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Human Cone Spectral Sensitivities and Color Vision Deficiencies
Andrew Stockman and Lindsay T. Sharpe
CONTENTS
INTRODUCTION
CONE SPECTRAL SENSITIVITIES
FACTORS THAT INFLUENCE SPECTRAL SENSITIVITY
CONGENITAL COLOR VISION DEFICIENCIES
CONCLUSIONS
REFERENCES
INTRODUCTION
Overview
A precise knowledge of the spectral sensitivities of the human cones is essential for the understanding and modeling of both normal and defective color vision. Here, we discuss cone spectral sensitivities, their relationship to color matching, and their derivation from psychophysical measurements obtained from normal trichromatic, dichromatic and monochromatic observers. We present a consistent set of mean 2-deg and 10-deg cone fundamentals based on the work of Stockman, Sharpe and Fach [1] and Stockman and Sharpe [2]. Along with the associated lens and macular pigment and photopigment templates, these can be used to model color vision at the cornea and retina and to account for individual differences. The common forms of congenital color vision deficiencies are due to alterations to the spectral sensitivities of the cones, or the loss of one or more cone photopigments. We consider these deficits, their symptoms and diagnosis.
Transduction
Human spectral sensitivity is determined at the very first step in the phototransduction cascade by the energy required to isomerize the visual chromophore in the rod and cone photoreceptors from 11-cis retinal to all-trans retinal. This isomerization triggers the rapid conformational change of the G protein-coupled-receptor-protein rhodopsin into the activated photoproduct, metarhodopsin II, which then activates transducin, the G protein molecule, by initiating the separation of the α-transducin from the trimer (see [3–8]).
From: Ophthalmology Research: Visual Transduction and Non-Visual Light Perception
Edited by: J. Tombran-Tink and C. J. Barnstable © Humana Press, Totowa, NJ
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The isomerization of the chromophore is energized by the absorption of a photon. The likelihood that a given photon will produce an isomerization depends on how closely its energy matches the energy required for the transition. Although the same chromophore is found in all the human photopigments, the isomerization energy is modified by its environment and, in particular, the identity of key amino acids in the parts of the opsin molecule that surround the chromophore. These modify the isomerization energy and thus the spectral sensitivity of the photoreceptor (for reviews of spectral tuning, see [9–11]). In most observers with normal color vision, there are four photoreceptor classes: three types of cone photoreceptors, which are referred to as long-, middle-, and short-wavelength sensitive (L, M, and S), and a single type of rod photoreceptor. Rods, which are more sensitive than cones, mediate vision at night when photons are relatively scarce, whereas cones mediate color vision during the day when photons are abundant. Those conditions under which the rods and the cones operate alone are known as scotopic and photopic, respectively, while those under which they operate jointly are known as mesopic (see Chapter 15 on luminous efficiency functions).
Although the cone spectral sensitivities, at the retina, depend mainly on the energy required to isomerize the chromophore, the spectral sensitivities of the human observer measured behaviorally, at the cornea, also depend on the absorption by the optical media and on the density of photopigment in the photoreceptor outer segment, which vary between observers (see the section on other factors that influence spectral sensitivity).
Univariance, Monochromacy, Dichromacy, and Trichromacy
When a photon causes the isomerization of the chromophore and triggers the phototransduction cascade, the effect on the output is independent of its wavelength. Thus, photoreceptors are effectively sophisticated photon counters, the outputs of which vary univariantly according to the number of photons that are absorbed (e.g., [12, 13]). If the rate of photon absorption changes, it is impossible to tell from the photoreceptor output whether the changes are due to a variation in light intensity or in wavelength. In other words, color and intensity are confounded, and individual rods or cones are color blind. Thus, human color perception requires comparisons between the activities of more than one type of photoreceptor that have distinct spectral sensitivities.
If only one photoreceptor type operates, vision is monochromatic or reduced to a single dimension: Two lights of any spectral composition can be made to match perfectly simply by matching their intensities. All normal observers are monochromatic under scotopic conditions (e.g., dim starlight), when only the rods are functioning. However, some rare human observers are monochromatic under photopic conditions because they have either no operating cone photoreceptors (rod monochromats, discussed in a separate section) or only one of the three types (cone monochromats, also discussed in a separate section). If only two cone photoreceptors operate, vision is dichromatic or reduced to two dimensions: Lights of any spectral composition can be matched by a mixture of two other lights. Human observers, who are dichromatic, fall into three classes: protanopes, deuteranopes, and tritanopes, depending on whether they are missing, respectively, the L, M, or S cones (see the sections on protan and deutan defects and on tritanopia). Most observers with normal color vision have three classes of cone photoreceptor and are therefore trichromatic (i.e., they have three dimensions of color vision).
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Fig. 1. Left: The amounts of each of the 444-, 526-, and 645-nm primaries (the tristimulus values) required to match monochromatic lights spanning the visible spectrum are known as the red (R), green (G), and blue (B) color-matching functions or CMFs (red, green, and blue lines respectively; the wavelengths of the primaries are shown by the vertical dashed lines). A negative sign means that the primary must be added to the target to complete the match. CMFs can be linearly transformed from one set of primaries to another. The only restriction on the choice of primary lights is that they must be independent—in the sense that no two will match the third. Right: CMFs for the imaginary cone fundamental L, M, and S primaries, primary lights that would uniquely stimulate the L, M, and S cones, respectively. The fundamentals are the 10° cone sensitivities of Stockman and Sharpe [2].
Trichromacy and Color-Matching Functions
Because normal human photopic vision is trichromatic, the color of any light can be defined or matched by just three variables: the intensities of three specially selected or “independent” primary lights, which are typically chosen to be red (R), green (G), and blue (B) (with the essential proviso that no two will match the third). The left panel of Fig. 1 shows examples of the red, green, and blue color-matching functions (CMFs; also known as tristimulus values) for RGB primaries of 645, 526, and 444 nm. Each CMF defines the amount of that primary required to match monochromatic targets throughout an equal energy visible spectrum.
The CMFs can be linearly transformed to any other set of real primary lights and, as illustrated in Fig. 1, to imaginary primary lights, such as the L, M, and S cone fundamental primaries (right panel). These three fundamental primaries (or Grundempfindungen, fundamental sensations) are the three imaginary primary lights that would uniquely stimulate each of the three cones to yield the L-, M-, and S-cone spectral sensitivity functions (such lights are not physically realizable because of the overlapping spectral sensitivities of the cone photopigments). The three CMFs corresponding to the three fundamental primaries are the cone fundamental CMFs or cone spectral sensitivities. For a further discussion of colorimetry and its link to the cone fundamentals, see the work of Stockman and Sharpe [2] and Stockman [14].
A knowledge of the linear transformation from the red, green, and blue CMFs to the three cone fundamental CMFs would allow us to define the mean human cone