Contrast detection in noise with positive and negative defocus in myopes

Purpose: Previous studies have shown that myopes produce an asymmetrical loss in contrast sensitivity with positive and negative defocusing lenses at low-medium spatial frequencies (1–7 c/deg) compared to non-myopes. The measurement of contrast detection in noise allows any given loss in visual sensitivity to be attributed to two components namely sampling efficiency and/or equivalent noise. Previous work has also shown that sampling efficiency gives an indication of the neural effects and in the absence of sampling efficiency changes, changes in equivalent noise levels are considered to be due to optical factors. We investigate whether the asymmetrical loss shown by myopes to defocus can be explained by changes in sampling efficiency or equivalent noise, or both. Methods: Contrast thresholds in four different levels of externally added noise were measured in five myopic and five non-myopic subjects for Gabors of 3 c/deg. Measurements were obtained under cycloplegia with 0 D, ±0.50 D, and ±1.00 D defocusing lenses. RMS wavefront aberrations were also measured using a Shack-Hartmann aberrometer. Results: In the absence of noise, there was no significant difference in contrast sensitivity without defocus between myopes and emmetropes. When sampling efficiency and equivalent noise were measured from Contrast Detection in Noise functions, analysis of variance showed no significant difference in sampling efficiency between myopes and non-myopes (p = 0.145). Myopes showed maximum levels of equivalent noise with +1.00 D defocus, decreasing to a minimum with −1.00 D defocus (p = 0.009). Non-myopes showed a symmetrical increase in equivalent noise from zero for both positive and negative defocus. A significant correlation was found between equivalent noise and the total RMS of Wavefront aberrations of the eye (p = 0.026). Conclusion: Optical factors explain the observed differences in contrast sensitivity with positive and negative defocus between myopes and non-myopes.