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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.
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