About 12% of women are carriers of an X-linked red/green hybrid gene that encodes a photopigment with a spectral sensitivity somewhere between those of the normal M and L cone photopigments. Owing to random X-chromosome inactivation (Lyon, 1961, Nature vol. 190), the retinal mosaic of such a carrier will contain more than three types of cone. We ask whether she can become a functional tetrachromat and exhibit enhanced colour discrimination.

Our subjects perform a three-alternative temporal forced-choice task, in which two of the stimuli are monochromatic (590 nm) while the third is a mixture of two monochromatic lights (550 nm plus 670 nm). The stimulus fields are 2-deg, circular patches centred on the fovea and are presented in rapid succession in Maxwellian-view. Chromatic aberrations are masked by an annulus containing temporal chromatic noise. Subjects are asked to identify which of the triplet of stimulus fields is the mixture. In a random sequence, the program tests different combinations of (a) the red/green mixture ratio and (b) the radiance of the monochromatic field. For the normal, there is a combination of (a) and (b), equivalent to the Rayleigh match, where performance is at chance. A tetrachromat should not fail at any combination.