3. DIFFERENCES BETWEEN MEN AND WOMEN

X-chromosome gene that encodes this type of photopigment. Serine vs alanine at this position makes a difference of 4 to 7 nm in the spectral peak of the encoded photopigment [37] (Sharpe et al. [38] give a detailed review of this topic in particular, and of the genetics of colour vision in general.)

In the case of men, who have only a single copy of the X chromosome, normally only one type of L cone and one type of M cone are expressed in the retina. Of the two types of L- pigment accepted as normal, approximately half of all men have the LA type (43.7%) expressed in their retina and the other half have the LS type (56.3%) [38].

In the case of women, who have two copies of the X chromosome, in early stages of embryo development each cell inactivates at random one of the copies [39] so that approximately half of the embryo's cells inactivate the copy from the mother and the other half the copy from the father. The result is that there are three population subtypes of women: about half of all women may express both L-pigments in their retina (heterozygotic), and the other half is split between expressing just one or the other L-pigment (homozygotic) [40].

Previous research proposed that such genetic heterozygosity may have perceptual consequences in individuals who actually express all four types of photopigment genes, because each gene type produces different retinal photopigment sensitivities, in effect yielding a four-cone-class retinal phenotype and a possible tetrachromacy. [41-48] However, such psychophysical variations are still a matter of dispute because there have been many studies that have failed to find significant differences between the two population groups [4950].

Two forms of tetrachromacy should be distinguishable according to the level to which the four signals remain independent. There is first the possibility that there are four types of cone photopigment but only three independent post-receptoral channels. This has been referred to as weak tetrachromacy by Jordan and Mollon [36]. The second possibility is that the four cones in the retina have access at the level of the cortex to four independent transformations of these signals. Jordan and Mollon refer to this possibility as strong tetrachromacy.

With the weak tetrachromacy hypothesis, if 50% of women who have both pigments behave like the average of the women who have only one photopigment, LA or LS, the average of all women must match the average of all men. But if this is not so, the differences in the expression of retinal L cones between men and women could give rise to sex differences in the perception of colour, so that they would have to be considered as different population groups. Testing the validity of this hypothesis was also an objective of this chapter.

When a trichromat observer is asked to judge the validity of a colour match in which two juxtaposed chromatic hemifields are presented, the observer evaluates unconsciously the equality of the colour attributes of the two hemifields – luminosity, hue, and saturation. If the colour stimuli that are presented are not subject to temporal or inductive effects that influence the two hemifields differently, the perceived attributes are directly related to the effective capture of photons by the photopigments of the three types of cone – L, M, and S – that are found in the retina of a trichromat.

For each of these three types of cone, this effective capture can be expressed in terms of the visible radiation reaching the eye, and of the response of each type of cone. For example, for the L cones:

This expression corresponds to Jordan & Mollon's weak tetrachromacy hypothesis mentioned above. From it, one can obtain the ratio of the radiances in the mixture of red and green spectral stimuli used to match a yellow spectral stimulus by a female observer whose retina has both L-cone photopigments. The question is whether this radiance ratio is equal to the average of the ratios used by a group of male observers 43.7% of whom have LA-type pigment and 56.3% the LS-type pigment.

Knowing the spectral intraocular lens [51] and macular pigment [52] optical densities, the template [26] of the photopigment optical densities for the L and M cones, and taking the usual values [53] for the wavelengths of the maximum photopigment optical densities (λmax) for the LS, LA, and M pigments – 563.2 nm, 554.2 nm, and 530.8 nm, respectively – one can estimate the cone response fundamental curve values for Eqs. (39) and (40). The Pr/Pg ratio can then be derived in both cases. The estimated difference between the average man and the average woman using this procedure is about 4.5%. The initial hypothesis thus remains open.

One way to confirm or reject this initial hypothesis is to determine experimentally the radiance ratio used by the average male observer and by the average female observer. Other workers have studied how variations corresponding to the presence of different types of photopigment in the human retina affect Rayleigh matching. The experimental method used was very similar to standard Rayleigh matching [54]. In this, the observer may adjust the proportion of 670 nm and 545 nm primary lights in the mixture field to match the 589 nm light in the reference field. The observer may also adjust the luminance of the 589 nm field to achieve the exact match. In this present work, we introduced slight modifications that make it advisable not to use the term Rayleigh matching for our procedure. In particular, the observers were allowed to change the luminances of the 546 nm and 671 nm stimuli, but not the 589 nm stimulus which was fixed at 21 cd/m2. Fixing the luminance of the reference stimulus reduces the number of variables in Eqs. (38) and (39) by converting one of them into a known constant.

A. STIMULUS GENERATING SYSTEM

The colour generating system was a new visual colorimeter recently constructed by the authors [55] by recycling an old fluorescence spectrophotometer with two monochromators. An optical system was designed that allows the fusion of two visual stimuli coming from the two monochromators of the spectrofluorimeter. Each of these stimuli has a wavelength and bandwidth control.

B. PSYCHOPHYSICAL TEST

The psychophysical test had to be simple and fast enough to present no problems when done by subjects with no prior experience in psychophysical testing. This would allow us to enlarge the size of the observer sample, which is usually small in this type of experiment. With these premises, we opted for metameric matching of a spectral stimulus to a mixture of another two spectral stimuli, presented in a 2° circular field divided vertically into equal halves by a fine separating line (Figure 9).