Effect of Blur Adaptation on Defocus Blur Sensitivity in Myopia

Abstract

Purpose: : Previous studies have demonstrated significant improvement in visual acuity in myopes after sustained periods of defocus blur in the distance, thus suggesting rapidly–acting neural compensation for a portion of the blur (i.e., blur adaptation). However, the effects of blur adaptation on defocus blur sensitivity in myopes remains unknown.

Methods: : Changes in three blur thresholds (i.e., noticeable, bothersome, and non–resolvable blur), and the subjective point of best focus, were assessed monocularly after 1 hour of blur adaptation in myopes, who viewed a far target binocularly through +2.50D spherical lenses worn over the distance refractive correction. To assess blur adaptation, changes in visual acuity were measured with an ETDRS LogMAR chart. A Badal optical system was used to present either an isolated 20/50 Snellen E (∼0.2 degrees H) or 20/50 lines of text (7 degrees H). Eight visually–normal adult myopes (–2 to –8D) were tested with their accommodation paralyzed. The psychophysical ascending method of limits was employed. Blur values were referenced to the subjective point of best focus.

Results: : All subjects exhibited blur adaptation, with a significant improvement in group mean visual acuity of –0.16 (LogMAR). There was a consistent and concurrent significant decrease in all three blur thresholds for the isolated 20/50 E. It was 0.15D for noticeable blur, 0.19D for bothersome blur, and 0.19D for non–resolvable blur, respectively. In addition, there was a 0.29D hyperopic shift in the subjective point of best focus after blur adaptation. For the 20/50 text, however, there was no significant effect of blur adaptation on either the blur thresholds or the subjective point of best focus, with large intersubject variability evident.

Conclusions: : Defocus blur sensitivity in myopes increased after blur adaptation for the isolated foveal 20/50 E. This may be attributed to the concurrent improvement of foveal visual acuity. This enhanced blur sensitivity explains a recent finding of increased accommodative accuracy following blur adaptation in myopes. A possible mechanism to explain the absence of blur threshold changes for the extended text target is as follows: since blur sensitivity decreases with retinal eccentricity, the same amount of retinal defocus will produce less blur in the periphery than in the fovea, and hence blur adaptation will be smaller and perhaps less consistent in the peripheral retina. Lastly, the shift in the subjective point of best focus suggests a neurological recalibration of the optimal point for image clarity.

Keywords: adaptation: blur • myopia • perception