Color blindness occurs when light-sensitive cells in the retina fail to respond appropriately to variations in wavelengths of light that enable people to see an array of colors.
Photoreceptors in the retina are called rods and cones. Rods are more plentiful (there are approximately 100 million rods in the human retina) and they are more sensitive to light, but rods are incapable of perceiving color.
The 6 to 7 million cones in the human retina are responsible for color vision, and these photoreceptors are concentrated in the central zone of the retina called the macula.
The center of the macula is called the fovea, and this tiny (0.3 mm diameter) area contains the highest concentration of cones in the retina and is responsible for our most acute color vision.
Inherited forms of color blindness often are related to deficiencies in certain types of cones or outright absence of these cones.
Besides differences in genetic makeup, other causes of color vision defects or loss include:
Parkinson's disease (PD). Because Parkinson's disease is a neurological disorder, light-sensitive nerve cells in the retina where vision processing occurs may be damaged and cannot function properly.
Cataracts. Clouding of the eye's natural lens that occurs with cataracts can "wash out" color vision, making it much less bright. Fortunately, cataract surgery can restore bright color vision when the cloudy natural lens is removed and replaced with an artificial intraocular lens.
Tiagabine for epilepsy. An antiepileptic drug known as tiagabine has been shown to reduce color vision in about 41 percent of those taking the drug, although effects do not appear to be permanent.
Leber's hereditary optic neuropathy (LHON). Particularly prevalent among males, this type of inherited optic neuropathy can affect even carriers who don't have other symptoms but do have a degree of color blindness. Red-green color vision defects primarily are noted with this condition.
Kallman's syndrome. This inherited condition involves failure of the pituitary gland, which can lead to incomplete or unusual gender-related development such as of sexual organs. Color blindness can be one symptom of this condition.