= Solution
\b[L, M and S describe the relative <spectral sensitivities> of cone classes. Calling them red, green and blue confuses receptor sampling with the <color appearance> constructed from comparisons and context.] The shorthand also wrongly suggests three narrow, nonoverlapping detectors or the primaries of a display.
First, each class has a broad sensitivity curve. A long-wavelength-sensitive cone responds to many <wavelengths>, and its sensitivity maximum is not a unique red stimulus. L- and M-cone sensitivities overlap extensively, while S <cone cells> are relatively more sensitive at shorter <wavelengths>. <Light> at one <wavelength> can therefore excite more than one class. <Andrew Stockman> and colleagues derived L- and M-cone sensitivity estimates using color-matching data and measurements from observers of known pigment genotype. Their analysis emphasizes broad overlap and the need for isolation procedures to measure one class without contamination from others.<The spectral sensitivities of the middle- and long-wavelength-sensitive cones derived from measurements in observers of known genotype by Andrew Stockman and others>[Vision Res 40, 1711–1737; spectral-sensitivity measurements and cone fundamentals]{ref} Exact curves depend on pigment variant and optical filtering, so the letters denote relative classes rather than three invariant <color> labels.
\Image[/past-exam-of-the-sensing-and-perceiving-light-colour-and-materials-course-of-the-university-of-oxford/2022-cone-comparisons.png]
{title=Original Gaussian toy receptor filters and an invertible opponent transform; the filters illustrate broad overlap and are not measured human cone fundamentals or a calibrated color chart}
Second, the principle of univariance prevents one cone from identifying <hue>. For a monochromatic stimulus, let mean <absorption> be $Q=I\,a(\lambda)$. Whenever $a(\lambda_1)$ and $a(\lambda_2)$ are positive, intensities satisfying $I_2=I_1a(\lambda_1)/a(\lambda_2)$ give the same $Q$. The single receptor cannot distinguish those two inputs on <absorption> count alone. Calling that receptor a red detector would assign a <color> judgment to a signal that cannot even uniquely distinguish the two <wavelengths>. It remains true that <spectral sensitivity> constrains the comparison; univariance does not mean all receptor types are identical.
Third, perceptual <color> requires combinations. Comparing L and M signals and comparing S with L+M supplies chromatic information; summing appropriately weighted signals contributes to luminance-related processing. An L-cone increment can be part of several stimulus changes depending on what happens to M and S and on the adapted background. Neither an L response nor a raw L:M ratio by itself specifies a context-independent <color> word. The retinal and cortical opponent circuitry demonstrated by <Dennis M. Dacey> and <Bevil R. Conway> provides mechanisms for comparisons, rather than a relabelling of three cone outputs as three sensations.<The blue-on opponent pathway in primate retina originates from a distinct bistratified ganglion cell type by Dennis M. Dacey and others>[Nature 367, 731–735]{ref}<Spatial Structure of Cone Inputs to Color Cells in Alert Macaque Primary Visual Cortex by Bevil R. Conway>[J Neurosci 21, 2768–2783]{ref}
Fourth, “blue” or “red” need not identify a single <wavelength>. Spectrally different <lights> can be metamers, and a purple appearance can arise from a mixture rather than a monochromatic <wavelength>. Conversely, the same physical spectrum can look different against another surround or after adaptation. Cone labels cannot replace the distinction between emitted or reflected spectrum, receptor catch, opponent representation and reported <hue>.
Fifth, the display analogy is backwards. An RGB screen emits mixtures from three selected primary spectra. The <cone cells> receive and integrate the resulting spectrum; their response curves are not identical to those primary spectra. If primary intensities are $p_k$ and cone sensitivities to each primary form <matrix> $B$, then $\mathbf Q=B\mathbf p$. Off-diagonal terms are generally nonzero because each primary can excite several cone classes. If $B$ is nonsingular, an intended catch change can be computed through $B^{-1}$ within the display's feasible <gamut>. It does not follow that every requested catch vector is realizable with nonnegative bounded primary intensities. Calibration and observer differences therefore matter in cone-isolating experiments.
Finally, <color> terms themselves vary across languages and tasks, whereas receptor class is a biological classification. Receptor genes, <color> matches, discrimination and linguistic categorization are related but distinct measurements. The labels L, M and S prevent an unwarranted assumption that one anatomical cell class directly instantiates one learned word or unique subjective <hue>.
\b[The useful statement is that trichromatic vision starts with three broad, overlapping receptor classes and extracts chromatic information by comparison. “Red, green and blue <cone cells>” conceals both the information loss at each receptor and the transformations needed to see <color>.]
Back to article page
