Lack of Covariation of the Effects of Luminance and Eccentricity on Contrast Sensitivity

Background and purpose. Qualitative individual differences in visual processing along various stimulus dimensions have been previously documented. For instance, as compared to the foveal contrast sensitivity function (CSF), the peripheral CSF of some subjects shifts toward lower frequencies, but it scales down for others; also, sensitivity to low spatial frequencies increases monotonically with luminance in some subjects, but it displays a decline at high luminances in others. Although these qualitatively distinct eccentricity‐ and luminance‐related patterns have been thoroughly described separately, their joint occurrence has never been studied. This study aimed at determining whether there is covariation between the effects of luminance and eccentricity on contrast sensitivity, i.e., whether each eccentricity‐related pattern occurs with one and only one of the luminance‐related patterns. Methods. We have measured contrast sensitivity to sine‐wave grating patches as a function of luminance and eccentricity in a sample of 18 subjects. Results. We found positive evidence of lack of covariation between the effects of eccentricity and luminance: we found subjects who show the same eccentricity‐related pattern but differ as to their luminance‐related patterns, and we have also found a subject who, unlike the rest, shows qualitatively distinct luminance‐related patterns at different eccentricities. Conclusion. The dependence of contrast sensitivity on eccentricity and luminance is subject to qualitative variations both across and within individuals, suggesting that meaningful conclusions on the effects of luminance and eccentricity on contrast sensitivity cannot be drawn when the data from all available subjects are aggregated.