SPATIAL-FREQUENCY TUNING OF SUSTAINED NONORIENTED UNITS OF THE RED-GREEN CHANNEL

The existence of nonoriented cells but not of sustained orthogonal mas king for achromatic stimuli led to an investigation of the spatial-fre quency (SF) tuning of sustained nonoriented color units. For this purp ose 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 ve rtical color tests and sinusoidal orthogonal color masks by the method of constant stimuli under Gaussian temporal presentation. Results sho w that (1) color CSF's are a low-pass function of SF, whereas TE curve s are a bandpass function of mask SF, and (2) a minimum of six SF-tune d color mechanisms (one low-pass and five bandpass functions of SF, wi th peak SF's of 0.13, 0.5, 2, 4, and 8 cycles per degree and bandwidth s of 3.9, 4.4, 2.9, 2.1, 1.1, and 1.2 octaves), similar to oblique-mas king color mechanisms, are extracted by the multiple-mechanism model. These data imply that (1) most of the SF tuning of the broadly oriente d color units is already present in the circularly symmetric units, an d (2) the latter may be an input to the former. (C) 1998 Optical Socie ty of America.