Input of long- and middle-wavelength cones to orientation discrimination

Orientation discrimination thresholds for Gabor stimuli as a function of spatial frequency [0.6, 1, 2, or 4 cycles per degree (cpd)] and contrast were determined for several directions in the color plane spanned by the long-wavelength-sensitive (L) and medium-wavelength-sensitive (M) cones. For Gabor stimuli with carrier frequencies of 2 or 4 cpd we do not find a systematic and robust advantage of a particular color direction in the LM-cone plane when the stimuli are equated in terms of the sum of the absolute L- and M-cone contrasts. Luminance (L+M) and equiluminant (L–2M) stimuli of identical overall L- and M-cone contrast yield identical orientation discrimination thresholds for the 2-cpd stimuli for the entire available contrast range (1–11%). For the very-low-spatial-frequency stimuli, orientation discrimination thresholds are lower for equiluminant stimuli than for luminance stimuli of the same cone contrast; for 4 cpd, orientation discrimination thresholds are slightly higher for equiluminant red–green stimuli than for luminance-defined stimuli.