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Effect of Positive and Negative Defocus on Contrast Sensitivity in Myopes

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Effect of Positive and Negative Defocus on Contrast Sensitivity in Myopes View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Vision Research 44 (2004) 1869–1878 www.elsevier.com/locate/visres Effect of positive and negative defocus on contrast sensitivity in myopes and non-myopes Hema Radhakrishnan *, Shahina Pardhan, Richard I. Calver, Daniel J. O’Leary Department of Optometry and Ophthalmic Dispensing, Anglia Polytechnic University, East Road, Cambridge CB1 1PT, UK Received 22 October 2003; received in revised form 8 March 2004 Abstract This study investigated the effect of lens induced defocus on the contrast sensitivity function in myopes and non-myopes. Contrast sensitivity for up to 20 spatialfrequencies ranging from 1 to 20 c/deg was measured with verticalsine wave gratings under cycloplegia at different levels of positive and negative defocus in myopes and non-myopes. In non-myopes the reduction in contrast sensitivity increased in a systematic fashion as the amount of defocus increased. This reduction was similar for positive and negative lenses of the same power (p ¼ 0:474). Myopes showed a contrast sensitivity loss that was significantly greater with positive defocus compared to negative defocus (p ¼ 0:001). The magnitude of the contrast sensitivity loss was also dependent on the spatial frequency tested for both positive and negative defocus. There was significantly greater contrast sensitivity loss in non-myopes than in myopes at low-medium spatial frequencies (1–8 c/deg) with negative defocus. Latent accommodation was ruled out as a contributor to this difference in myopes and non-myopes. In another experiment, ocular aberrations were measured under cycloplegia using a Shack– Hartmann aberrometer. Modulation transfer functions were calculated using the second order term for defocus as well as the fourth order Zernike term for sphericalaberration. The theoreticalmaximalcontrast sensitivity based on aberration data predicted the measured asymmetry in contrast sensitivity to positive and negative defocus that was observed in myopic subjects. The observed asymmetry in contrast sensitivity with positive and negative defocus in myopes may be linked to the altered accommodative re- sponse observed in this group. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Myopia; Contrast sensitivity; Defocus; Aberration; Accommodation 1. Introduction 1988). Recent reports also suggest that the accommo- dative response mechanism plays an important role in It is known that opticaldefocus guides severalvisual myopia development/emmetropization (Wildsoet & processes including accommodation and emmetropiza- Schmid, 2001). tion (Diether & Schaeffel, 1997; Kruger & Pola, 1987; It is well known that reduced accommodative re- Schmid & Wildsoet, 1997). The human eye constantly sponse to negative lenses occurs in myopes (Gwiazda, encounters opticaldefocus in the normalvisualenvi- Thorn, Bauer, & Held, 1993; O’Leary & Allen, 2001; ronment as a result of various factors including refrac- Seidel, Gray, & Heron, 2003). A possible explanation tive error and microfluctuations in accommodation. for the reduced accommodative response in myopes was Variation in the retinalimage quality with changing given by Jiang (1997). He proposed a modelof static levels of defocus is of considerable interest as the defo- accommodation and evaluated it by substituting mea- cused image is thought to provide feedback for emme- sured accommodative response values from a group of tropization. In emmetropes the feedback mechanism is late-onset myopes and emmetropes. This model pre- considered to act normally and guide the growth of the dicted that the lowered accommodative response in eye such that there is minimalrefractive error (Hung, myopes is due to a reduction in blur sensitivity. Since Crawford, & Smith, 1995; Shaeffel, Glasser, & Howland, then, some researchers have investigated blur sensitivity in myopes directly. * Corresponding author. Tel.: +44-1223-363271x2237; fax: +44- Rosenfield and Abraham-Cohen (1999) measured 1223-417712. defocus thresholds in myopes and emmetropes with the E-mail address: [email protected] (H. Radhakrishnan). BadalOptometer system. Adultsubjects were asked to 0042-6989/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.visres.2004.03.007 1870 H. Radhakrishnan et al. / Vision Research 44 (2004) 1869–1878 report when they first noticed a difference in clarity be- an overestimation of the aberrations in myopes in this tween two parts of a bipartite target when the movable study. If indeed myopes have higher magnitudes of half of the target was oscillated. Their results showed ocular spherical aberrations, this should result in a significantly higher defocus detection thresholds in more negative focus for peak contrast at low-medium myopes (±0.19D) when compared to non-myopes spatialfrequencies in this group (Charman & Jennings, (±0.11D). However, they did not differentiate between 1976). blur thresholds for target moved towards and away So far, no study has compared contrast sensitivity from the subject, which would simulate positive and with positive and negative defocus in myopes and non- negative defocus, respectively. myopes. We investigated the effect of lens-induced de- Schmid, Iskander, Li, Edwards, and Lew (2002) focus on contrast sensitivity in myopes and non-myopes. measured blur thresholds in children with simulated blur The study was conducted in three parts. In the first part targets obtained by Fourier transformation of the we examined the effect of sign of defocus on contrast opticaltransfer function. They found no significant sensitivity; in the second part we studied the effect of difference in thresholds between myopes and non-myo- different magnitudes of defocus on contrast sensitivity, pes for simulated positive and negative defocused tar- and thirdly we predicted the contrast sensitivity of the gets. This study does not provide information on the subjects based on the aberration data. Our results re- interaction between defocus and the eye’s opticalprop- vealed significant differences in contrast sensitivity in the erties, but shows that myopic children do not have an presence of positive and negative lens-induced defocus advantage over emmetropes in interpreting details in a between myopes and non-myopes. picture that have been blurred by external factors. The spatialfrequency content of the target is an important characteristic affecting accommodation 2. Methods (Charman & Heron, 1979; Owens, 1980) and emme- tropization (Schmid & Wildsoet, 1997). Studies on 2.1. Part 1: Effect of type of defocus on contrast contrast sensitivity with defocus show that the optimum sensitivity focus is dependent on spatialfrequency (Green & Campbell, 1965). The optimum focus is more myopic for 2.1.1. Subjects low and medium spatial frequencies relative to the high Eight myopic and eight non-myopic subjects took spatialfrequencies, a resultthat is attributed to ocular part in the study. The relevant information about the aberrations (Green & Campbell, 1965). Equal magni- subjects is given in Table 1. All subjects had visual tudes of positive and negative defocus can therefore acuity of at least 6/5. Subjects with )1.00D myopia or result in different thresholds in the presence of spherical more following cycloplegia were included in the myopic aberration. Charman and Jennings (1976) and Jansonius group, and those with cycloplegic spherical equivalent and Kooijman (1998) calculated the effect of spherical refractive error ranging between )0.25D and +1.25D aberration on the modulation transfer function and were considered non-myopic. The subjects included in found that in the presence of sphericalaberration the the non-myopic group had non-cycloplegic refractive modulation transfer function for intermediate spatial error ranging between Plano to +0.50D. All subjects frequencies is much higher with negative defocus when were screened to exclude astigmatism greater than compared to positive defocus. 1.25D, myopic retinaldegeneration, amblyopia or any Some previous studies which have measured aber- ocular disease. rations have shown that the ocular aberrations are The measurements were carried out on the left eye higher in myopes when compared to emmetropes (He only. Two drops of cyclopentolate hydrochloride 1%, et al., 2000, 2002). On the other hand, Cheng, Bradley, were instilled with a 3 min interval in the left eye. One Hong, and Thibos (2003) found that myopic eyes do drop of cyclopentolate hydrochloride 0.5% was instilled not have significantly different amounts of monochro- every 2 h during the experimentaltrial.Thirty minutes matic aberrations compared with emmetropes. Al- after the instillation of the first drop of cycloplegic the though Collins, Wildsoet, and Atchison (1995) showed pupildiameter had increased to 7 mm or more. The that fourth order aberrations were lower in some refractive error was initially determined with a cyclo- myopic subjects compared to emmetropes, in a signifi- plegic AutoRefractor (Nidek AR600-A) reading fol- cant number of myopic subjects aberrations were so lowed by a full subjective refraction (to an accuracy of great that measurement was not possible. Applegate ±0.12D) with an artificialpupil(6 mm diameter). The (1991) using a subjective single-pass aberroscope had refraction was determined for both 1 and 6 m test dis- also found dramatically increased coma and spherical tance. The end point of refraction was duochrome bal- aberration in some myopic eyes. However, the failure to ance at 1 m and a
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