For discussions of the genetics of color vision and the color vision of heterozygous women, see W. Jaeger, "Genetics of congenital colour deficiencies," in Handbook of Sensory Physiology, D. Jameson and L. M. Hurvich, eds. (Springer-Verlag, Berlin, 1972), Vol. VII/4, pp. 625–642; P. J. Waardenburg, in Genetics and Ophthalmology, P. J. Waardenburg, A. Franceschetti and D. Klein, eds. (Charles C. Thomas, Springfield, III, 1963), Vol. II, p. 1425; H. Kalmus, Diagnosis and Genetics of Defective Colour Vision (Pergamon, New York, 1965); T. P. Piantanida, "Polymorphism of human color vision," Am J. Optom. Physiol. Opt. 53, 647–657 (1976).
For a recent discussion on this view of trichromatic color matching, the additivity laws, and a summary of the evidence, see G. S. Brindley, Physiology of the Retina and Visual Pathway, 2nd ed. (Arnold, London, 1970), Chap. 8, pp. 199–223; R. M. Boynton, Human Color Vision (Holt, Rinehart, and Winston, New York, 1979), Chap. 5, pp. 97–155.
For a discussion of the breakdown of additivity in the parafovea and a summary of recent work on parafoveal color vision see J. D. Moreland, "Peripheral colour vision," in Handbook of Sensory Physiology, D. Jameson and L. M. Hurvich, eds. (Springer-Verlag, Berlin, 1972), Vol. VII/4, pp. 517–536.
Rayleigh matches are often used to diagnose red-green color deficiencies. Two aspects of the match are useful in differentiating different types of color vision: the value of the red-green ratio required to match a yellow test light and the width of the range of red-green ratios acceptable as a match. See L. M. Hurvich, "Color vision deficiencies," in Handbook of Sensory Physiology, D. Jameson and L. M. Hurvich, eds. (Springer-Verlag, Berlin, 1972), Vol. VII/4, pp. 582–624; R. M. Boynton, Human Color Vision (Holt, Rinehart, and Winston, New York, 1979), p. 377.
Because of spatial summation, color matches are typically more precise with larger fields. The annular test configuration was used to avoid retinal inhomogeneities from fovea to parafovea that sometimes cause difficulties with large-field color matches. See W. S. Stiles, "The basic data of colour-matching: 18th Thomas Young Oration," Phys. Soc. Yearbook (The Physical Society, London), pp. 44–65 (1955). The test annulus had an inner diameter of 40° and an outer diameter of 12°. Matches were made at a retinal illuminance of approximately 20 trolands. The optical system used to deliver stimuli to the observer was conventional in design and is described in A. L. Nagy, "Large-field substitution Rayleigh matches of dichromats," J. Opt. Soc. Am. 70, 778–784 (1980).
The bleaching field, which was 16° in diameter with a retinal illuminance of 6.6 log scotopic trolands, was obtained from a 750-W Kodak projector with a 500-nm interference filter taped over the lens. The observer placed his eye near the front of the lens and looked into the light beam for 10 sec. Two control experiments were done to determine the stimulus presentation interval. Thresholds for the test stimuli were determined after a bleach in order to determine when the cones had recovered from the bleach and when the rods became sensitive enough to determine the threshold. Cones had recovered 3 min after the bleach, but extrapolation of the rod branch of the threshold curve upward above the cone plateau portion of the curve indicated that rods were not able to detect the test stimuli less than 10 min after the bleach. In the second experiment observers adjusted a Rayleigh match soon after the cones had recovered from a bleach and then observed the match until the two halves of the field began to look dissimilar. When rods begin to see the test field, the mixture half of the field becomes much less saturated than the monochromatic half. The breakdown in the match always occurred more than 10 min after the bleach. Therefore we asked observers to make matches between the third and the tenth minute after the bleach. The matches made were stable throughout this interval. Bleaches were repeated to obtain a sufficient number of observations from each observer.
The red and blue background fields were 20° in diameter with a retinal illuminance of approximately 20 photopic trolands.
The procedure used is similar to one suggested by A.Linkz, An Essay on Color Vision and Clinical Color- Vision Tests (Grune and Stratton, New York, 1964),. The experimenter adjusted a particular 546–660-nm-mixture ratio into the upper half of the field and then asked the observer to attempt a match by varying the brightness of the lower half of the field with circular graded neutral filter. The observer then reported whether the match was a perfect color match or whether the upper half of the field was redder or greener than the lower half of the field. This procedure was repeated until the experimenter had determined values of the mixture ratio that could be discriminated reliably from the 588-nm field and values that could not.
This view was confirmed for one heterozygote whose son was a dichromat. In these women one might expect a pattern of retinal mocaicism in which areas of the retina containing both normal cone types alternate with areas in which only one of the normal cone types is detectable. See A. Krill and E. Beutler, "Red-light thresholds in heterozygote carriers of protanopia: genetic implications," Science 149, 186–188 (1965); P. Grutzner, G. Born, and H. J. Hemminger, "Coloured stimuli within the central visual field of carriers of dichromatism," Mod. Probl. Ophthalmol. 17, 147–150 (1976); see also a study by B. Wooten and G. Wald, Department of Psychology, Brown University, Providence, R.I. 02912 (in preparation). It is interesting that several of the heterozygous women for whom additivity held complained that the matches were difficult because the test fields appeared to be composed of tiny red and green dots. The patchy appearance could be due to mocaicism. In contrast, none of the male observers made similar complaints.
Any variability in absorption spectrum among cones of the same type should generate minor additivity failure even in normal observers. With the help of plausible assumptions, additivity data like these can be used to compute a constraint on the extent of such variability. These data permit the conclusion that the three absorption spectra of normal vision are replicated with high precision from cone to cone. If Weber's law is assumed to hold at the level of the cones, an adapting light of (say) long wavelength will bias sensitivity in favor of those individual cones with shorter than average λmas. This will shorten the effective λmax of each cone type by an amount proportional to both (1) the variance of the distribution of λmax in that cone type (assumed Gaussian) and (2) the slope of the spectral sensitivity curve at the adapting wavelength. The ratio of red to green light required for a match in our apparatus increases by about 0.033 log unit/nm reduction in the λmax of either red- or green-sensitive cones [compare J. Pokorny, V. C. Smith, and I. Katz, "Derivation of the photopigment absorption spectra in anamalous trichromats," J. Opt. Soc. Am. 63, 232–237 (1973)]. In our blue- and red-adapted conditions, the measured ratios for the 36 nonshifting observers differed by less than 0.01 log unit (90% confidence interval is 0.001–0.016 log unit). It follows that changes in effective λmax because of adaptation did not exceed 0.25 nm. The corresponding upper limit for the standard deviation of λmax for cones of a given type is only 1.6 nm. Unfortunately, the Weber's law assumption has not been tested for our conditions, so further work on this point would be desirable.
M. Alpern, Vision Research Laboratory, University of Michigan, Ann Arbor, Mich. 48104, personal communication.