Spatial-frequency tuning of sustained nonoriented units of the Red–Green channel

The existence of nonoriented cells but not of sustained orthogonal masking for achromatic stimuli led to an investigation of the spatial-frequency (SF) tuning of sustained nonoriented color units. For this purpose the Red-Green channel was isolated by the minimum flicker and hue cancellation techniques. Chromatic contrast sensitivity functions (CSF's), threshold elevation (TE) curves, and contrast nonlinearities (TE-versus-mask-contrast curves) were measured with spatially localized vertical color tests and sinusoidal orthogonal color masks by the method of constant stimuli under Gaussian temporal presentation. Results show that (1) color CSF's are a low-pass function of SF, whereas TE curves are a bandpass function of mask SF, and (2) a minimum of six SF-tuned color mechanisms (one low pass and five bandpass functions of SF, with peak SF's of 0.13, 0.5, 2, 4, and 8 cycles per degree and bandwidths of 3.9, 4.4, 2.9, 2.1, 1.1, and 1.2 octaves), similar to oblique-masking color mechanisms, are extracted by the multiple-mechanisms model. These data imply that (1) most of the SF tuning of the broadly oriented color units is already present in the circularly symmetric units, and (2) the latter may be an input to the former.