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L. S. Gray, A. Demberg, D. Seidel, N. C. Strang; The Relative Contribution of Contour Interaction and Fixational Eye Movements to the Crowding Effect in Visually Normal Children. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5512.
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© ARVO (1962-2015); The Authors (2016-present)
The crowding effect is a term used to describe the combined effect of contour interaction and fixational eye movements upon visual acuity (VA). Previous work has shown that the crowding effect is greater in children, although the relative contribution of the two factors is unknown. The purpose of the present study is to quantify the contribution of contour interaction and fixational eye movements to the crowding effect in a group of visually normal children.
82 visually normal children (mean±SD age = 70±4 months) participated in the study. Informed consent was obtained from the parents after the purpose of the study had been explained to them. The effects of contour interaction and fixational eye movements upon VA were quantified using three test charts designed specifically to isolate these factors. These were 1. Single Letter Chart (SLC), 2. Linear Letter Chart (LLC) and 3. Repeat Letter Chart (RLC). All charts had a logarithmic progression with 5 letters presented at each acuity level. Letter spacing in charts 2 and 3 was always 0.5 of a letter diameter and the charts were all high contrast (90%). VA was measured with all charts which were presented randomly to each subject. Subjects were encouraged to keep guessing until no more letters could be identified. The VA scores were recorded per letter in standard logMAR format. The effect of contour interaction can be quantified as the difference between VA(RLC) and VA(SLC). Similarly the effect of fixational eye movements is quantified by VA(LLC) - VA(RLC). The total crowding effect is quantified by VA(LLC) - VA(SLC).
Mean±SD logMAR VA values for the 3 charts were as follows (data for RE): VA(SLC) = -0.02±0.09; VA(LLC) = 0.21±0.07 and VA(RLC) = 0.06±0.10. VA(LLC) was significantly worse than VA(SLC, p<0.01) and VA(RLC, p<0.01). VA(RLC) was significantly worse than VA(SLC, p<0.01). The effect of contour interaction (VA(RLC) - VA(SLC)) was 0.08±0.07 logMAR. The effect of fixational eye movements upon VA (VA(LLC) - VA (RLC)) was 0.15±0.07 logMAR. The total crowding effect (VA(LLC) - VA(SLC)) was -0.23±0.08 logMAR.
The crowding effect in visually normal children produces a reduction of about 2.5 lines of logMAR VA when using linear test charts designed to produce maximal crowding effect. This reduction in VA is produced by the combined effects of contour interaction (reduction of 1 line logMAR VA) and fixational eye movements (reduction of 1.5 lines logMAR VA). These results show clearly that test chart design is a critical parameter to consider in the measurement of VA in children.
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