The time course of blur adaptation in emmetropes and myopes.

Purpose/Background This study examined the effect of myopic defocus on visual acuity (VA) over time, with attention being paid to the first point at which blur adaptation had a significant and measurable effect on defocused VA. Visual acuity was sampled at a higher rate than previous studies in order to assess the time course of blur adaptation processes in myopic and emmetropic observers. Methods Participants were 24 normally-sighted observers (12 emmetropes and 12 myopes, median age: 22.5years). All ametropic participants wore their full refractive correction throughout the experiment. 1 D and 3 D of myopic defocus were introduced in two separate, randomised sessions. Visual acuity was measured using Test Chart 2000 at 2min intervals over a 30min session whilst looking through defocus lenses. Recovery clear VA was also measured every 2min for a further 20min. Results Defocused VA was found to improve significantly within 4min after the introduction of defocus for both 1 D (P<0.0001) and 3 D conditions (P<0.0001). The improvements reached a plateau shortly after, with no significant further improvements in defocused VA after 6min. There were no significant differences found in the temporal blur adaptation profiles between emmetropes and myopes (P=0.267). Data were fitted with an exponential decay function; the lowest R2 value for this fit was 0.95. Conclusions Blur adaptation has a clinically significant and measurable effect on VA within 4min of exposure to defocus. This finding indicates that the visual system instigates the neural compensatory mechanisms shortly after the appearance of defocus. Our results relate particularly to real-life vision of uncorrected myopes or myopes who remove their correction for part of the day.