ORIENTATION TUNING OF THE SPATIAL-FREQUENCY-TUNED MECHANISMS OF THE RED-GREEN CHANNEL

Significant orthogonal masking for color stimuli [Invest. Ophthalmol. Visual Sci. 34, 782 (No. 405-54) (1993)] but not for achromatic stimul i [J. Opt. Soc. Am. A 1, 226 (1984)] under sustained presentation led us to investigate the orientation tuning of the spatial-frequency-(SF- ) tuned color mechanisms. The Red-Green channel was isolated from the achromatic channel by the minimum flicker technique and from the Yello w-Blue channel by the hue cancellation technique. Contrast sensitivity functions, threshold elevation versus mask orientation curves (measur ed by orientation masking), and threshold elevation versus mask contra st curves were measured by the method of constant stimuli and a two-in terval forced-choice technique on two normal observers. Test targets w ere spatially localized (D6), vertical color patterns, and masks were sinusoidal color patterns oriented 15 degrees-90 degrees from the vert ical in 15 degrees steps and had the same SF's as those of test patter ns. Mask contrasts were varied between 1.2% and 60%. The orientation t uning curves of the SF-tuned color mechanisms were extracted by obtain ing the best fit to contrast sensitivity and threshold elevation data simultaneously at a given SF with use of the masking model. Results sh ow that threshold elevations depend on test SF, mask SF, mask orientat ion, and mask contrast. Half-bandwidths at half-height (with respect t o 15 degrees from the vertical) of threshold elevation versus mask ori entation curves range from 90 degrees to 29 degrees depending on SF's. The slopes of threshold elevation versus mask contrast curves range f rom 0.76 to 0.29 on octave-octave coordinates depending on SF's. Orien tation half-bandwidths at half-height of orientation tuning curves of the SF-tuned color mechanisms (1) range from 79 degrees to 28 degrees and (2) average from 68 degrees to 30 degrees for SF's 0.063-8 cycles per degree (cpd). Data suggest that the orientation tuning curves of t he SF-tuned chromatic mechanisms are broader (except at 2 cpd) than th ose of the achromatic mechanisms (orientation half-bandwidths: 32 degr ees-15 degrees for 0.5-11.3 cpd [J. Opt. Soc. Am. A 1, 226 (1984)]); m oreover, the orientation bandwidths are SF dependent. (C) 1997 Optical Society of America.