Contribution of Objective Depth of Focus to Reduced Blur Sensitivity in Myopes

Abstract

Purpose: : Depth of focus can be assessed by determining the limits of perceptual clarity of a target and subjective depth of focus has previously been found to be larger in myopes than emmetropes. This may result in less accurate accommodation control which has been implicated in the development of myopia. The change in target vergence that is required before a compensatory accommodative response is initiated gives an objective measure of depth of focus and is therefore not affected by individual differences in subjective perception of clarity. Objective depth of focus was investigated in different refractive error groups.

Methods: : Participants were classified following subjective refraction as emmetropes (n = 8; mean Rx 0.03D SD 0.34) low myopes (n = 5; mean Rx -2.58 D, SD 0.85) and moderate myopes (n = 6; mean Rx -6.19D, SD 2.1). Pupil sizes and accommodation responses were measured with the Plusoptix SO4 infra-red auto-refractor modified to record continuously with a sampling rate of 12.5 Hz. A target was viewed at 33cm then moved proximally or distally at a constant speed three times each from the starting position. Objective depth of focus (DOF) was determined by identifying the time point of the first change in accommodation in response to altered target vergence and this was converted to a dioptric distance from the starting position. Variability in accommodation response was examined by performing a Fast Fourier Transform using Origin 8.1 software to determine amplitude of fluctuations versus frequency.

Results: : Total DOF was significantly larger in moderate myopes than emmetropes (p = .05). Distal, proximal and total DOF were significantly negatively correlated with refractive error (p ≤ .036). Mean distal DOF was smaller (emmetropes: 0.27D; low myopes: 0.30D; moderate myopes: 0.38D) than mean proximal DOF (emmetropes: 0.34D; low myopes: 0.44D; moderate myopes: 1.01D) for each group. Moderate myopes showed more variable accommodation responses than emmetropes and greater variability between trials.

Conclusions: : The larger objective DOF in myopes shows that at least a portion of the higher subjective DOF reported previously is also shown by changes in the accommodation control system, indicating a neural control which is independent of subjective perception of blur. The relationship between the subjective and objective depth of focus is being investigated.

Keywords: myopia • accommodation • depth