Eye, Brain, and Vision

COLOR VISION

The hundreds of dollars extra that consumers are willing to pay for color TV
in preference to black and white must mean that we take our color sense
seriously. A complex apparatus in the eye and brain can discriminate the dif-
ferences in wavelength content of the things we see, and the advantages of this
ability to our ancestors are easy to imagine. One advantage must surely have
been the ability to defeat the attempts of other animals to camouflage them-
selves: it is much harder for a hunted animal to blend in with the surroundings
if its predator can discriminate the wavelength as well as the intensity of light.
Color must also be important in finding plant food: a bright red berry standing
out against green foliage is easily found by a monkey, to his obvious advantage
and presumably to the plant's, since the seeds pass unharmed through the
monkey's digestive tract and are widely scattered. In some animals color is
important in reproduction; examples include the bright red coloration of the
perineal region of macaque monkeys and the marvelous plumage of many
male birds.
In humans, evolutionary pressure to preserve or improve color vision
would seem to be relaxing, at least to judge from the 7 or 8 percent of human
males who are relatively or completely deficient in color vision but who seem
to get along quite well, with their deficit often undiagnosed for years, only to
be picked up when they run through red lights. Even those of us who have
normal color vision can fully enjoy black-and-white movies, some of which
are artistically the best ever made. As I will discuss later, we are all color-blind
in dim light.
Among vertebrates, color sense occurs sporadically, probably having been
downgraded or even lost and then reinvented many times in the course of
evolution. Mammals with poor color vision or none at all include mice, rats,
rabbits, cats, dogs, and a species of monkey, the nocturnal owl monkey.
Ground squirrels and primates, including humans, apes, and most old world
monkeys, all have well-developed color vision. Nocturnal animals whose vi-
sion is specialized for dim light seldom have good color vision, which suggests
that color discrimination and capabilities for handling dim light are somehow
not compatible. Among lower vertebrates, color vision is well developed in
many species of fish and birds but is probably absent or poorly developed in
reptiles and amphibia. Many insects, including flies and bees, have color vi-
sion. We do not know the exact color-handling capabilities of the overwhelm-
ing majority of animal species, perhaps because behavioral or physiological
tests for color vision are not easy to do.
The subject of color vision, out of all proportion to its biologic importance
to man, has occupied an amazing array of brilliant minds, including Newton,
Goethe (whose strength seems not to have been science), and Helmholtz. Nev-
ertheless color is still often poorly understood even by artists, physicists, and
biologists. The problem starts in childhood, when we are given our first box
of paints and then told that yellow, blue, and red are the primary colors and
that yellow plus blue equals green. Most of us are then surprised when, in
apparent contradiction of that experience, we shine a yellow spot and a blue
spot on a screen with a pair of slide projectors, overlap them, and see in the
overlapping region a beautiful snow white. The result of mixing paints is
mainly a matter of physics; mixing light beams is mainly biology.
In thinking about color, it is useful to keep separate in our minds these
different components: physics and biology. The physics that we need to know
is limited to a few facts about light waves. The biology consists of psycho-
physics, a discipline concerned with examining our capabilities as instruments
for detecting information from the outside world, and physiology, which ex-
amines the detecting instrument, our visual system, by looking inside it to
learn how it works. We know a lot about the physics and psychophysics of
color, but the physiology is still in a relatively primitive state, largely because
the necessary tools have been available for only a few decades.

THE NATURE OF LIGHT
Light consists of particles called photons, each one of which can be
regarded as a packet of electromagnetic waves. For a beam of electromagnetic
energy to be light, and not X-rays or radio waves, is a matter of the wave-
length—the distance from one wave crest to the next—and in the case of light
this distance is about 5 X 10 to the -7 meters, or 0.0005 millimeter, or 0.5 micrometer, or 500 nanometers.
Light is defined as what we can see. Our eyes can detect electromagnetic
energy at wavelengths between 400 and 700 nanometers. Most light reaching

our eyes consists of a relatively even mixture of energy at different wave-
lengths and is loosely called white light. To assess the wavelength content of a
beam of light we measure how much light energy it contains in each of a series
of small intervals, for example, between 400 and 410 nanometers, between 410
and 420 nanometers, and so on, and then draw a graph of energy against
wavelength. For light coming from the sun, the graph looks like the left illus-
tration on this page. The shape of the curve is broad and smooth, with no very
sudden ups or downs, just a gentle peak around 600 nanometers. Such a broad
curve is typical for an incandescent source. The position of the peak depends
on the source's temperature: the graph for the sun has its peak around 600
nanometers; for a star hotter than our sun, it would have its peak displaced
toward the shorter wavelengths—toward the blue end of the spectrum, or the
left in the graph—indicating that a higher proportion of the light is of shorter
wavelength. (The artist's idea that reds, oranges, and yellows are warm colors
and that blues and greens are cold is related to our emotions and associations,
and has nothing to do with the spectral content of incandescent light as related
to temperature, or what the physicists call color temperature.)
If by some means we filter white light so as to remove everything but a
narrow band of wavelengths, the resulting light is termed monochromatic (see
the graph at the right on this page).

PIGMENTS
When light hits an object, one of three things can happen: the light
can be absorbed and the energy converted to heat, as when the sun warms
something; it can pass through the object, as when the sun's rays hit water or
glass; or it can be reflected, as in the case of a mirror or any light-colored
object, such as a piece of chalk. Often two or all three of these happen; for