Cones function to perceive color and
require 50 to 100 times more light for
stimulation than do rods. Consequently,
night vision is almost totally
rod vision. Unlike humans, who have
both day and night vision, some vertebrates
specialize for one or the other.
Strictly nocturnal animals, such as bats
and owls, have pure rod retinas. Purely
diurnal forms, such as the common
gray squirrel and some birds, have only
cones; they are virtually blind at night.
In 1802 the English physician and
physicist Thomas Young speculated that
we see color by relative excitation of
three kinds of photoreceptors: one each
for red, green, and blue. In the 1960s
Young’s prescient hypothesis was eventually
supported through the combined
work of several groups of researchers.
Humans have three types of cones,
each containing a visual pigment that
responds to a particular wavelength of
light (Figure 35-34). Blue cones absorb
the most light at 430 nm, green cones at
540 nm, and red cones at 575 nm. Variation
in the structure of opsin produces
the different visual pigments found in
rods and the three types of cones. Colors
are perceived by comparing levels of
excitation of the three different kinds of
cones. For example, a light having a
wavelength of 530 nm would excite
green cones 95%, red cones about 70%,
and blue cones not at all. This comparison
is made both in nerve circuits in the retina and in the visual cortex of the
brain, and the brain interprets this combination
Color vision occurs in some members
of all vertebrate groups with
the possible exception of amphibians.
Bony fishes and birds have particularly
good color vision. Surprisingly, most
mammals are color blind; exceptions
are primates and a few other species
such as squirrels.
|Figure 35-34 The absorption spectrum of human vision.
Three types of visual pigments in cones absorb maximally
430 nm (blue cones), 540 nm (green cones), and 575 nm (red cones).