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A. Demberg, L.S. Gray; The Relative Contribution of Fixational Instability and Contour Interaction to the Crowding Effect in Amblyopic and Non–Amblyopic Eyes . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5364.
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The crowding effect has two main components; contour interaction and fixational instability. Crowding is known to be exaggerated in amblyopic eyes. Previous studies have not examined the separate components of the crowding phenomenon. The aim of this study was to measure the relative contribution of contour interaction and fixational instability to the crowding phenomenon in both amblyopic (A) and non–amblyopic (NA) eyes.
20 subjects (mean ± SD age 43.2 ± 13.1 years) with amblyopia participated with informed consent in the study. To isolate contour interaction and fixational instability three test charts were constructed for use in the study. All charts were high contrast (80%) and letter spacing was set at half a letter diameter in all charts to ensure a constant level of contour interaction. The charts all employed a logMAR scale and all had 5 letters at each level. Letters were chosen from a set of 10 letters of approximately equal legibility. The first chart was a linear chart, the second chart was a repeat letter chart similar to that proposed by Regan, and the third chart was a single letter chart. The difference in corrected acuity between the amblyopic and the non–amblyopic eyes for all subjects was more than five letters as measured with the Bailey–Lovie chart. Subjects wore their full spectacle correction for testing.
The total mean reduction in logMAR acuity due to the crowding effect was A: 0.30 ± 0.26 (range 0 to 0.68) log units; NA: 0.09 ± 0.05 log units. The mean reduction in logMAR acuity due to contour interaction was A: 0.13 ± 0.13 (range 0 to 0.34) log units; NA: 0.04 ± 0.03. The mean reduction in logMAR acuity due to unsteady fixation was A: 0.18 ± 0.17 (range 0 to 0.52); NA: 0.05 ± 0.02 log units. The effect of crowding in the amblyopic eyes showed considerable variation between subjects due to variations in either the degree of contour interaction or fixational instability.
Accurate test chart design allows the measurement of the relative contribution of fixational instability and contour interaction to the overall crowding effect. The effect of crowding upon logMAR acuity in amblyopic eyes varies considerably between individuals. The relative contribution of contour interaction and fixational instability to the overall crowding effect varies widely between amblyopic subjects. These findings may have implications for the monitoring of the amblyopic eye during treatment.
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