Opponent-color receptive-field profiles determined from large-area psychophysical measurements

By compensating for retinal inhomogeneity and using the opponent-color characteristics of retinal receptive fields, some of the problems associated with previous methods of inferring local spatial-processing properties from sine-wave contrast-sensitivity measurements can be eliminated. Forced-choice contrast detection thresholds were obtained for circular frequency-modulated patterns, over a wide range of spatial and temporal frequencies, with three subjects. Eye-movement artifacts were minimized by image stabilization. The luminance contrast sensitivity (L) for a given isochromatic stimulus and the chromatic contrast sensitivity (C) for the corresponding isoluminance stimulus were measured in rapid succession, under identical conditions, using red and green components with accurately known spectral distributions. Receptive-field center (E) and surround (I) frequency functions were then calculated from the L and C data by solving two linear equations in two unknowns, with coefficients given by the spectral characteristics of the stimuli and the long-wave and middle-wave cone pigments. The results depend on the small differences of large numbers inherent in opponent-color responses, so they are somewhat noisy, but the forms of these (E-I) spatial profiles are similar to those obtained by other methods.