Response saturation of monochromatic increments on intense achromatic backgrounds: implications for color-opponent organization in human vision

Abstract

We present evidence that steady achromatic adapting fields can produce response saturation in color-opponent pathways. We measured tvi (log increment threshold illuminance versus log background illuminance) functions at four test wavelengths (430, 490, 575, and 660 nm) and nine background illuminances from 4.0 to 5.6 log Td. Foveal, 2 degrees diameter, 1 s duration test stimuli were presented on a concentric, perceptually white (5128 degrees K color temperature), 7 degrees diameter, steady background. Thresholds were obtained by the method of adjustment, after which the test stimulus illuminances were increased 0.6 log unit and the subject estimated percentages of red, yellow, green, blue, and white. Average tvi slopes for two subjects were 2.06 for 430 nm, 1.6 for 490 nm, 1.11 for 575 nm and 1.34 for 660 nm, consistent with the estimated ratios of chromatic to achromatic sensitivity at the same wavelengths. Also, the percentage of white seen in the suprathreshold increments increased with increasing background illuminance despite increases in excitation purity. These findings imply that steady, intense, achromatic backgrounds can produce response saturation in color-opponent mechanisms at wavelengths across the visible spectrum. The saturation was more extreme at short wavelengths than at middle or long wavelengths, producing a tritanopic condition at the highest background illuminances. The tritanopia reduced color space to a predominately red-blue dichromacy, in agreement with previous findings. The results support a multistage opponent-color model in which precortical koniocellular and parvocellular opponent pathways interact to produce the observed red-green and yellow-blue color-opponent channels at a cortical level.