Color perception is a
fascinating series of physical and chemical reactions which allow some
organisms to see in color. The process of color perception is literally all in
the mind, with the eye containing the equipment which responds to light so that
the brain can process it. The number of colors an organism can distinguish can
vary considerably, from animals that can see a very wide array of millions of
colors to animals that see in a much more limited range.
Two
types of cells in the eye are responsible for vision: rods and cones. Both
cells are located in the retina, and they respond to light when it enters the
eye. Rods are highly light-sensitive, allowing for vision in a range of light
levels, while cones are sensitized to colors of particular wavelength ranges.
Humans have three different types of cones sensitized to short, medium, and
long wavelengths, and they are especially sensitive to yellow and green light.
Organisms with three types of cones are known as trichromatic, and other
animals may have two types of cones (dichromatic), while others have up to five
(pentachromatic).
Theories of
Colour Vision
There are two major theories that explain
and guide research on colour vision: the trichromatic theory also known as the
Young-Helmholtz theory, and the opponent-process theory. These two theories are complementary and
explain processes that operate at different levels of the visual system.
Young-Helmholtz
theory or Trichromatic Theory
In
1802, Thomas Young proposed that all human vision occurred through the combination
of sensitivity to red, green, and blue. This theory, modified by Hermann von
Helmholtz in 1852, came to be known as the Young-Helmholtz or trichromatic
(three-color) theory of color vision. The basic idea was that the eye responded
to three primary colors, and combining the three primary colors of additive
color mixing formed all the other colors.
The
finding that there are three types of color-sensitive cone receptors in the
retina supported the three-color theory. One set of receptors is sensitive to
long wavelengths such as red, one to medium wavelengths such as green, and one
is sensitive to short wavelengths such as blue.
The
Young-Helmholtz theory was inspired by the observation that varying the amounts
of red, blue and green can produce any colour. They speculated that the retina
might consist of three different types of colour-detecting cells, each
sensitive to red, blue or green wavelengths of light. They further speculated
that different rates of firing of these cells gives rise to the perception of
different colours.
There are two
phenomena that this theory cannot explain easily:
• Colour blindness
This
occurs when a person is unable to distinguish between at least two certain
wavelengths of light (e.g. shades of red from shades of green). In some cases
there is no perception of colour at all. It is difficult to see how a theory
based on different types of cone cells for red, blue and green could account
for colour blindness.
• Negative
after-effects
Stare
at a red patch for a couple of minutes or so, and then look at a white sheet of
paper. An after-effect will appear in the form of green patch of colour.
After-effects reflect the opposite of the stimulus to which they have been
exposed (the opposite of the pairs red/green, blue/yellow or light/dark). The
Young- Helmholtz theory cannot explain after-effects.
Opponent-process
theory
An
alternative theory of colour perception was suggested by Hering (1878–1964) in
an attempt to explain colour blindness and negative after-effects. It is based
on the idea that three types of cells in the retina respond to pairs of
opposite colours: red/green, blue/ yellow and light/dark. In its anabolic
phase, a cell processes one colour of the pair it is responsive to and
processes the opposite colour in its catabolic phase. Negative after effects
can be explained by assuming that cells become fatigued by prolonged
stimulation of the same colour and that they will work in the opposite way as
they recover. De Valois, Abramov and Jacobs (1966) provide support for the
opponent-process theory.
They
found bipolar cells in the second layer of the retina and also in the thalamus.
However, MacNichol (1986) found three different types of cells in the retina
that respond maximally to one of the three different wavelengths of light as
predicted by the Young- Helmholtz theory. It seems then that processes
described by both theories are evident in the visual system.
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