Abstract
A rectangular achromatic grating presented against a coloured background may appear tinged with either yellow-green (e.g., when against a yellow background), or blue (e.g., when against a pink background) provided its spatial frequency is high enough. This colour illusion, which is referred to as high-spatial-frequency tritanopia (HSFT), can be observed against backgrounds of only a limited set of colours. We investigated which colours do not produce HSFT. Such a no-illusion set (NIS) of colours is quite indicative of a possible colour mechanism of HSFT. If, as recently suggested (Hutchinson and Logvinenko, 2001 Perception 30 Supplement, p. 8), it is a result of poorer spatial characteristics of either (i) the opponent yellow-blue linear colour channel, or (ii) two unipolar (e.g., yellow and blue) linear channels each of which is followed by a rectifier, the NIS should be either (i) a plane, or (ii) a dihedral, through an achromatic locus, respectively. Generally, any explicit quantitative model of the S-cone pathway makes a specific prediction of the NIS. We have explored the equi-luminant and ML planes in the SML cone contrast space for a grating of 10.1 c/deg. In the ML-plane the NIS was found to be a narrow strip through the origin, with an angle of approximately 65 deg with the M-axis. In the equi-luminant plane the NIS was found to be a curvilinear line for M − L < 0. However, when M − L > 0 the NIS was not a line but a whole curvilinear sector. These results clearly indicate that at least two non-linear unipolar post-receptor chromatic mechanisms, making up an opponent pair, are involved in producing HSFT. Both mechanisms are driven by a difference between a S-cone input and a linear combination of M- and L-cone inputs transformed by static non-linearity. With both being half-wave rectified, one mechanism, produces a bluish illusory colour and the other produces a yellow-greenish illusory colour.