Acquired Colour Vision Deficiencies


"... as soon as he entered, he found his entire studio, which was hung with brilliantly colored paintings, now utterly grey and void of color. His canvases, the abstract color paintings he was known for, were now greyish or black and white. His paintings--once rich with associations, feelings, meanings--now looked unfamiliar and meaningless to him. At this point the magnitude of his loss overwhelmed him.

"He had spent his entire life as a painter; now even his art was without meaning, and he could no longer imagine how to go on."

     Oliver Sacks, The Case of the Colorblind Painter, 1995
      In An Anthropologist On Mars, p.6



It is not fully clear if the brain damage that caused the "Colorblind Painter" to lose his vision was caused by a car accident in which he was involved, or perhaps by carbon monoxide poisoning that also contributed to the car accident. His acquired loss of colour vision, however, was sudden, complete and life-changing, particularly so given his profession.

Acquired Versus Congenital Colour Deficiencies
The tragic case of the Colorblind Painter as described by Sacks is an example of an acquired colour vision deficiency--one that is due to life events such as brain trauma, disease, or the effects of a toxic agent. Such "acquired" deficiencies also can be distinguished functionally from inborn or "congenital" deficiencies in a number of ways.

Congenital deficiencies typically involve red-green confusions, whereas acquired deficiencies more often than not are a blue-yellow problem (called Köllner’s Rule). Also, because some of the most common congenital defects are linked to the X female chromosome, they are more prevalent in males than females. Acquired defects, in contrast, are not related to gender except by gender differences in the experience of trauma or toxic exposure. Acquired colour deficiencies are more likely to be asymmetric between the two eyes than are hereditary defects; they are also less likely to be stable with time. Congenital defects are usually easier to detect with standard clinical colour vision tests, while some acquired ones can be more subtle and thus are difficult to diagnose. Finally, those with acquired colour deficiencies are also more likely to display colour-naming errors because, unlike those with congenital deficiencies, they lack the life-long experience with defective colour perception.

Types of Acquired Colour Deficiencies
Because acquired colour vision deficiencies are due to life events, they can be due to any of a number of different causes that affect the optic media of the eye, retina, visual pathways, or areas of the brain that process colour information.

Cataracts
As we age, the crystalline lens of the eye hardens, becomes more opaque, and tends to yellow over time. Exposure to UV-A (320-400 nm) and UV-B (230-320 nm) high-energy wavelengths can contribute to these changes. As a result of yellowing (known as xanthopsia), the lens selectively absorbs short-wavelength light (blues and greens), gradually making discriminations in this part of the spectrum more difficult. Other secondary contributors to cataracts include glaucoma and diabetes.

Although the cornea absorbs most of the energy from the infra-red part of the electromagnetic spectrum (from 800 nm to 106 nm), if excessive and continued, it too can induce cataracts and a yellowing of the lens. Occupations at risk for this include welding and glass blowing, especially in the absence of appropriate eye protection; referred to as glassblower's cataract. The image below simulates how a person with cataracts (left) may experience a normal colour scene (right).




Corneal Edema
Corneal edema refers to a swelling in the tissues of the cornea, which can cause scarring of its inner layer. Causes of the swelling include infection, allergic reactions and irritation from contact lenses. A person with corneal edema is likely to see coloured rainbows or halos around bright light sources, especially at night when glare is more pronounced. The image below is a simulation of how a visual scene might be experienced by a person suffering from corneal edema.


Age-Related Maculopathy (A.R.M.)
This disease, also referred to as age-related macular degeneration (A.M.D.), involves a loss of the cone-rich area of central vision (i.e., the macula) where acuity and colour vision are best. A.R.M. is a leading cause of blindness, and the most prevalent form of acquired colour vision deficiency in the developed world. Approximately twenty-three percent of people over the age of 65 display some form of A.R.M. There are two forms of the disease, a "dry" and a "wet" form.

Approximately ninety percent of A.R.M. patients have the slow-developing "dry" form of the disease, wherein small yellowish waste deposits called drusen accumulate underneath the macula (see image above right). These drusen are associated with the breakdown of cone photoreceptors in the macula, which can result in a loss of acuity and colour vision.

The remaining ten percent or so of A.R.M. patients have the fast-developing "wet" form of the disease, where tiny blood vessels begin to grow behind the retina toward the macula (called neovascularization). These new vessels are fragile and prone to bleeding, leaking blood into the vitreous and surround tissue of the macula. This causes rapid and severe vision loss, and colour vision is often distorted as the disease progresses to blindness.

The visual distortions that a person with A.R.M. would experience are simulated in the image to the left. Patients often report that objects in their central field of vision become distorted, changing shape, size, or colour, and may even seem to move or disappear.

 

Diabetic Retinopathy
By elevating blood sugar levels, diabetes can induce changes in the retinal capillaries sufficiently severe to affect vision. Diabetic retinopathy, another leading cause of age-related blindness, involves the swelling of blood vessels, and in a minority of cases, the abnormal growth of new vessels (i.e., neovascularization). These new vessels are fragile, and may leak blood into the vitreous, reducing the light reaching the retina (see accompanying figure). They may also fail to supply adequate oxygen to meet the metabolic needs of the photoreceptors. The associated death of cones in the area can impair both acuity and colour vision.

 

 

The image below simulates the vision loss a person with diabetic retinopathy would experience. Notice the diffuse and irregular pattern of vision loss.


 

Migraines
Migraines, which result from changes in brain blood flow, can produce severe headaches as well as disturbing visual experiences. They affect about ten percent of the general population. There are several different kinds of migraines, two of which can produce colour vision deficiencies or distortions: migraine headache with visual prodrome and ophthalmic migraine. In both cases, they are due to temporary spasms in blood vessels that cause them to constrict. This vasoconstriction reduces oxygen delivery to the photoreceptors, especially to cones, which have higher metabolic needs than rods. This in turn results in a transient loss or distortion of colour vision and central acuity for periods of several minutes up to an hour. This "migraine" that occurs does not necessarily involve any experience of a headache--often only visual distortions are present. The accompanying image simulates the flashes and ribbons of colour that someone suffering from migraine might experience.

While patient experiences can vary considerably, a common syndrome is a small central blind spot (scotoma) with shimmering zig-zags of light within it. As the migraine progresses, the crescent-shaped blind spot gradually grows larger and may shift around the central field of view. As the visual distortion subsides, a headache may develop. This form is known as a migraine headache with visual prodome. In the absence of a headache during the vasoconstriction, it is termed an ophthalmic migraine.

 

Optic Neuritis
Optic neuritis refers to an inflammation of the optic nerve which can result in blurred vision and a distortion or lack of colour vision. Although the cause of optic neuritis is not known, it is thought to begin with the formation of plaques around the myelin sheath of the optic nerve. It has been diagnosed in young children following an illness such as the measles or mumps; it can also indicate a neurological impairment. It is most prevalent in adults in their thirties, and over one third of patients suffering from optic neuritis proceed to develop multiple sclerosis later in life. Multiple sclerosis is the result of a degeneration of the myelin sheath of nerve fibres in the central nervous system, impairing nerve transmission. After plaque formation the onset of the visual symptoms of optic neuritis is rapid, ranging from hours to days.

 

People suffering from optic neuritis have reported an increased sensitivity to light, pain in eye movements, scotomas, and a loss of colour vision. Although A.R.M. and diabetic retinopathy have similar symptoms, visual distortions in these two diseases are manifest in a person's central vision; in contrast, optic neuritis produces these symptoms across the entire visual field. The accompanying image simulates the loss of colour that might accompany optic neuritis.

Cerebral Achromatopsia
Oliver Sacks' The Case of the Colorblind Painter
documents eloquently the history and experience of a professional painter who had the great misfortune to suffer cerebral achromatopsia. This rare type of colour vision loss is due to damage to the brain areas responsible for processing colour, usually to area V4 in the temporal lobe. Because areas V2 and V4 are among the most metabolically active areas of the cerebral cortex, they are therefore among the first to suffer from the effects for reduced oxygen delivery. This could be due to a variety of causes, including carbon monoxide poisoning and stroke. Unlike the rod monochromat whose loss of colour is due to the absence of cones, since the retina is not damaged, the achromatopsia patient may have excellent acuity.

The simulation below gives some idea how a scene viewed someone with cerebral achromatopsia (left panel) might compares with the same landscape viewed by someone with normal colour vision (right panel).

 

Because brain damage due to trauma is seldom confined to one specific area, functions other than colour perception can also be affected. For example, objection recognition defects (termed agnosias) are common in these patients. One such defect, colour anomia, is the inability to name colours appropriately.

The loss of colour vision may occur for one (hemiachromatopsia) or both halves of the visual field (bilateral achromatopsia). If the cortical lesions are confined to one hemisphere only, hemiachromatopsia for the visual field contralateral to the side of damage would result (simulated below).

 

Transient Achromatopsia
Transient achromatopsia, a temporary loss of colour vision, is caused by a short-lived vascular insufficiency, apparently to V1 blobs and the thin stripes of V2 in the occipital cortex. People suffering from strokes or mild cerebral infarctions have been observed rarely to display this temporary loss of colour vision. While perceptually identical to the cerebral form of achromatopsia, it only persists during the temporary constriction of blood vessels in the brain.

 

Chromatopsia
Chromatopsias are more of a colour distortion than an outright deficiency. Patients suffering from chromatopsias simply do not perceive certain colours as well as others. Chromatopsias take two forms. One of these is distinguished by the colour that predominate in vision (cyanopsia or xanthopsia); the other is even more rare that is experienced by some blind people (phantom chromatopsia).

Cyanopsia is characterized by the patient's illusory perception of a penetrating blueness in the scene. It is frequently observed in patients who have had recent cataract surgery in which the natural lens is replaced with a clear plastic implant. After living with the yellowing filtering effects (i.e., xanthopsia) of cataracts for so many years, the visual cortex apparently compensates by adding blue to the visual scene. This mechanism may be similar to the those that underlie colour constancy. The bluish tinge may persist for weeks or months but gradually it gives way to normal colour vision. The following image is a simulation of how cyanopsia may affect someone's colour vision.



Xanthopsia refers to the predominance of yellow in the visual scene due to the yellowing of the optic media of the eye. This yellow filtering effect can be induced by cataracts as well as the chronic use of the drug digitalis. It has been suggested by some that the prevalent use of yellow in the work of the famous painter Vincent Van Gogh (1853-1890) may be due to digitalis, used at the time as a treatment for epilepsy. Xanthopsia can also be induced by the chemical fluorescein used in fluorescein angiography, although this form is very short-lived. The image below is a simulation of how a person with xanthopsia may experience colour.

Phantom Chromatopsia. This rare disorder can occur in patients who are blind or nearly blind. Zeki (1993) has documented this condition in only a few individuals, but it may be somewhat underreported in the literature.

Patients report a sensation of colour (usually golden or purple) that can occur even during sleeping. For most of these people it is apparently a haunting, even terrifying experience that can lead to suicide or psychotic symptoms. 

Phantom achromatopsia may be due to simulation of cells in area V4 by cortical irritation or other trigger factors (memory, experience, or situational factors). The image at right is an artist's impression of what such a visual experience might be like.


 

   


Bases of Colour Vision created by 
Brian Thomas Wagner 
and Donald Kline
University of Calgary