Sensitivity to S-Cone Stimuli and the Development of Myopia

Abstract

Purpose: Longitudinal chromatic aberration (LCA) is a color signal available to the emmetropization process that causes greater myopic defocus of short wavelengths than long wavelengths. We measured individual differences in chromatic sensitivity to explore the role LCA may play in the development of refractive error.

Methods: Forty-four observers were tested psychophysically after passing color screening tests and a questionnaire for visual defects. Refraction was measured and only subjects with myopia or hyperopia without severe astigmatism participated. Psychophysical detection thresholds for 3 cyc/deg achromatic, L-, M-, and S-cone-isolating Gabor patches and low-frequency S-cone increment (S+) and decrement (S-) blobs were measured. Parametric Pearson correlations for refractive error versus threshold were calculated and nonparametric bootstrap 95% percentage confidence intervals (BCIs) for r were computed.

Results: S-cone Gabor detection thresholds were higher than achromatic, L-, and M-cone Gabors. S-cone Gabor thresholds were higher than either S+ or S- blobs. These results are consistent with studies using smaller samples of practiced observers. None of the thresholds for the Gabor stimuli were correlated with refractive error (RE). A negative correlation with RE was observed for both S+ (r = -0.28; P = 0.06; BCI: r = -0.5, -0.04) and S- (r = -0.23; P = 0.13; BCI = -0.46, 0.01) blobs, although this relationship did not reach conventional statistical significance.

Conclusions: Thresholds for S+ and S- stimuli were negatively related to RE, indicating that myopes may have reduced sensitivity to low spatial frequency S-cone stimuli. This reduced S-cone sensitivity might have played a role in their failure to emmetropize normally.

Figure 1

The top row shows high-contrast versions of the achromatic, L-, M-, and S-cone Gabors. The bottom row shows S+ increment and S− decrement blobs. The colors are only approximately correct owing to the reproduction process.

Figure 2

Box and whisker plot of log₁₀(threshold), in cone contrast units, as defined in Equations 1 and 2, for the six chromatic conditions. Boxes cover the interquartile range, whiskers are 1.5 times the interquartile range, and outliers are shown as black circles.

Figure 3

Refractive error versus log₁₀(threshold) for the achromatic, L-, M-, and S-Gabor conditions. Each point is an observer's threshold, the line the best-fitting regression line, and the gray shaded region the 95% confidence interval of the fit (Chambers and Hastie⁴⁷).

Figure 4

Refractive error (best sphere) versus threshold for the S+ increment and S− decrement blob conditions. Each point is an observer's threshold, the line the best-fitting regression line, and the gray shaded region the 95% confidence interval of the fit (Chambers and Hastie⁴⁷).