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Esther Gonzalez, Agnes Wong, Manokaraananthan Chandrakumar, Luke Gane, Martin Steinbach; Characteristics of Ocular Drifts During Fixation in Amblyopia. Invest. Ophthalmol. Vis. Sci. 2013;54(15):178.
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We previously showed that fixation in patients with amblyopia is unstable, and primarily due to ocular drifts, rather than microsaccades or blinks. In this study, we investigated the characteristics of these ocular drifts. We hypothesize that the spatiotemporal deficits in amblyopia are associated with increased ocular drift magnitude and velocity.
Using a video-based eye tracker, the positions of both eyes of 13 patients with amblyopia and 20 visually normal controls were recorded simultaneously in three viewing conditions: binocular, monocular with fellow eye viewing (right eye for controls), and monocular with amblyopic eye viewing (left eye for controls). For the monocular conditions, the movements of the covered eye behind the eye patch were also recorded (open loop testing). Fixation epochs were obtained by deleting saccades and blinks from the eye position records. For each group and viewing condition, we analyzed the mean drift magnitude; that is, eye eccentricity from fixation in Euclidean distance (deg) as well as the mean drift velocity (deg/s) during the epochs.
For mean eye eccentricity, there were no statistically significant differences between the fellow eye (FE) in patients with amblyopia and normal eye (NE) in control participants under all three viewing conditions. Eye eccentricity of the amblyopic eye (AE) was significantly higher than those of FE and NE whether it was the viewing eye (AE: 6.57±8.73, FE: 0.75±0.76, NE: 0.0.44±0.19), or the covered eye (AE: 7.34±8.34, FE: 3.01±3.19, NE: 1.86±1.16). Interestingly, during binocular viewing, although the eccentricity of AE was significantly higher than that of FE and NE (AE: 0.98±0.78, FE: 0.44±0.26, NE: 0.43±0.20), the magnitude of the difference was much smaller than in monocular viewing. Patients with amblyopia exhibited higher drift velocity than control participants. During binocular viewing, the drift velocity in both AE (0.91±1.06) and FE (1.05±1.57) was higher than that in NE (0.42±0.36). During monocular viewing, the FE exhibited higher drift velocity than AE and NE when it was the viewing eye; however, when the AE was the non-viewing eye, it exhibited higher drift velocity than FE and NE.
The ocular drift (and hence fixation instability) in patients with amblyopia is characterized by increased drift magnitude and velocity than normal, and this varies with viewing conditions.
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