Page 369 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Color Vision 420 nm); (2) M cones, which absorb medium-
wave (M) blue-green to yellow light (λ max =
White light passing through a prism is split 535 nm), and (3) L cones, which absorb long-
into a color spectrum ranging from red to vio- wave (L) yellow to red light (λ max = 565 nm).
let (colors of the rainbow). Red is perceived at a (The physiological sensitivity curves shown in
wavelength (λ) of 650–700 nm, and violet at E1 make allowances for light absorbed by the
around 400–420 nm (! A). The eye is sensitive lens.) Ultraviolet rays (λ max ! 400 nm) and in-
to waves in this λ range. Perception of white
frared rays (λ max " 700 nm) are not visible.
Central Nervous System and Senses plementary colors, e.g., orange (612 nm) and trichromatic theory of color vision) and trans-
light does not require the presence of all colors
Sensory information relayed by the three
of the visible spectrum. It is sufficient to have
types of cones (peripheral application of the
the additive effects (mixing) of only two com-
duction of these visual impulses to brightness
blue light (490 nm).
and opponent color channels ( ! E2 and p. 354)
in the retina and lateral geniculate body (LGB)
A color triangle (! B) or similar test panel can be
enables the visual cortex (! p. 358) to recog-
used to illustrate this effect. The two upper limbs of
nize different types of colors. The human eye
the triangle show the visible spectrum, and a white
can distinguish 200 shades of color and differ-
dot is located inside the triangle. All straight lines
ent degrees of color saturation. The absolute
passing through this dot intersect the upper limbs of
the triangle at two complementary wavelengths
(! D, “normal”).
(! C): The color yellow is obtained by mixing roughly
equal parts of red and green. Orange is produced by
Color perception is more complex. White paper, for
example, will look white in white light (sunlight), yel-
12 (e.g., 612 and 490 nm). Additive color mixing differential threshold for color vision is 1–2 nm
using a higher red fraction, and yellowish green is ob-
tained with a higher green fraction. These colors lie low light (light bulb) and red light. We also do not
between red and green on the limbs of the color tri- perceive the different shades of color in a house that
angle. Similar rules apply when mixing green and vio- is partially illuminated by sunlight and partially in the
let (! B and C). The combination of red with violet shade. This color constancy is the result of retinal
yields a shade of purple not contained in the spec- and central processing of the retinal signal.
trum (! B). This means that all colors, including There is a similar constancy of size and shape:
white, can be produced by varying the proportions Although someone standing 200 meters away
of three colors—e.g. red (700 nm), green (546 nm) makes a much smaller image on the retina that at
and blue (435 nm) because every possible pair of 2 meters’ distance, we still recognize him or her as a
complementary colors can be obtained by mixing person of normal height, and although a square table
these three colors of the spectrum. may appear rhomboid in shape when viewed from
Subtractive color mixing is based on the op- the side, we can still tell that it is square.
posite principle. This technique is applied when color Color blindness occurs in 9% of all men and in
paints and camera filters are used. Yellow paints or 0.5% of women. The ability to distinguish certain
filters absorb (“subtract”) the blue fraction of white colors is impaired or absent in these individuals, i.e.,
light, leaving the complementary color yellow. they have a high differential threshold for color (! D).
Various types of color blindness are distinguished:
Light absorption. Photosensors must be able to protanopia (red blindness), deuteranopia (green
absorb light to be photosensitive. Rods blindness), and tritanopia (blue-violet blindness).
(! p. 348) contain rhodopsin, which is re- Protanomaly, deuteranomaly and tritanomaly are
sponsible for (achromatic) night vision. characterized by decreased sensitivity of the cones
Rhodopsin absorbs light at wavelengths of ca. to colored, green and blue, respectively. Color vision
400–600 nm; the maximum absorption value is tested using color perception charts or an anomalo-
(λ max) is 500 nm (! E1). Relatively speaking, scope. With the latter, the subject has to mix two
color beams (e.g., red and green) with adjustable in-
greenish blue light therefore appears brightest tensities until their additive mixture matches a
and red appears darkest at night. Wearing red specific shade of color (e.g. yellow, ! C) presented
glasses in daylight therefore leaves the rods for comparison. A protanomal subject needs a too
adapted for darkness. Three types of color-sen- high red intensity, a deuteranomal person a too high
sitive cones are responsible for (chromatic) green intensity. Protanopes perceive all colors with
day vision (! E1): (1) S cones, which absorb wavelengths over approx. 520 nm as yellow.
356 short-wave (S) blue-violet light (λ max =
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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