Contrast sensitivity for isoluminant interference fringes in human foveal vision

In this dissertation, I examined the limitations imposed by the post-receptoral visual system on contrast sensitivity, to determine whether they affect performance differently for isoluminant and isochromatic modulations. I developed a new psychophysical technique for measuring isoluminant contrast sensitivity without the influence of diffraction and aberrations in the eye. With this technique, the red-green isoluminant contrast sensitivity measurements were extended to 20-25 c/deg, substantially higher than has previously been reported. The data at such spatial frequencies allow a comparison of the efficiency of the visual system for detecting isoluminant and isochromatic modulations.

For both isoluminant and isochromatic gratings, the post-receptoral limitations are estimated by comparing the spatial contrast sensitivity of real and ideal observers under identical stimulus conditions. The comparison allows us to estimate the degree to which the post-receptoral visual system contributes to the detection task, taking account of the differences in sensitivity loss for isoluminant and isochromatic stimuli at the receptoral stage of the visual system. I find that the post-receptoral visual system modifies the shape of the contrast sensitivity functions differently for isoluminant and isochromatic gratings. The difference can be explained by a bandwidth that is 1.8 times greater for isochromatic than isoluminant stimuli. On the other hand, the properties of receptors are all that is required to explain differences in performance to two different directions of isoluminant modulations in color space, the red-green and S cone isolation directions. This suggests that the visual system has equal spatial bandwidth for two different isoluminant modulations and that these are narrower than that for isochromatic modulations.