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CHAPTER 13 Looking At Vision andHearing 251
Good vibrations: Gathering and
transmitting sound to the brain
The auditory system has three major structures: the outer ear, middle ear, and
inner ear. Figure 13-6 shows its anatomy. The outer and middle ears involve
mechanical collection and transformations of sound waves to help detect and dis-
criminate sounds. The inner ear is where neurons respond to sound and produce
a neural signal that is sent to the brain through the auditory nerve.
The auditory system starts with mechanical elements that capture, direct, and
amplify sound.
The rst mechanical transformations happen in the outer ear, which has three
parts:
»
Pinna
»
Auditory canal
»
Eardrum (which transitions to the middle ear)
FIGURE13-6:
The anatomy of
the auditory
system.
© John Wiley & Sons, Inc.
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252 PART 3 Understanding Perception and Cognition
Tuning and directing in the outer ear
The rst part of the outer ear that sound waves encounter is the pinna. The pinna
is the technical name for what is commonly called the ear. The pinna reects
sound waves into the auditory canal. Specically, the pinna does two things to
sound:
»
It concentrates sound by reecting it from the larger pinna area to the smaller
auditory canal opening.
»
It changes the frequency content of the incoming sound based on the
elevation it came from, based on the pinna’s complex shape.
Each person’s pinna is slightly dierent. As you develop, you learn your own
pinna’s directional frequency transform function to better localize sound.
The auditory canal has a resonance (at which it transmits better) at midrange
frequencies that are important for hearing human voices. When sound reaches the
end of the auditory canal it causes the eardrum (called the tympanic membrane) to
vibrate. The eardrum is attached to the three bones of the middle ear.
Amplifying in the middle ear
On the other side of the eardrum from the auditory canal are three tiny bones that
constitute the middle ear:
»
Hammer (Latin name: malleus)
»
Anvil (Latin name: incus)
»
Stirrup (Latin name: stapes)
The vibrations of the eardrum move the attached hammer, which moves the anvil,
which moves the stirrup, which is attached to the oval window at the entrance to
the cochlea. We discuss the cochlea in the next section.
The hammer, anvil, and stirrup are the smallest bones in the body, and they com-
prise a lever system. The ball-like head of the hammer— where it connects to the
anvil — is the fulcrum in this system. An eardrum movement that has larger
vibrations, but weaker force, causes a smaller but stronger movement of the stir-
rup. This leverage system exists because uid in the cochlea, which is going to be
vibrated, is stier than air at the eardrum.
Another mechanical advantage in this system is that the area of the eardrum
is larger than the area of the oval window. That means the force per area is ampli-
ed by this ratio as well. The total leverage ratio of the middle ear bones and
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CHAPTER 13 Looking At Vision andHearing 253
dierence in eardrum versus oval window area produces a leverage ratio roughly
around 20 to 1.
Responding to waves with hair cells
The oval window vibrates the cochlea, which is lled with uid. The Organ of Corti
is the structure inside the cochlea that contains the hair cells that respond to
sound vibrations. Vibrations create pressure waves that bend the cilia on the audi-
tory hair cells in the Organ of Corti. These cilia are called stereocilia, small actin-
rich protrusions from the cell body. Bending of the cilia leads to depolarization of
hair cells, which releases neurotransmitter onto auditory neurons that re action
potentials along the auditory nerve.
The stereocilia that extend into the cochlea uid are derived— evolutionarily—
from motor-type cilia, such as those that some single-celled organisms use to
move. Auditory hair cells are so sensitive to small vibrations that they respond to
deections of their cilia by movements the size of an atom.
Transducing waves into electrical
responses
Within the auditory inner hair cell membrane are specialized ion channels, celled
mechanosensitive ion channels that are activated when the cilia bend. When these
nonselective cation channels open, an inux of positive ions produces depolariza-
tion of the hair cell, which leads to a release of its neurotransmitter glutamate.
Hair cells show a graded response to the stimulus that modulates glutamate
release. Hair cells synapse on and release glutamate onto neurons called spiral
ganglion neurons (or more informally, auditory neurons) that re action potentials
that are conducted along the auditory nerve to the cochlear nucleus in the
brainstem.
Toning up: Frequency transduction
in the Organ of Corti
The Organ of Corti contains two types of hair cells: inner and outer. There are
fewer inner hair cells than outer ones, but the inner hair cells make more connec-
tions to the auditory nerve, so they generate most of the sound transduction. The
outer hair cells function to control the stiness of the membrane near the hair cell
cilia. This boosts the hair cell response to low amplitude sounds, particularly at
high frequencies, but weakens the response for very loud sounds to prevent hair
cell damage.
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254 PART 3 Understanding Perception and Cognition
Spiral ganglion/auditory neurons resemble — in structure— mechanoreceptors
in the dorsal root ganglia of the spinal cord (refer to Chapter14). The cell body of
the spiral ganglion auditory neurons (whose axons make up the auditory nerve)
are in the spiral ganglion. Auditory signals from spiral ganglion/auditory nerve
axons travel to the cochlear nucleus and onto two areas of the brainstem before
reaching the thalamus.
The cochlea has a spiral structure that causes high-frequency vibrations to be
concentrated near the oval window, while low-frequency vibrations travel to the
end of the spiral. The cochlea and auditory nerve are frequency coded, such that
dierent auditory bers along the cochlea respond best to specic frequencies.
The auditory nerve codes frequency in two ways:
»
Fibers are active according to their location on the cochlea (more important
for high than low frequencies).
»
For low frequencies— fewer than 500 Hertz (Hz)— the nerves can re at the
frequency of the sound. This can’t occur for higher frequencies in the human
hearing range (20 to 20,000 Hz) because neurons can’t re action potentials
faster than about 500Hz. The hair cells re at a particular phase of the sound,
such as the peak amplitude, in a process called phase locking that is important
for later neural circuits to determine location by interaural time dierence.
Auditory bers also code for the amplitude of the sound by the number of action
potentials per sound wave and by having hair cells with dierent thresholds, so
that some only re to very weak or very loud sounds.
The auditory nerve has no explicit code for locating the sound source— unlike the
retina, for example, where position on the retina corresponds precisely to visual
eld angle. Instead, in the auditory system, the auditory pathway computes the
location of the sound in space by combining information from the two ears. This
is something like how the visual system computes depth— not explicitly repre-
sented on the two dimensional retinas, but via binocular dierence detectors in
visual cortex.
Channeling Sounds to the Brain
The spiral ganglion/auditory nerve sends axons to the brainstem cochlear nucleus
(dorsal and ventral cochlear nuclei). From there, auditory information then goes
to the superior olivary nucleus (superior olive), then to the inferior colliculus, and
nally to the auditory thalamic area, called the medial geniculate nucleus. Here we
describe how the relay of sound information is involved inlocalizing sound.
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CHAPTER 13 Looking At Vision andHearing 255
Comparing and relaying
Most axons from the right cochlear nucleus cross over to the superior olive nucleus
on the left; axons from the left project to the superior olive on the right. However,
a few axons stay ipsilateral, so that some areas in the superior olive and around it
have binaural input. This binaural input is part of the mechanism for localizing
sound.
Relay neurons in the superior olive project to the inferior colliculus, located just
below the superior colliculus in the midbrain. The inferior colliculus neurons
project to the medial geniculate nucleus of the thalamus. Virtually all the projec-
tions after the contralateral projection to the superior olive are ipsilateral. So, the
right medial geniculate receives input mostly from the left ear, and the left medial
geniculate receives input mostly from the right ear.
Localizing by intensity and delay comparisons
Neural comparisons between the outputs from each ear make up the neural coding
for localizing sound. In the auditory system, neural mechanisms sensitive to the
dierences in sound volume and arrival time between the two ears mediate how
horizontal sound is localized. Computations based on dierences in arrival time
between the two ears require precise timing, so it must occur very early in the
auditory processing stream. Sound elevation is done primarily by the change in
frequency content by dierent reections from the pinna, as we mention in the
section “Tuning and directing in the outer ear” earlier in this chapter.
Relaying at the medial geniculate
nucleus of the thalamus
Medial geniculate neurons respond similarly to the responses in the auditory
nerve. The thalamic relay for the auditory system is similar to that for vision
described in the section “Relaying at the thalamus” earlier in this chapter. Func-
tions that gate for attention in the thalamus occur in the medial geniculate, as in
other thalamic areas. For example, if you hear a suspicious noise, your attention
will be drawn to that noise and other similar sounds.
Analyzing sounds: Superior temporal lobe
The medial geniculate nucleus projects to the superior temporal lobe at an area
slightly posterior to the middle of the superior sulcus— known as Heschl’s gyrus.
This area is called the primary auditory cortex, or A1. A1 has a tonotopic map, in
which position on the cortical surface corresponds to the frequency to which neu-
rons respond.
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256 PART 3 Understanding Perception and Cognition
Sound localization also is processed by neurons in A1 using subcortical timing and
intensity comparisons. Lesions in A1 compromise the ability to locate the direction
of a sound source.
Perceiving tones in voices and music
Around auditory area A1 are several higher-order areas, referred to as auditory
association areas. Neurons in these areas respond best to complex or environmen-
tally relevant sounds such as chirps or rattles, rather than pure tones.
The selectivity for higher-order patterns in auditory areas is common in cortical
processing. At lower sensory levels, most neurons respond to most stimuli— the
neural signal is carried by a relatively small number of signals. But in the cortex,
neurons prefer more complex patterns, and only a minority of cortical neurons
responds to any specic sound conguration. But many more neurons are availa-
ble to parcel out this sparser representation in which similar sounds drive com-
pletely dierent cells. Humans achieve these specic neural responses by learning
and experience that gives neurons the ability to dierentiate complex stimuli-
based sound feature extraction.
Understanding language
Wernicke’s area is located at the border between the temporal lobe and parietal
lobe (around Brodmann area 22). This area is crucial for processing speech. People
with damage to Wernicke’s area have problems understanding language, and the
language they produce is often nonsensical.
Wernicke’s area has many connections with Broca’s area, another language area
in the frontal lobe. Broca’s area is located just anterior to the primary motor areas
that control the tongue, vocal cords, and other parts needed for speaking. Damage
to Broca’s area makes producing speech and understanding complex sentences
(such as passive-voice constructions) dicult.
Both Wernicke’s area and Broca’s area are on the left side of the brain in about
95 percent of right-handed people and about 60 percent of left-handed people.
Damage to the brain area on the right that is mirror symmetric to Wernicke’s area
causes problems with processing meaningful tones in spoken language— the
changes in tone and rhythm. People with this damage have trouble distinguishing
sarcasm from a question.
Music is a complex stimulus for audio processing. A clinical syndrome called amu-
sia is associated with bilateral damage in higher-order auditory areas. People with
this condition can’t recognize melodies, although they can understand speech and
other complex sounds around them.
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CHAPTER 13 Looking At Vision andHearing 257
Losing Hearing
Hearing loss can range from complete deafness, the loss of only some frequencies,
or some hearing loss at all frequencies.
Deafness or hearing loss can be congenital, resulting from errors in development,
or the result of disease or damage. An impact (such as putting a cotton swab too
far in the ear) or even a loud noise can damage the eardrum. Loud noise can also
kill auditory hair cells. The following sections discuss the dierent types of hear-
ing loss and their treatments.
Conductive versus neural hearing loss
Conductive hearing loss occurs when sound vibrations aren’t mechanically trans-
mitted from the eardrum to the cochlea. Neural hearing loss is mostly due to dam-
age to the auditory hair cells, typically from loud noises. However, tumors or
strokes that aect any part of the auditory pathway can cause neural hearing loss
as well.
Inner ear infections can cause conductive deafness by destroying the three middle
ear bones (refer to section “Amplifying in the middle ear” earlier in this chapter).
Another cause of conductive hearing loss is otosclerosis, the growth of spongy
masses on the middle ear bones. This is a hereditary conductive hearing loss that
can happen between the ages of 15 and 40. A pregnant woman with rubella (also
called German measles) can transmit the virus to her baby, causing the child to be
born deaf.
Items entering the auditory canal can physically damage eardrums, as can high
pressure, such as diving into deep water.
Eh? Aging, environment, and hearing loss
As humans age, the high-frequency sensitivity declines (presbycusis, which occurs
in almost everyone). Young people can hear in the range of 20 to 20,000 Hertz, but
the elderly typically hear little above 15,000Hz.
Some age-related hearing loss is probably inevitable, but the most frequent cause
of hearing loss in older people is due to damage to the auditory hair cells. The
louder the noise, the more quickly it damages a person’s hearing.
Being exposed only once to very loud sounds— such as standing by the speakers
at a concert— can cause signicant hearing loss no matter your age. On the other
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258 PART 3 Understanding Perception and Cognition
hand, consistently working in a loud environment such as a construction site can
cause hearing damage that accumulates over decades.
Auditory hair cell damage is permanent.
Aiding hearing: Amplifying and replacing
Hearing aids can treat conductive hearing loss and some neural loss. Hearing aids
amplify sound, usually matching in a complementary way the hearing loss prole.
This is why good hearing aids are custom programmed.
If a person has severe neural loss, amplifying sound is useless because nothing is
left to transduce it. Within the last few decades, hundreds of thousands of audi-
tory prostheses have been installed in people with profound deafness. These are
linear sets of about 30 electrodes threaded into the cochlea to electrically stimu-
late the cochlear auditory bers and restore some hearing. This works well because
of the cochlea’s inherent dierent frequency sensitivity along its length.
Cochlear inserts can’t treat deafness that’s caused by damage higher in the ner-
vous system. Electrode arrays are being developed with the intent to implant them
into brain structures in the auditory pathway. But so far, they’re experimental and
not generally available.
Ringing and tinnitus
The perception of ringing in the years is called tinnitus. Chronic tinnitus is psy-
chologically debilitating. It has multiple causes that include ear infections, aller-
gies, reactions to certain drugs, and exposure to loud noise (tinnitus is common
among rock musicians).
Muscle spasms in the ears cause objective tinnitus. The condition is diagnosed with
special ear-specic instruments that detect the actual noise made by the spasms.
Subjective tinnitus can’t be diagnosed objectively. It can have numerous causes and
is typically associated with hearing loss. Tinnitus can appear suddenly and then
disappear within a few months.
One treatment for tinnitus has people wear noise-producing earphones that mask
the ringing sound with electronically generated white noise. In extreme cases,
people need to have the auditory nerve surgically destroyed to get some relief, but
this compromises normal hearing.
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CHAPTER 13 Looking At Vision andHearing 259
Balancing via the Vestibular System
The vestibular system starts with the three semicircular canals and otoliths in the
inner ear, which are sensitive to gravity and acceleration. These canals are ori-
ented in three right-angled planes. Hair cells in these canals work like nearby
cochlear hair cells. However, instead of responding to sound vibrations, they
respond to the movement of uid in the cochlea that happens when you move or
turn your head. The moving uid acts on a structure called the cupula, which con-
tains the hair cells that convert the physical movement to vestibular nerve action
potentials.
The semicircular canal system transduces rotational movements while the oto-
liths sense linear accelerations such as occur during the beginning of forward
movement. The vestibular system projects to neural structures that control eye
movements, which generates the vestibulo-ocular reex (VOR)— a compensatory
eye movement that allows you to see stable images.
The three semicircular canals are called the horizontal (lateral), anterior (supe-
rior), and posterior (inferior) semicircular canals. The vestibular nuclei project to
the cerebellum, spinal cord, thalamus, reticular formation, and the nuclei of cra-
nial nerves III, IV, and VI.These nerves allow the muscles to compensate for
things such as uneven terrain when running, so that you keep your balance. Some
nuclei combine vestibular with visual signals from directionally selective ganglion
cells that also inform the brain about rotations and translations.
The VOR stabilizes images on the retina when you move your head by producing
an opposite, compensatory eye movement. A related reex, called the optokinetic
reex (OKN), is mediated by inputs from directionally selective ganglion cells in
the retina.
Meniere’s disease is a condition of the cochlea and semicircular canals that aects
hearing and balance. It likely has multiple causes, but at this point the disease is
poorly understood.
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