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Sevda Agaoglu, Avigael M Aizenman, Preeti Verghese, Dennis M Levi; Similar multiplicative improvements in fixation stability in normal vision and amblyopia. Invest. Ophthalmol. Vis. Sci. 2018;59(9):5793. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Amblyopia is a developmental disorder, which leads to impaired form vision as well as oculomotor anomalies including eccentric and unsteady fixation. In this study, we investigated the effect of the presence of visual cues on fixation stability for amblyopic and normally sighted populations.
Five individuals with amblyopia (2 strabismic, 2 anisometropic, and 1 mixed) and 11 normally sighted individuals (the control group) participated in this study. The observers were asked to look at the center of a gray blank screen with or without a fixation target (0.3 deg in diameter) at the center. The control group completed the task binocularly whereas amblyopic observers were tested under both binocular and monocular viewing conditions. The target luminance was adjusted for amblyopic observers such that the target was visible 100% of the time regardless of the viewing eye. Eye movements were recorded with an Eyelink II eye tracker. The duration of a single trial was 30 sec, and each session for the target and no-target conditions consisted of 10 trials.
The 68% iso-line area (ISOA) values, similar to the to more-commonly used bivariate contour ellipse metric but without the normality assumption, were calculated as a measure of fixation stability. As expected, we found that the fixation stability becomes worse without a fixation target. In addition, fixation stability was worse for amblyopic observers when they viewed the stimulus monocularly with the amblyopic eye compared to binocular viewing or viewing with the fellow eye. However, when we computed the ISOA ratio in the no-target and target conditions, the differences between the controls and amblyopic observers vanished, regardless of whether the latter group viewed stimuli binocularly or with their amblyopic eye.
Fixation stability under the no-target condition, where observers try to maintain steady fixation without nearby visual cues, can be considered as a proxy for the internal noise of the oculomotor system. From this perspective, our results suggest that the presence of the target helps to reduce the internal noise, and more importantly, that this effect is multiplicative. Interestingly, this mechanism works the same for amblyopic and normally sighted individuals; improvements in fixation stability due to the presence of a target can be explained by a single multiplicative factor.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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