Eye, Brain, and Vision

example, some light may be absorbed and some reflected. For many objects,
the relative amount of light absorbed and reflected depends on the light's
wavelength. The green leaf of a plant absorbs long- and short-wavelength
light and reflects light of middle wavelengths, so that when the sun hits a leaf,
the light reflected back will have a pronounced broad peak at middle wave-
lengths (in the green). A red object will have its peak, likewise broad, in the
long wavelengths, as shown in the graph on this page.
An object that absorbs some of the light reaching it and reflects the rest is
called a pigment. If some wavelengths in the range of visible light are absorbed
more than others, the pigment appears to us to be colored. What color we see, I
should quickly add, is not simply a matter of wavelengths; it depends on
wavelength content and on the properties of our visual system. It involves
both physics and biology.

VISUAL RECEPTORS
Each rod or cone in our retina contains a pigment that absorbs
some wavelengths better than others. The pigments, if we were able to get
enough of them to look at, would therefore be colored. A visual pigment has
the special property that when it absorbs a photon of light, it changes its
molecular shape and at the same time releases energy. The release sets off a
chain of chemical events in the cell, described in Chapter 3, leading ultimately
to an electrical signal and secretion of chemical transmitter at the synapse. The
pigment molecule in its new shape will generally have quite different light-
absorbing properties, and if, as is usually the case, it absorbs light less well
than it did before the light hit it, we say it is bleached by the light. A complex
chemical machinery in the eye then restores the pigment to its original confor-
mation; otherwise, we would soon run out of pigment.
Our retinas contain a mosaic of four types of receptors: rods and three types
of cones, as shown in the illustration at the top of the facing page. Each of
these four kinds of receptors contains a different pigment. The pigments differ
slightly in their chemistry and consequently in their relative ability to absorb
light of different wavelengths. Rods are responsible for our ability to see in
dim light, a kind of vision that is relatively crude and completely lacks color.
Rod pigment, or rhodopsin, has a peak sensitivity at about 510 nanometers, in
the green part of the spectrum. Rods differ from cones in many ways: they are
smaller and have a somewhat different structure; they differ from cones in
their relative numbers in different parts of the retina and in the connections
they make with subsequent stages in the visual pathway. And finally, in the
light-sensitive pigments they contain, the three types of cones themselves dif-
fer from each other and from rods.
The pigments in the three cone types have their peak absorptions at about
430, 530, and 560 nanometers, as shown in the graph on this page; the cones
are consequently loosely called "blue", "green", and "red", "loosely" because
(i) the names refer to peak sensitivities (which in turn are related to ability to
absorb light) rather than to the way the pigments would appear if we were to
look at them; (2) monochromatic lights whose wavelengths are 430, 530, and
560 nanometers are not blue, green, and red but violet, blue-green, and
yellow-green; and (3) if we were to stimulate cones ofjust one type, we would
see not blue, green, or red but probably violet, green, and yellowish-red in-
stead. However unfortunate the terminology is, it is now widely used, and

efforts to change embedded terminology usually fail. To substitute terms such
as long, middle, and short would be more correct but would put a burden on
those of us not thoroughly familiar with the spectrum.
With peak absorption in the green, the rod pigment, rhodopsin, reflects blue
and red and therefore looks purple. Because it is present in large enough
amounts in our retinas that chemists can extract it and look at it, it long ago
came to be called visual purple. Illogical as it is, "visual purple" is named for the
appearance of the pigment, whereas the terms for cones, "red", "green", and
"blue", refer to their relative sensitivities or abilities to absorb light. Not to
realize this can cause great confusion.
The three cones show broad sensitivity curves with much overlap, espe-
cially the red and the green cones. Light at 600 nanometers will evoke the
greatest response from red cones, those peaking at 560 nanometers, but will
likely evoke some response, even if weaker, from the other two cone types.
Thus the red-sensitive cone does not respond only to long-wavelength, or red,
light; it just responds better. The same holds for the other two cones.
So far I have been dealing with physical concepts: the nature of light and
pigments, the qualities of the pigments that reflect light to our eyes, and the
qualities of the rod and cone pigments that translate the incoming light into
electrical signals. It is the brain that interprets these initial signals as colors. In
conveying some feel for the subject, I find it easiest to outline the elementary
facts about color vision at the outset, leaving aside for the moment the three-
century history of how these facts were established or how the brain handles
color.

GENERAL COMMENTS ON COLOR
It may be useful to begin by comparing the way our auditory sys-
tems and our visual systems deal with wavelength. One system leads to tone
and the other to color, but the two are profoundly different. When I play a
chord of five notes on the piano, you can easily pick out the individual notes
and sing them back to me. The notes don't combine in our brain but preserve
their individuality, whereas since Newton we have known that if you mix two
or more beams of light of different colors, you cannot say what the compo-
nents are, just by looking.
A little thought will convince you that color vision has to be an impover-
ished sense, compared with tone perception. Sound coming to one ear at any
instant, consisting of some combination of wavelengths, will influence thou-
sands of receptors in the inner ear, each tuned to a slightly different pitch than
the next receptor. If the sound consists of many wavelength components, the
information will affect many receptors, all of whose outputs are sent to our
brains. The richness of auditory information comes from the brain's ability to
analyze such combinations of sounds.