Color Vision

Sight is a complex process that results when visible light, a narrow band of the electromagnetic spectrum between 400 and 700 nanometers (nm), is converted into signals that can be interpreted by the brain. This process involves special light-sensitive cells called photoreceptors that are located in the retina, a thin structure that lines the inside of the eye. These cells capture packets of light, called photons, and transform their energy into signals that are transported from the eye to the occipital cortex, the portion of the brain that allows us to interpret these signals as sight.

Normal human color vision is trichromatic (based on the perception of three primary colors) and requires three types of photoreceptor cells, called cones, each of which contains a different photopigment. Each photopigment

Table 1. Adapted from American Academy of Ophthalmology, 1995.

absorbs particular wavelengths of light in the short (blue, 440-nm), middle (green, 545-nm), or long (red, 560-nm) wavelength region of the visible spectrum. About 7 percent of all cones are blue-sensitive, 37 percent are green-sensitive, and 56 percent are red-sensitive. These cones are the basic mediators of color vision. If one or more of their pigments is missing, color blindness results. Rod cells, unlike cones, detect light intensity but not color. The photopigment in rod cells is called rhodopsin.

The spectral sensitivity of the cone photopigments is intimately related to the structure of the pigment molecules. These are concentrated in the photoreceptor outer segment, the portion of the cell containing the photo-transduction machinery. Each pigment molecule consists of an opsin protein and a chromophore (11-cis-retinal), which is a derivative of vitamin A. Photon absorption by the pigment molecules causes a change in the shape of the chromophore, which initiates the processes that lead to vision.

The different opsins of the cone photopigments and of the rod photopigment are encoded by four separate genes, the BCP (blue cone pigment), GCP (green cone pigment), RCP (red cone pigment), and RHO (rhodopsin) genes. The genes encoding the blue cone and rod pigments reside on the long arms (called the q arms) of chromosome 7 and chromosome 3, respectively. The genes encoding the red and green cone pigments reside on the q arm of the X chromosome.

Color vision defects may be divided into two groups, hereditary and acquired. Hereditary color vision defects are almost always "red-green" and affect 8 percent of males and 0.5 percent of females. Acquired defects are more often "blue-yellow," and affect males and females equally. Hereditary defects are typically bilateral (affecting both eyes), while acquired defects may affect one eye only and are often asymmetric. Hereditary color vision defects tend to remain stable throughout life and are usually not associated with other retinal or optic nerve pathology. Acquired defects, however, may have a more variable course and are frequently associated with observable

Table 2. Adapted from American Academy of Ophthalmology, 1995.

ocular pathology. A common cause of acquired color-vision loss is optic nerve disease, such as optic neuritis.

Inherited color blindness usually results from the loss of one of the photopigments and reduces color vision to two dimensions, or dichromacy. Other less common conditions reduce color vision to one dimension (monochromacy), or may completely extinguish it (achromacy). Vision in this last circumstance is purely dependent on the rods, which function primarily in dim conditions and do not contribute to color vision.

The most common forms of hereditary color blindness are protanopia/anomaly and deuteranopia/anomaly, both of which are caused by defects in the red (L) and green (M) cones. Also known as red-green color vision deficiencies, they typically demonstrate an X-linked recessive pedigree pattern. The incidence of X-linked color-vision defects varies between human populations of different racial origin, with some of the highest rates appearing in Europeans and some populations in India.

The incidence of these common forms of color blindness is much lower in females than in males because the defects are inherited as X-linked recessive traits. Males, who have only one X chromosome, are hemizygous (meaning that they have only one gene present for the trait) and they will always manifest a color vision deficiency if they inherit an abnormal gene from their mother. Females, on the other hand, have two X chromosomes, one inherited from each parent, so they will not usually show a complete manifestation of the typical color defect unless they are homozygous, though a partial manifestation of color blindness may be present in heterozygotic carriers. A variety of special tests are used to screen for these red-green colorvision defects.