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 ) and nine background illuminances from . Foveal, 2° diameter, duration test stimuli were presented on a concentric, perceptually white ( color temperature), 7° diameter, steady background. Thresholds were obtained by the method of adjustment, after which the test stimulus illuminances were increased unit and the subject estimated percentages of red, yellow, green, blue, and white. Average tvi slopes for two subjects were 2.06 for , 1.6 for , 1.11 for and 1.34 for , 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.
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