52 Biological Psychology
facts of human color vision, could deduce the physiological
processes in the eye and brain.
An interesting chapter in the development of color-vision
theory is the work of Christine Ladd-Franklin (Hilgard,
1987). She completed her PhD in mathematics at Johns
Hopkins in 1882 but was not awarded the degree because she
was a woman. Later she spent a year in Müller’s laboratory in
Göttingen, where he gave her private lectures because, as a
woman, she was not allowed to attend his regular lectures.
She developed a most interesting evolutionary theory of
color vision based on the color zones in the retina. The center
of the fovia has all colors and the most detailed vision. The
next outer zone has red and green sensitivity (as well as blue
and yellow), the next outer zone to this has only blue and yel-
low sensitivity (and black-white), and the most peripheral
regions have only black-white (achromatic) sensitivity.
She argued that in evolution, the achromatic sensitivity
(rods) developed first, followed by evolution of blue and yel-
low receptors and finally red and green receptors. The fact
that red-green color blindness is most common is consistent
with the idea that it is the most recent to evolve and hence the
most “fragile.”
Modern molecular biology and genetics actually provide
support for Ladd-Franklin’s evolutionary hypothesis. The
Old World monkey retina appears to be identical to the
human retina: Both macaques and humans have rods and
three types of cones. It is now thought that the genes for the
cone pigments and rhodopsin evolved from a common ances-
tral gene. Analysis of the amino acid sequences in the differ-
ent opsins suggest that the first color pigment molecule was
sensitive to blue. It then gave rise to another pigment that in
turn diverged to form red and green pigments. Unlike Old
World monkeys, New World monkeys have only two cone
pigments, a blue and a longer wavelength pigment thought to
be ancestral to the red and green pigments of humans and
other Old World primates. The evolution of the red and green
pigments must have occurred after the continents separated,
about 130 million years ago. The New World monkey retina,
with only two color pigments, provides a perfect model for
human red-green color blindness. Genetic analysis of the var-
ious forms of human color blindness, incidentally, suggests
that some humans may someday, millions of years from now,
have four cone pigments rather than three and see the world
in very different colors than we do now.
The modern field of vision, encompassing psychophysics,
physiology, anatomy, chemistry, and genetics, is one of the
great success stories of neuroscience and biological psychol-
ogy. We now know that there are more than 30 different
visual areas in the cerebral cortex of monkeys and humans,
showing degrees of selectivity of response to the various
attributes of visual experience, for example, a “color” area, a
“movement” area, and so on. We now have a very good un-
derstanding of phenomena of visual sensation and perception
(see the chapter by Coren in this volume). The field con-
cerned with vision has become an entirely separate field of
human endeavor, with its own journals, societies, specialized
technologies, and NIH institute.Pitch DetectionAs we noted, Helmholtz published a most influential work on
hearing in 1863 (On the Sensation of Tone). The fundamental
issue was how the nervous system codes sound frequency
into our sensation of pitch. By this time, much was known
about the cochlea, the auditory receptor apparatus. Helmholtz
suggested that the basilar membrane in the cochlea func-
tioned like a piano, resonating to frequencies according to the
length of the fibers. The place on the membrane so activated
determined the pitch detected; this view was called the place
theory of pitch. The major alternative view was the frequency
theory (Rutherford, 1886), in which the basilar membrane
was thought to vibrate as a whole due to the frequency of
the tone activating it. Boring (1926) presented a comprehen-
sive theoretical analysis of these possibilities.
One of Boring’s students, E. G. Wever, together with C. W.
Bray, recorded from the region of the auditory nerve at the
cochlea and found that the recorded electrical signal followed
the frequency of the tone up to very high frequencies, many
thousands of Hertz (Wever & Bray, 1930). So the frequency
theory was vindicated. But there were problems. A single
nerve fiber cannot fire at much greater than 1,000 Hertz. The
attempted answer was the volley theory: Groups of fibers al-
ternated in firing to code higher frequencies.
Wever and Bray’s discovery is an interesting example of a
perfectly good experiment fooled by biology. As it happens,
there is a process in the cochlea much like the pizoelectric
effect—a tone generates electrical activity at the same fre-
quency as the tone, now termed the cochlear microphonic. It
is thought to be an epiphenomenon, unrelated to the coding
functions of the auditory system.
The solution to the question how the cochlea coded tone
frequency was provided by Georg von Békésy. Born in
Budapest, he received his PhD in physics in 1923 and was a
professor at the University of Budapest from 1932 to 1946. In
1947, he accepted a research appointment in the psychology
department at Harvard, where he worked until 1964. During
his time at Harvard, he was offered a tenured professorship
but did not accept it because he disliked formal teaching.