saccade frequency were similar between the two groups, but participants with anisometropic amblyopia exhibited more variable steady state gain (P = 0.045).

Conclusions: This study provides evidence of temporally delayed smooth pursuit initiation in anisometropic amblyopia. After initiation, the smooth pursuit velocity profile in anisometropic amblyopia participants is similar to visually normal controls. This finding differs from what has been observed previously in participants with strabismic amblyopia who exhibit reduced smooth pursuit velocity gains with more catch-up saccades.

Figure 1

Desaccaded position (top graph) and velocity (bottom graph) plots of 20 leftward pursuit movements shown for the anisometropic amblyopia participant number 5 (red) and a representative visually normal participant (blue) during the amblyopic/nondominant eye viewing condition. The target trace is shown in black.

Figure 2

Mean smooth pursuit latencies of 11 participants with anisometropic amblyopia (black) and 14 visually normal individuals (white) shown for all three viewing conditions. The amblyopic eye latency was significantly longer than the nondominant eye latency in controls (*P = 0.002). Error bars indicate SEMs.

Figure 3

Mean open-loop smooth pursuit gains of 11 participants with anisometropic amblyopia (black) and 14 visually normal individuals (white) shown for all three viewing conditions. No comparisons were significant. Error bars indicate SEMs.

Figure 4

Mean steady state smooth pursuit gains of 11 participants with anisometropic amblyopia (black) and 14 visually normal individuals (white) shown for all three viewing conditions. No comparisons were significant. Error bars indicate SEMs.

Figure 5

Mean steady state smooth pursuit gains of 11 participants with anisometropic amblyopia (black) and 14 visually normal individuals (white) shown for the nasalward (solid bars) and temporalward (striped bars) pursuit movements during the two monocular viewing conditions. No comparisons were significant. Error bars indicate SEMs.