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Y.C. Tea, E.E. Hartmann, B. Jiang; Objective Measure of Defocus Thresholds: Differences in the Transient VEP Between Individuals With Emmetropia vs. Myopia . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2715.
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Purpose: Previous studies looking at defocus thresholds across individuals with different refractive errors are based on subjective measurements. The purpose of this study is to use the transient visually evoked potentials (VEP) to objectively determine the defocus threshold and evaluate the differences in this threshold between individuals with emmetropia and myopia. Methods: Eight optometry students have participated in this study to date: 4 with emmetropia, 4 with myopia. Transient VEPs were measured to contrast-reversing checkerboard patterns using the VENUS system. Testing was conducted monocularly (OD only) at a viewing distance of 1m. The subject wore no correction (for emmetropes) or his/her best correction (for myopes). VEPs were recorded under 4 defocus conditions, using lenses as follows: +1.00, +1.25, +1.50, and +1.75 D. Amplitude and latency of the VEP were measured for each viewing condition. Results: The VEP latencies are constant across defocus conditions for each subject. The peak amplitude of the VEP waveforms vs. defocus is plotted for each subject. The slope of a linear regression fit to the data points is used to estimate the defocus threshold of the subject. The average slopes for emmetropes and myopes are -13.42 ± 5.37(S.D.) (µv/D) and -6.64± 5.98(S.D.) (µv/D), respectively. While the differences between these slopes are not statistically significant, only one of the four individuals with myopia showed a much lower slope than the other three subjects, which may account for these results. Conclusions: This study suggests that the transient VEP may be used to objectively estimate an individual’s defocus threshold. The data are suggestive that individuals with myopia are less sensitive to defocus or blurred image than individuals with emmetropia. If these findings are confirmed with additional subjects, the results support the defocus theory for the development of myopia.
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