We have used the fitting of ERG-derived spectral sensitivity functions to individualized L-cone spectral absorption curves as a method for removing variation in the λ
max of the underlying photopigments as a source of error. To test the effectiveness of this method in removing the error, we compared the individual cone ratio estimates to Rayleigh matches on the Nagel anomaloscope. The Rayleigh match midpoint is linearly related to the spectral sensitivities of the underlying cone pigments, and thus, the match midpoint is highly correlated to variation in λ
max among the normal pigments (
Neitz & Jacobs, 1986). If the variability in λ
max introduces error into the estimates of cone proportion derived from ERG spectral sensitivity, then the estimates of L:M proportion should also be significantly correlated with the Rayleigh match midpoints because both measures will share a strong component from variability in λ
max.
Figure 5A shows the results for the 62 males prior to being corrected by using the individualized L-cone spectral sensitivities. To obtain the results for
Figure 5A, a single L-cone fundamental was used for all subjects. As expected from the fact that the variability in the cone pigments is normally a strong source of error in the cone contribution estimates obtained from spectral sensitivity, there is a strong correlation between the two measures that is highly statistically significant (
r2 = 0.30,
p < .0001). However, in
Figure 5B, when we use corrected L-cone spectral sensitivity functions, based on the deduced amino acid sequence for each subject’s L pigment, the correlation is reduced to near zero and the extremely low residual correlation is not statistically significant (
r2 = 0.04,
p = .11). Thus, at least from this analysis, there is no measurable error associated with variations in λ
max remaining after corrections have been made using the genetic data.
In the sample used here, 35 subjects had S at position 180, whereas the remaining 27 had A180. In the absence of any other amino acid substitution, the S180 subjects have a pigment with a λ
max of 559 nm. However, 2 of the S180 subjects had polymorphisms in exon 4, which are known to shift the spectral sensitivity of the pigment toward the shorter wavelengths (
Neitz et al., 1991;
Merbs & Nathans, 1992;
Asenjo et al., 1994;
Sharpe et al., 1998). Thus, nearly one half of the subjects (35 − 2 = 33) had 559-nm L pigments, whereas the other half had pigments with λ
max values 3.5 nm shorter or more. This means that in a sample where no genetic information was known and an average L-cone spectral sensitivity was used for all subjects, all estimates would be subject to error from the λ
max variation. Alternatively, if an L pigment with a 559-nm spectral peak was used, nearly one half of the subjects would have accurate L:M estimates, and the other half would show L:M estimates that were incorrect. To determine how inaccurate these estimates would be, we derived a %L estimate using a 559-nm L pigment for all 29 of the subjects who had either A180 or some other polymorphism that shifted the spectrum of their L pigment. For one subject, the %L estimate was 25%L lower with the 559-nm curve than his individualized spectrum with a λ
max of 554 nm. This is the equivalent to misidentifying a person with a 3:1 L:M ratio as having only a 1:1 ratio. The average error was about 15%L, roughly the difference between a 1:1 and a 2:1 L:M ratio.