There has been much controversy as to whether there are sex-related differences in human color vision. While previous work has concentrated on testing the central visual field, this study compares male versus female color vision in the near peripheral retina. Male (*n* = 19) and female (*n* = 19) color normal observers who exhibited no significant differences either in the midpoints or the ranges of their Rayleigh matches were tested with a color matching paradigm. They adjusted hue and saturation of a 3° test spot (18° eccentricity) until it matched a 1° probe (1° eccentricity). Both groups demonstrated measurable shifts in the appearance of the peripheral color stimuli similar to those that have been previously reported. However, females showed substantially less saturation loss than males (*p* < 0.003) in the green–yellow region of color space. No significant differences were found in other regions of color space. This difference in the perceived saturation of color stimuli was minimally affected either by the inclusion or exclusion in the analysis of potential heterozygous female carriers of deutan color vision deficiencies. We speculate that this advantage of female over male color vision is conferred by M-cone polymorphism.

*SD*) and 19 female (24 ± 6 years) color normals participated in the main asymmetric matching experiment. Written informed consent was obtained from all subjects and the study was approved by the Ethics of Research on Human Beings Committee of the University of Manchester (Ref. No. 09169).

^{2}) was kept constant during the experimental procedure. The 0°–180° and 90°–270° axes used in these experiments coincide with the cardinal L–M and S–(L + M) axes, respectively (Derrington, Krauskopf, & Lennie, 1984). Chromatic axis, which is the physical equivalent of hue, is defined as the rotation of a vector (spanning 360°) that originates from the background illuminant C (

*x*= 0.31,

*y*= 0.316 at 12.5 cd/m

^{2}). Purity, the physical equivalent of saturation, is the length of that vector. Since there is no absolute value for purity, we defined a vector of length 0.0739 as having purity of 1, following the work of De Valois, De Valois, Switkes, and Mahon (1997). The parafoveal spot was always displayed with purity equal to 0.5. The two spots were presented simultaneously for 380 ms following which the observer had unlimited time to adjust either purity or chromatic axis in increments of 5° (axis) or 0.1 (purity). After the observer's response, the two stimuli were presented again and a new match was required. Hence, the ISI depended on the observer's response time.

*SD*and the light gray lines are the cardinal axes.

*R*

^{2}) to the data. For the number of data points (

*N*= 950),

*R*

^{2}= 0.696 (males) and

*R*

^{2}= 0.725 (females) are high, revealing a high level of association (

*p*< 0.0001 for both graphs). Both groups show the same pattern of hue rotation. As has been reported previously (Parry et al., 2006), some hues are remarkably stable while others undergo obvious and systematic shifts. These major hue shifts occur especially around the 90° and 270° cardinal axes. For other hues, there is no hue distortion (rotation = 0), that is, observers do not need to change the hue of the peripheral spot to match it with the probe spot. These are what Parry et al. (2006) referred to as peripherally invariant hues. Males have a slightly greater interindividual variability compared with the females, according to the confidence bounds. As it is not possible to compare these two graphs quantitatively by eye, the following statistical analysis was conducted.

*θ*and a concentration parameter

*k*(

*θ*and

*k*are analogous to the mean and variance of a linear normal distribution). The hue rotation data shown in Figure 2 failed the test for “circular normality” and so parametric tests are not applicable. The Wilcoxon–Mann–Whitney rank sum test was, therefore, applied (Batschelet, 1981). This showed no statistically significant difference in the hue rotation between the two groups for any chromatic axis. The statistical

*p*-value ranges between 0.175 and 0.965 (

*a*= 0.05) for the 24 chromatic axes.

*R*

^{2}= 0.196 and

*R*

^{2}= 0.161 for males and females, respectively (

*p*< 0.0001 for both graphs).

*SD*(95% confidence interval). Thus, the points that lie outside the dashed lines differ significantly from the others. Hence, these data suggest that there is a statistically significant difference between males and females in saturation match in the green–yellow region of the color space between 225° and 240° axes. The saturation of only these two axes between males and females is compared with an independent sample

*t*-test, having first checked for normality using the Kolmogorov–Smirnov non-parametric test (

*p*= 0.433 for males and

*p*= 0.0913 for females). This statistical analysis confirms the analysis performed on the data shown in Figure 3. The

*t*-test shows a statistically significant difference in saturation match between males and females in the region of 221°–244° with

*p*= 0.003 and post hoc power of 85%. The mean ± 1

*SD*saturation match of males is 0.82 ± 0.32 and that of females is 0.63 ± 0.19. These numbers give a mean saturation loss of 64% for males and 26% for females with the difference between the two groups being 38%.

*p*= 0.519,

*α*= 0.05, independent sample

*t*-test) and no statistically significant difference in their matching ranges, which, according to Pokorny, Smith, Verriest, and Pinckers (1979), is a measure of hue discrimination (

*p*= 0.465,

*α*= 0.05, independent sample

*t*-test). Jordan and Mollon (1993) found that the matching range of the female carriers was significantly greater than that of color normal females. Hood et al. (2006) found that, if heterozygous females and any clearly color-deficient subjects are removed from a sample, any differences in color perception between males and females disappear. We wanted to ascertain whether similar exclusions from our analysis had any effect on the differences in perceived saturation changes that were measured here. In Figure 4, one can see that there are a number of female observers with wider matching ranges than the rest (see arrows in Figure 4). The prevalence of total carriers in the color normal population is approximately 15% (Jordan & Mollon, 1993; Sharpe, Stockman, Jagle, & Nathans, 1999), which in our group of 19 females is equivalent to 3 observers. So three female observers with the widest matching range can be excluded because of the possibility of their being carriers of a color defect. A similar approach was adopted by Rodriguez-Carmona et al. (2008). Because female observers No. 2 and No. 11 have the same matching range (7 units), both of them have been excluded, resulting in a total of 15 color normal female observers.

*p*= 0.007,

*α*= 0.05, independent sample

*t*-test, power of 78%). Following the argument above, excluding the 4 females who performed worst in the anomaloscope task should have reduced the peripheral saturation loss and thus increased the difference between the males and females. In fact, the mean saturation loss for the female group remained unchanged at 26% so that the overall male–female saturation difference was unaffected. Note, however, that the

*p*-value for the comparison of the 19 males versus the 15 residual females increased from 0.003 to 0.007. For the sake of completeness, the same 4 females were excluded from the hue rotation data, but it did not have any significant effect on the results.

*n*= 19) and female (

*n*= 19) observers who exhibited no significant differences in either the midpoints or the ranges of their Rayleigh matches. Both groups demonstrated measurable shifts in the appearance of color stimuli that were presented in more eccentric regions of the retina, similar to those that have been previously reported (e.g., Parry et al., 2006). We measured chromatic axis-dependent shifts in perceived hue with increasing retinal eccentricity, but there were no significant sex-related differences. However, there was a statistically significant difference (

*p*= 0.003,

*α*= 0.05) between male and female observers in the perceived saturation of stimuli in the green–yellow region of color space between the chromatic axes of 225° and 240°. Females were found to exhibit a decrease in the perceived saturation of peripheral color stimuli, which was 38% less than that experienced by males. This difference remained virtually unchanged when potential (deutan) heterozygous carrier females (i.e., those with the largest Rayleigh matching ranges) were excluded from the analysis.