Coding Sounds

101
Hearing
genes that might direct the regrowth of damaged hair cells (Izumikawa et al., 2005).
This achievement could revolutionize the treatment of nerve deafness, which cannot be
overcome by conventional hearing aids. In the meantime, scientists have developed an
artificial cochlea that can be implanted in the human ear to stimulate the auditory nerve
(Seghier et al., 2005; Zeng, 2005).
The brain region known as the primary auditory cortex
is larger in trained musicians than in people whose jobs are less focused on fine
gradations of sound. How much larger this
area becomes is correlated with how long
the musicians have studied their art. This
finding reminds us that, as described in
the chapter on biology and behavior, the
brain can literally be shaped by experience
and other environmental factors.
SHAPING THE BRAIN
auditory nerve The bundle of axons
that carries messages from the hair cells
of the cochlea to the brain.
place theory A theory of hearing stating that hair cells at a particular place
on the basilar membrane respond most
to a particular frequency of sound.
volley theory A theory of hearing
stating that the firing rate of an auditory nerve matches a sound wave’s frequency. Also called frequency-matching
theory.
Auditory Pathways to the Brain Before sounds can be heard, the information
coded in the firing of the many axons that make up the auditory nerve must be sent
to the brain for further analysis. This transmission process begins when the auditory
nerve conveys the information to the thalamus. From there, the information is relayed
to the primary auditory cortex, an area in the temporal lobe of the brain (see Figure
2.10). Cells in the auditory cortex have preferred frequencies. That is, individual cells
there respond most vigorously to sounds of a particular frequency. Each neuron in the
auditory nerve also has a “favorite,” or characteristic, frequency, though each also
responds to some extent to a range of frequencies (Schnee et al., 2005). The auditory
cortex examines the pattern of activity of many neurons to determine the frequency of
a sound. Some parts of the auditory cortex are devoted to processing certain types of
sounds. One part, for example, specializes in information from human speech (Belin,
Zatorre, & Ahad, 2002); others are particularly responsive to sounds coming from animals, tools, or musical instruments (Lewis et al., 2005; Zatorre, 2003). The auditory
cortex receives information from other senses as well. For example, the primary auditory cortex is activated when you watch someone say words (but not when the person
makes other facial movements). This is the biological basis for the lip reading that helps
you to hear what people say (van Wassenhove, Grant, & Poeppel, 2005).
Coding Sounds
Most people can hear a wide range of sound intensities. The faintest sound that can be
heard barely moves the ear’s hair cells. Sounds more than a trillion times more intense
can also be heard. Between these extremes, the auditory system codes intensity in a
rather simple way: The more intense the sound, the more rapid the firing of a given
neuron. We are also very good at detecting differences between sound frequencies that
allow us to hear differences in pitch (Shera, Guinan, & Oxenham, 2002). Information
about frequency differences appears to be coded in two ways: by their location on the
basilar membrane and by the rate at which the auditory neurons fire.
As sound waves move down the basilar membrane, they reach a peak and then taper
off, much like an ocean wave that crests and then dissolves. High-frequency sounds
produce a wave that peaks soon after it starts down the basilar membrane. Lowerfrequency sounds produce a wave that peaks farther down the basilar membrane.
According to place theory, the greatest response by hair cells occurs at the peak of the
wave. Because the location of the peak varies with the frequency of sound, it follows
that hair cells at a particular place on the basilar membrane are most responsive to a
particular frequency of sound. When cells with a particular characteristic frequency fire,
we sense a sound of that frequency.
But place theory cannot explain the coding of very low frequencies, such as deep
bass notes, because there are no auditory nerve fibers that have very low preferred frequencies. Humans can hear frequencies as low as twenty hertz, though, so they must
be coded somehow. The answer appears to be frequency matching, a process in which
the firing rate of a neuron in the auditory nerve matches the frequency of a sound
wave. Frequency-matching theory is sometimes called the volley theory of frequency
coding, because the outputs of many cells can combine to create a volley of firing.
The nervous system apparently uses more than one way to code the range of audible
frequencies. The lowest frequencies are coded by frequency matching. Low to moderate
frequencies are coded by frequency matching, as well as by the place on the basilar membrane at which the wave peaks. And high frequencies are coded solely by the place at
which the wave peaks. (“In Review: Hearing” summarizes the coding process and other
aspects of the auditory system.)