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D. Seidel, L.S. Gray, G. Heron; The Minimum Blur Threshold for Accommodation in Emmetropia and Myopia . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2726.
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Purpose: The objective blur threshold for an accommodation response is smaller (~0.1 D) than the ocular depth of focus (~0.3D). It has been reported that the subjective blur threshold is significantly reduced in myopia. The aim of this experiment is to compare the objective blur threshold for accommodation in emmetropia and myopia. Methods: 5 emmetropic and 7 myopic subjects took part with informed consent in this study. The emmetropes (mean age: 22.2 ± 5.3 years) had a mean refractive error of 0.18 D (range -0.12 to +0.37 D) whilst the myopic group (mean age: 21.9 ± 4.2 years) had refractive errors ranging from -0.75 to -5.00 D with a mean of -2.55 D. Accommodation was recorded continuously using a specially modified Canon R1 optometer. Pupil diameter was monitored concurrently to ensure that pupil size was always >4 mm. The stimulus consisted of a backlit high contrast Maltese cross target mounted on an XY plotter, which oscillated sinusoidally at a frequency of 0.6 Hz. This was viewed in a Badal system (+3 D) with auxiliary lens to incorporate the subject’s refractive error. The amplitude of the oscillations was varied randomly between 0D and 0.4D in 0.1D steps. For each trial 10 continuous measures of accommodation of 10 s duration were obtained and a mean power spectrum calculated. A response to the stimulus was defined as a statistically significant increase in power at the driving frequency compared to the power obtained from the stationary target. Results: 1. All emmetropic subjects produced significant responses to stimuli ≥ 0.1 D. 2. Early - onset myopes produced significant responses to stimuli ≥ 0.1 D. 3. Late-onset myopes responded to stimuli ≥ 0.3 D. Conclusions: 1. The objective blur threshold for an accommodation response is significantly greater in late-onset myopes compared with emmetropes and early-onset myopes. 2. This suggests that decreases in subjective blur sensitivity, previously reported in myopia, are paralleled by increases in the blur detection threshold of the accommodation controller. 3. These increases in the objective blur threshold might explain much of the accommodative dysfunction in late-onset myopia.
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