COLOR AND LUMINANCE SPATIAL TUNING ESTIMATED BY NOISE MASKING IN THE ABSENCE OF OFF-FREQUENCY LOOKING

We assessed the contribution of off-frequency looking for pattern dete ction and obtained bandwidths for chromatic and luminance mechanisms i n conditions free from this effect. We used a simultaneous spatial mas king technique with Gaussian enveloped sinusoidal test stimuli (0.5 cy cle/deg) and filtered one-dimensional static-noise masks whose spectra l power was uniformly distributed per octave. Stimuli were modulated i n the chromatic (isoluminant red-green) or the luminance( yellow-black ) domain. Color and luminance detection thresholds were compared for l ow-pass, high-pass, and notch- (band-stopped) filtered noise. We obtai ned the following results: (1) at high-noise spectral densities, maski ng by notched noise is greater than the summed masking of the high- an d low-pass noise, indicating the presence of off-frequency looking for both color and luminance detection. There is no evidence for off-freq uency looking at lower power densities. (2) Using notch-filtered noise , which avoids the problem of off-frequency looking, we found that col or processing is subserved by bandpass channels with bandwidths simila r to those revealed for luminance processing. (3) Both color and lumin ance mechanisms appear to have bandwidths proportional to their center frequency (constant in octaves). (4) The lower and upper sides of the color and luminance tuning functions were estimated individually by u se of high-pass and low-pass noise of a low power density and are reve aled to be asymmetric, with the lower side declining more steeply than the upper side.