Abstract
Abstract: :
Purpose: To investigate L– and M–cone contributions to signals for reflex accommodation and their relationship to refractive error. Methods: Accommodation was measured continuously while subjects viewed a sine wave grating (2.2 c/d) that isolated positions in cone contrast space. Seven conditions tested loci that form a circle with equal vector length (0.27) at 0, 22.5, 45, 67.5, 90, 120, and 145 deg. The eighth condition was an empty field stimulus (CIE (x,y ) co–ordinates (0.4554, 0.3835)). Each of the gratings moved at 0.2 Hz sinusoidally between 1.00 D and 3.00 D for 40 secs, while the effects of longitudinal chromatic aberration were neutralized with an achromatizing lens. Subjects were divided into two groups: Myopes and Non–myopes. Results: Data show significant changes in the mean accommodation level that correspond to changes in the ratio of L– and M–cone contrast (p<0.01; F=4.28; Two Factor ANOVA). The mean accommodation level is higher when M–cone contrast is high and L–cone contrast is low, and lower when L–cone contrast is high and M–cone contrast is low. There was a significant difference in the mean accommodation response to the L/M cone contrast ratio of the image with refractive error (p<0.03; F=5.99; Two Factor ANOVA). Both Myopes and Non–myopes showed a significant difference in accommodation level between conditions at 67.5 deg and 22.5 deg (p=0.03 and 0.001; t–test: one tail). Both Myopes and Non–Myopes showed an anti–clockwise rotation (in cone contrast space) of the "static" reflex accommodation responses compared to that predicted by a luminance controlled mechanism. Rotation was greater for Non–myopes than Myopes, indicating a greater L–cone weighting to the controlling mechanism for Myopes. There was a significant change in gain with a change in the cone contrast ratio of the stimulus (p=0.03; F=7.44; Two factor ANOVA), but the difference between Myopes and Non–myopes did not reach significance (p=0.08; F=3.32; Two factor ANOVA). The iso–response contour for gain for Myopes and Non–myopes showed a shift in the vector angle of the iso–gain contour with refractive error. The vector angle was 23 deg for Myopes, and 48 deg for Non–myopes. These values indicate a L/M cone weighting ratio to the luminance component of 2.2:1 for Myopes, and 0.8:1 for Non–myopes. Conclusion: The results support previous findings that both chromatic and luminance components are a necessary part of the stimulus for accommodation. In addition, the results suggest that the weighting factor for the L–cone contrast component of the luminance stimulus is greater for Myopes than Non–Myopes.
Keywords: emmetropization • refractive error development