Functional consequences of the relative numbers of L and M cones

Abstract

Direct imaging of the retina by adaptive optics allows assessment of the relative number of long-wavelengthsensitive (L) and middle-wavelength-sensitive (M) cones in living human eyes. We examine the functional consequences of variation in the relative numbers of L and M cones (L/M cone ratio) for two observers whose ratios were measured by direct imaging. The L/M cone ratio for the two observers varied considerably, taking on values of 1.15 and 3.79. Two sets of functional data were collected: spectral sensitivity measured with the flicker electroretinogram (ERG) and the wavelength of unique yellow. A genetic analysis was used to determine L and M cone spectra appropriate for each observer. Rayleigh matches confirmed the use of these spectra. We determined the relative strength of L and M cone contributions to ERG spectral sensitivity by fitting the data with a weighted sum of L and M cone spectra. The relative strengths so determined (1.06 and 3.38) were close to the cone ratios established by direct imaging. Thus variation in L/M cone ratio is preserved at the sites tapped by the flicker ERG. The wavelength of unique yellow varied only slightly between the two observers (576.8 and 574.7 nm). This small variation indicates that neural factors play an important role in stabilizing unique yellow against variation in the L/M cone ratio.

Organization of the Human Trichromatic Cone Mosaic

“…Unique yellow, the wavelength that appears neither reddish nor greenish and represents the neutral point of the red-green color mechanism, is thought to be driven mainly by differences in L and M cone excitation. Several investigators have noted that whereas estimates of L:M cone ratio vary widely, the wavelength that subjects judge uniquely yellow is nearly constant, varying with a SD of only 2-5 nm (Pokorny et al, 1991;Jordan and Mollon, 1997;Miyahara et al, 1998;Brainard et al, 2000;Neitz et al, 2002). In agreement with these studies, measures of unique yellow did not correlate with direct measurements of L:M cone ratio in six of our subjects (HS, YY, MD, JP, JC, and BS; data not shown).…”

“…They found the ERGderived estimates agreed with those obtained from direct imaging with AO (Brainard et al, 2000). Their finding supports the hypothesis that individual variability in L:M cone ratio underlies individual variability in photopic luminous efficiency (Brainard et al, 2000). Here, by examining a larger number of subjects with more diverse L:M cone ratios, we were able to more rigorously test this hypothesis.…”

“…Eight subjects were from this study (filled circles), and two subjects were from the study by Roorda and Williams (1999) (open triangles). ERG data for these two subjects were published previously (Brainard et al, 2000). Shown on the abscissa is a histogram of ERG-derived L:M ratios for 62 color-normal males (including JC, BS, and RS from the current study).…”

“…For example, the relative contributions of the Land M-cone derived signals to the flicker photometric ERG have been used as an index of the variability in cone ratio among males with normal color vision (Jacobs and Neitz, 1993;Carroll et al, 2002). Recently, Brainard et al (2000) used the flicker photometric ERG to estimate L:M cone ratio in the two subjects from the original Roorda and Williams (1999) study. They found the ERGderived estimates agreed with those obtained from direct imaging with AO (Brainard et al, 2000).…”

Ecological influences on individual differences in color preference

“…Even typical trichromats differ in their relative numbers of retinal L, M, and S cones (Roorda & Williams, 1999), suggesting that such differences might underlie individual differences in color preferences. However, individual differences in the relative numbers of different cone types among normal trichromats have little effect on sensitive psychophysical measurements such as the locus of unique hues and color appearance ratings (e.g., Brainard et al, 2000;Webster, Miyahara, Malkoc, & Raker, 2000). The visual system appears to have mechanisms that largely compensate for such differences, keeping color perception relatively invariant across the normal range of trichromatic retinal physiology.…”