Spatial frequency tuned covariance channels for red-green and luminance-modulated gratings: psychophysical data from human adults

Both chromatic and luminance-modulated stimuli are served by multiple spati al-frequency-tuned channels. This experiment investigated the independence versus interdependence of spatial frequency channels that serve the detecti on of red-green chromatic versus yellow-black luminance-modulated stimuli a t low spatial frequencies. Contrast thresholds for both chromatic and lumin ance-modulated gratings were measured within 12 individual subjects using a repeated-measures design. Spatial frequencies ranged from 0.27 to 2.16 c/d eg. A covariance structure analysis of individual differences was applied t o the data. We computed statistical sources of individual variability, used them to define covariance channels, and determined the number and frequenc y tuning of these channels. For luminance-modulated gratings, two covarianc e channels were found, including one above and one below 1 c/deg [cf. Peter zell, Sz Teller (1996). Individual differences in contrast sensitivity func tions: the coarsest spatial pattern analyzer. Vision Research, 36, 3077-308 5]. For chromatic gratings, correlations between thresholds for most spatia l frequencies:were uniformly high, yielding a single covariance channel,cov ering all but the highest spatial frequency tested. A combined analysis of both data sets recovered the same three: covariance channels, and showed th at detection thresholds for low-frequency red-green chromatic and luminance -modulated stimuli are served by separate, statistically independent proces ses. (C) 1999 Elsevier Science Ltd. All rights reserved.