FoundationalConceptsNeuroscience

of incident light. Short-wavelength visible light activates primarily S
cones, with M and L cones activated to a lesser extent; we perceive this
light to be violet and blue. We perceive medium-wavelength visible
light, activating primarily M cones, as green and yellow, and long-
wavelength visible light, activating primarily L cones, as orange and
red. Thus, as one moves from 400 nm to 700 nm across the spectrum
of visible light, our perception generates the familiar rainbow of col-
ors: violet, blue, green, yellow, orange, and red. Colors do not exist “out
there” in the world—they are mental experiences related in some still
mysterious way to how our nervous system responds to electromag-
netic radiation having different energies.
We humans, as well as many other species of primate animals, have
what is called trichromatic color vision, mediated by three different
types of cone photoreceptors with different light-sensitivity ranges.
(Rod cells and rhodopsin appear to have little input into color vision.)
The visual system compares the relative amounts of stimulation
among the three cone photoreceptor types, and from this comes our
ability to perceive a rich variety of colors. Many nonprimate mammals
have only two cone photoreceptor types; this dichromatic color vision
comes with a lesser ability to discriminate wavelength differences,
and presumably a less rich experience of color. On the other hand,
many birds have tetrachromatic color vision: four different cone types
allowing for a more nuanced discrimination of wavelength anda
presumably richer experience of color. That is, it is possible that birds
experience colors that we humans have never seen.
The human opsin proteins each have around 350 amino acids, the
exact sequence of which differs among the various cone opsin pro-
teins (and rhodopsin). The genes for the M and L opsins are found on
the X chromosome, one of the human sex chromosomes. The genes
for S opsin and rhodopsin are found on other, nonsex chromosomes.