Sensory Physiology 303only one of these genes to produce only one of these photo-
psins. Humans and Old World primates (including chimpanzees,
gorillas, and gibbons) have trichromatic color vision. We are
trichromats, with three different types of cones. These may be
designated blue, green, and red, according to the region of the
visible spectrum in which each cone’s pigment absorbs light
best ( fig. 10.42 ). This is each cone’s absorption maximum.
The absorption maximum for the blue cones at 420 nano-
meters (nm) is in the short wavelengths, so these are also known
as S cones. The absorption maximum for the green cones (at
530 nm) is in the middle wavelengths, and so these are called
M cones. The red cones (with an absorption maximum of
562 nm) absorb best in the longer wavelengths, and so are
L cones. This trichromatic vision is exploited by television and
computer screens, which have only red, green, and blue pixels
and yet provide us with the multitude of colors we can perceive.
The gene for the S cone pigment is located on autosomal
chromosome number 7, whereas the genes for the M and L
cones are located in the X chromosome. Most mammals other
than humans and Old World primates have only two types of
cones, M (green) and S (blue). Because of this, they are dichro-
mats. Scientists believe that our trichromatic vision evolved
in an ancestral species with dichromatic vision after the gene
for the M cone pigment duplicated on the X chromosome.
The duplicate could then have given rise to the third type of
cones, the L cones able to absorb light best in the longer (red)
wavelengths.
Suppose that a person has become dark adapted but
wishes to be able to see (a star map, for example) without
losing the dark adaptation. Because rods do not absorb red
light but red cones (L cones) do, a red flashlight will allow
vision due to excitation of the red cones but will not cause
bleaching of the dark-adapted rods. When the red light is
turned off, the rods will still be dark-adapted and the person
will still be able to see.
An individual cone’s response to light depends on both
the wavelength (color) of the light and its intensity. For exam-
ple, a green (M) cone is stimulated effectively by a weaker
green light ( fig. 10.42 ), but it can be equally stimulated by
a more intense red light. The color we perceive actually
depends on neural computations of the effects of a light on
different types of cones. Certain ganglion cells have inputs
from their receptive fields arranged into a central excitatory
(or “on”) region surrounded by an antagonistic “off ” surround
(see fig. 10.47 ). This allows the effects of different cones
to oppose each other. There are two classes of such opposi-
tion: (1) L 2 M contrasts the activity of L and M cones; and
(2) S 2 (L 1 M) compares the activity of S cones with the
combined activity of L and M cones. This provides informa-
tion about color and light intensity. The ganglion cells pro-
ject to the lateral geniculate nuclei of the thalamus, which are
arranged in layers that preserve this information and relay it
to the primary visual cortex. The neural pathways of vision
are described in more detail shortly.
Figure 10.42 The three types of cones. Each type
contains retinene, but the protein with which the retinene is
combined is different in each case. Thus, each different pigment
absorbs light maximally at a different wavelength. Color vision is
produced by the activity of these blue cones, green cones, and
red cones.
*See the Test Your Quantitative Ability section of the Review
Activities at the end of this chapter.
S (blue) cone M (green) cone L (red) cone400 500 600 700
Wavelength (nanometers)100500Relative sensitivity (%)CLINICAL APPLICATION
Color blindness is caused by the congenital lack of one
or more types of cones. Most commonly this involves
either the L (red) or M (green) cones, producing red-green
color blindness. People with this condition have only two
functioning types of cones (are dichromats), have trouble
distinguishing red from green, and see colors like purple
and brown—which are combinations of colors—differently
than normal. The absence of M cones (called deuteran-
opia ) is more common than the absence of L cones (called
protanopia ). The absence of S cones ( tritanopia ) is the
least common condition.
The genes for the M and L cone photopsins (pigments)
are located on the X chromosome. Because men have only
one X chromosome and cannot carry a mutation in these
genes as a recessive trait, red-green color blindness is far
more common among men than women (with incidences
of 8% and 0.5%, respectively). Gene therapy, by inserting
the gene for the defective cone photopsin into the retina
using a virus vector, has been reported to be successful in
squirrel monkeys, but is not presently available as a medical
treatment.