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Concept
Monochromatisme
Résumé
Monochromacy (from Greek mono, meaning "one" and chromo, meaning "color") is the ability of organisms or machines to perceive only light intensity without respect to spectral composition. Such organisms and machines are colorblind in most the literal sense of the word. Organisms with monochromacy are called monochromats.
Many mammals, such as cetaceans, the owl monkey and the Australian sea lion (pictured at right) are monochromats.
In humans, absence of color vision is one among several other symptoms of severe inherited or acquired diseases, including achromatopsia or blue cone monochromacy, together affecting about 1 in 30,000 people. The affected can distinguish light, dark, and shades of gray but not color.
Human vision relies on a duplex retina, comprising two types of photoreceptor cells. Rods are primarily responsible for dim-light scotopic vision and cones are primarily responsible for day-light photopic vision. For all known vertebrates, scotopic vision is monochromatic, since there is typically only one class of rod cell. However, the presence of multiple cone classes contributing to photopic vision enables color vision during daytime conditions.
Most humans have three classes of cones, each with a different class of opsin. These three opsins have different spectral sensitivities, which is a prerequisite for trichromacy. An alteration of any of these three cone opsins can lead to colorblindness.
Anomalous trichromacy, when all three cones are functional, but one or more is altered in its spectral sensitivity.
Dichromacy, when one of the cones is non-functional and one of the red-green or blue-yellow opponent channels are fully disabled.
Cone monochromacy, when two of the cones are non-functional and both chromatic opponent channels are disabled. Vision is reduced to blacks, whites, and greys.
Rod Monochromacy (Achromatopsia), when all three of the cones are non-functional and therefore photopic vision (and therefore color vision) is disabled.
Source officielle
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