Spatial frequency discrimination: a comparison of achromatic and chromatic conditions.

In this study, we have compared foveal SF discriminations for luminance and color-defined stimuli using two different tasks (criteria): in criterion-A, the discrimination is based on spatial (size of the stimuli) and/or spatial frequency; in criterion-B, it is based on apparent motion (contraction/expansion). We used high contrast (75%) spatially localized D6 stimuli and cosine gratings (0.25-9.5 cpd). The SF discrimination was measured by the method of constant stimuli with a two-interval forced-choice procedure. Data show that: (i) for criterion-A, the discrimination thresholds for color stimuli were lower than that for luminance stimuli at low SFs, but similar or higher at higher SFs; for criterion-B, the thresholds to chromatic stimuli were higher than that to achromatic stimuli for all SFs; (ii) SF discrimination was best at inter-stimulus-interval (ISI) of about 200 ms for color stimuli and at ISI of 0 ms for luminance stimuli; (iii) SF discrimination got better with stimulus duration and reached to plateau at 200 ms (or more) for color stimuli and at 67 ms (or more) for luminance stimuli; (iv) SF discrimination threshold (mean Delta(f)=0.19 octaves) is about one-tenth of the full bandwidth (mean=1.96 octaves) of SF tuned mechanisms and is in hyperacuity range; both (discrimination and hyperacuity) can be explained by the relative activities within a population of tuned mechanisms. We conclude that color and luminance SF discrimination thresholds have a different SF dependence. While color appears to perform better than luminance vision at low SFs, this effect is lost or even reversed at high SFs. Data imply that color and form interact, but color and motion are largely segregated (i.e. they weakly interact).