The effects of simulated lens yellowing and opacity on detection and resolution of short wavelength gratings in the peripheral retina.

Abstract

Abstract: : Purpose: Contrast sensitivity mediated by short–wavelength sensitive (SWS) cones might be significantly affected by the ageing of the crystalline lens. We separately simulated the effects of lens yellowing and opacification on SWS–cone acuity and contrast sensitivity for both detection and resolution tasks. We wanted to determine which task, if any, is more affected by the simulated ageing of the lens. Methods: Two subjects (29 yrs and 54 yrs) were tested. The SWS–isolating stimuli were blue sinusoidal gratings (1.7 log Td) superimposed on a bright yellow field (4.7 log Td) and presented at 8 deg eccentricity. A set of yellow filters of increasing short–wavelength absorption was applied to simulate the typical spectral absorption changes of the lens with age. A set of white opaque filters of increasing density was used to simulate the increased light scatter with age. Acuity and contrast sensitivity at the high spatial frequency limb were measured under each condition for detection task (detection of the grating contrast) and resolution task (orientation identification). Factorial analysis of variance was used to assess statistical significance. Results: The lens yellowing simulation affected the contrast sensitivity for both subjects, more significantly the older subject (0.1 log unit on average with the strongest filter), compared with the younger subject (0.06 log unit). The acuity declined slightly only for the older subject. No systematic change of the detection/resolution ratio was found with the increased short–wavelength absorption. The simulation of lens opacity produced results typical for a loss of contrast due to the optical image deterioration. Both acuity and contrast sensitivity declined with the increased opacity to a greater extent compared to the decline caused by lens yellowing. The detection was more affected than resolution, thus reducing the detection/resolution ratio close to 1. Conclusions: The effect of the lens yellowing on detection and resolution SWS–contrast sensitivity is smaller than, and qualitatively different from, the effect of the simulated lens opacity. The first is related to the loss of SWS–cone contrast due to the selective short–wavelength absorption but not to optical deterioration, while the second is related to loss of contrast due to the increased light scatter.

Keywords: aging • contrast sensitivity • visual acuity