The detection and discrimination of categorical yellow

Threshold detection of most spectral lights presented on a white background is subserved by colour opponent mechanisms which produce distinct percepts of colours. Five ranges of wavelengths can be discriminated: red, yellow, green, blue and violet [Mullen and Kulikowski (1990) Wavelength discrimination at detection threshold. J. Opt. Soc. Am. A7, 733-742]. However, under most viewing conditions yellow can also be detected and discriminated by activating the achromatic mechanism. Using 2-alternative forced-choice we studied detection and discrimination between spots (1 deg) which were either spectral colours or white (matching the background in colour temperature), with and without masking the achromatic mechanisms. For low colour temperatures of white (2700 K), yellow could be discriminated from white at slight suprathreshold levels of detection. However, at a colour temperature of 6800 K and with masking, the yellow-white discrimination threshold for 565-574 nm was also close to detection threshold (as for other wavelengths). We conclude that it is possible to demonstrate the role of the blue-yellow opponent mechanism in categorical perception of yellow (at threshold) by sensitizing its yellow branch and by suppressing the achromatic mechanism.