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Chapter 11:
Sound, The Auditory System,
and Pitch Perception
Overview of Questions
• If a tree falls in the forest and no one is there
to hear it, is there a sound?
• What is it that makes sounds high pitched or
low pitched?
• How do sound vibrations inside the ear lead
to the perception of different pitches?
• How are sounds represented in the auditory
cortex?
Pressure Waves and Perceptual Experience
• Two definitions of “sound”
– Physical definition - sound is pressure
changes in the air or other medium.
– Perceptual definition - sound is the
experience we have when we hear.
Sound Waves
• Loud speakers produce sound by:
– The diaphragm of the speaker moves out,
pushing air molecules together called
condensation.
– The diaphragm also moves in, pulling the
air molecules apart called rarefication.
– The cycle of this process creates
alternating high- and low-pressure regions
that travel through the air.
Figure 11.1 (a) The effect of a vibrating speaker diaphragm on the surrounding air. Dark areas represent
regions of high air pressure, and light areas represent areas of low air pressure. (b) When a pebble is
dropped into still water, the resulting ripples appear to move outward. However, the water is actually
moving up and down, as indicated by movement of the boat. A similar situation exists for the sound waves
produced by the speaker in (a).
Sound Waves - continued
• Pure tone - created by a sine wave
– Amplitude - difference in pressure between
high and low peaks of wave
• Perception of amplitude is loudness
• Decibel (dB) is used as the measure of
loudness
• Number of dB = 20 logarithm(p/po)
• The decibel scale relates the amplitude
of the stimulus with the psychological
experience of loudness.
Figure 11.2 (a) Plot of sine-wave pressure changes for a pure tone; (b) Pressure changes are indicated, as
in Figure 11.1, by darkening (pressure increased relative to atmospheric pressure) and lightening (pressure
decreased relative to atmospheric pressure.)
Figure 11.3 Three different amplitudes of a pure tone. Larger amplitudes are associated with the perception
of greater loudness.
Table 11.1 Relative amplitudes and decibels for environmental sounds
Figure 11.7 Loudness of a 1,000 Hz tone as a function of intensity, determined using magnitude estimation.
(Adapted from Gulick, Gescheider, & Frisina, 1989.)
Sound Waves - continued
• Frequency - number of cycles within a given
time period
– Measured in Hertz (Hz) - 1 Hz is one cycle
per second
– Perception of pitch is related to frequency.
– Tone height is the increase in pitch that
happens when frequency is increased.
Figure 11.4 Three different frequencies of a pure tone. Higher frequencies are associated with the
perception of higher pitches.
Complex Periodic Sounds
• Both pure and some complex tones are
periodic tones.
• Fundamental frequency is the repetition rate
and is called the first harmonic.
• Periodic complex tones consist of a number
of pure tones called harmonics.
– Additional harmonics are multiples of the
fundamental frequency.
Complex Periodic Sounds - continued
• Additive synthesis - process of adding
harmonics to create complex sounds
• Frequency spectrum - display of harmonics of
a complex sound
• Attack of tones - buildup of sound at the
beginning of a tone
• Decay of tones - decrease in sound at end of
tone
Figure 11.5 Left: Waveforms of (a) a complex periodic sound with a fundamental frequency of 200 Hz; (b)
fundamental (first harmonic) = 200 Hz; (c) second harmonic = 400 Hz; (d) third harmonic = 600 Hz; (e)
fourth harmonic = 800 Hz. Right: Frequency spectra for each of the tones on the left. (Adapted from Plack,
2005)
Complex Periodic Sounds - continued
• Timbre - all other perceptual aspects of a
sound besides loudness, pitch, and duration
– It is closely related to the harmonics, attack
and decay of a tone.
• Effect of missing fundamental frequency
– Removal of the first harmonic results in a
sound with the same perceived pitch, but
with a different timbre.
• This is called periodicity pitch.
Figure 11.10 Frequency spectra for a guitar, a bassoon, and an alto saxophone playing a tone with a
fundamental frequency of 196 Hz. The position of the lines on the horizontal axis indicates the frequencies
of the harmonics and their height indicates their intensities. (From Olson, 1967)
Figure 11.6 (a) The complex tone from Figure 11.5A, with its frequency spectrum; (b) the same tone with its
first harmonic removed (Adapted from Plack, 2005)
Musical Scales and Frequency
• Letters in the musical scale repeat.
• Notes with the same letter name (separated
by octaves) have fundamental frequencies
that are multiples of each other.
– These notes have the same tone chroma.
– We perceive such notes as similar to one
another.
Figure 11.8 A piano keyboard, indicating the frequency associated with each key. Moving up the keyboard
to the right increases the frequency and tone height. Notes with the same letter, like the A’s (arrows) have
the same tone chroma.
Range of Hearing
• Human hearing range - 20 to 20,000 Hz
• Audibility curve - shows the threshold of
hearing in relation to frequency
– Changes on this curve show that humans
are most sensitive to 2,000 to 4,000 Hz.
• Auditory response area - falls between the
audibility curve and and the threshold for
feeling
– It shows the range of response for human
audition.
Range of Hearing - continued
• Equal loudness curves - determined by using
a standard 1,000 Hz tone
– Two dB levels are used - 40 and 80
– Participants match the perceived loudness
of all other tones to the 1,000 Hz standard.
– Resulting curves show that tones sound
• Almost equal loudness at 80 dB.
• Softer at 40 dB for high and low
frequencies than the rest of the tones in
the range.
Figure 11.9 The audibility curve and the auditory response area. Hearing occurs in the light
green area between the audibility curve (the threshold for hearing) and the upper curve (the
threshold for feeling). Tones with combinations of dB and frequency that place them in the
light red area below the audibility curve cannot be heard. Tones above the threshold of
feeling result in pain. The places where the dashed line at 10 dB crosses the audibility
function indicate which frequencies can be heard at 10 dB SPL. (From Fletcher & Munson,
1933)
The Ear
• Outer ear - pinna and auditory canal
– Pinna helps with sound location.
– Auditory canal - tube-like 3 cm long
structure
• It protects the tympanic membrane at
the end of the canal.
• The resonant frequency of the canal
amplifies frequencies between 1,000
and 5,000 Hz.
Figure 11.11 The ear, showing its three subdivisions -- outer, middle, and inner. (From Lindsay & Norman,
1977)
The Middle Ear
• Two cubic centimeter cavity separating inner
from outer ear
• It contains the three ossicles
– Malleus - moves due to the vibration of the
tympanic membrane
– Incus - transmits vibrations of malleus
– Stapes - transmit vibrations of incus to the
inner ear via the oval window of the
cochlea
Figure 11.12 The middle ear. The three bones of the middle ear transmit the vibrations of the tympanic
membrane to the inner ear.
Function of Ossicles
• Outer and inner ear are filled with air.
• Inner ear is filled with fluid that is much
denser than air.
• Pressure changes in air transmit poorly into
the denser medium.
• Ossicles act to amplify the vibration for better
transmission to the fluid.
• Middle ear muscles dampen the ossicles’
vibrations to protect the inner ear from
potentially damaging stimuli.
Figure 11.13 Environments inside the outer, middle, and inner ears. The fact that liquid fills the inner ear
poses a problem for the transmission of sound vibrations from the air of the middle ear.
Figure 11.14 (a) A diagrammatic representation of the tympanic membrane and the stapes, showing the
difference in size between the two. (b) How lever action can amplify the effect of a small force, presented on
the right, to lift the large weight on the left. The lever action of the ossicles amplifies the sound vibrations
reaching the tympanic inner ear. (From Schubert, 1980).
The Inner Ear
• Main structure is the cochlea
– Fluid-filled snail-like structure (35 mm long)
set into vibration by the stapes
– Divided into the scala vestibuli and scala
tympani by the cochlear partition
– Cochlear partition extends from the base
(stapes end) to the apex (far end)
– Organ of Corti contained by the cochlear
partition
Figure 11.15 (a) A partially uncoiled cochlea. (b) A fully uncoiled cochlea. The cochlear partition, indicated
here by a line, actually contains the basilar membrane and the organ of Corti, which are shown in Figure
11.16 and 11.17.
The Organ of Corti
• Key structures
– Basilar membrane vibrates in response to
sound and supports the organ of Corti
– Inner and outer hair cells are the receptors
for hearing
– Tectorial membrane extends over the hair
cells
The Organ of Corti - continued
• Transduction takes place by:
– Cilia bend in response to movement of
organ of Corti and the tectorial membrane
– Movement in one direction opens ion
channels
– Movement in the other direction closes the
channels
• This causes bursts of electrical signals.
Figure 11.16 (a) Cross-section of the cochlea. (b) Close-up of the organ of Corti, showing how it rests on
the basilar membrane. Arrows indicate the motions of the basilar membrane and tectorial membrane that
are caused by vibration of the cochlear partition. (Adapted from Denes and Pinson, 1993.)
Figure 11.18 (a) Movement of hair cilia in one direction opens ion channels in the hair cell, which results in
the release of neurotransmitter onto an auditory nerve fiber; (b) Movement in the opposite direction closes
the ion channels so there is no ion flow and no transmitter release.
Neural Signals for Frequency
• There are two ways nerve fibers signal
frequency:
– Which fibers are responding
• Specific groups of hair cells on basilar
membrane activate a specific set of
nerve fibers;
– How fibers are firing
• Rate or pattern of firing of nerve
impulses
Békésys’ Place Theory of Hearing
• Frequency of sound is indicated by the place
on the organ of Corti that has the highest
firing rate.
• Békésy determined this in two ways:
– Direct observation of the basilar membrane
in cadavers.
– Building a model of the cochlea using the
physical properties of the basilar
membrane.
Figure 11.20 Hair cells all along the cochlea send signals to nerve fibers that combine to form the auditory
nerve. According to place theory, low frequencies cause maximum activity at the apex end of the cochlea,
and high frequencies cause maximum activity at the base. Activation of the hair cells and auditory nerve
fibers indicated in red would signal that the stimulus is in the middle of the frequency range for hearing.
Békésys’ Place Theory of Hearing continued
• Physical properties of the basilar membrane
– Base of the membrane (by stapes) is:
• Three to four times narrower than at the
apex.
• 100 times stiffer than at the apex.
• Both the model and direct observation
showed that the vibrating motion of the
membrane is a traveling wave .
Figure 11.21 A perspective view showing the traveling wave motion of the basilar membrane. This picture
shows what the membrane looks like when the vibration is “frozen,” with the wave about two thirds of the
way down the membrane. (From Tonndorf, 1960).
Figure 11.22 A perspective view of an uncoiled cochlea, showing how the basilar membrane gets wider at
the apex end of the cochlea. The spiral lamina is a supporting structure that makes up for the basilar
membrane’s difference in width at the stapes and the apex ends of the cochlea. From Schubert, 1980.
Békésys’ Place Theory of Hearing continued
• Envelope of the traveling wave
– Indicates the point of maximum
displacement of the basilar membrane
– Hair cells at this point are stimulated the
most strongly leading to the nerve fibers
firing the most strongly at this location.
– Position of the peak is a function of
frequency.
Figure 11.23 Vibration of the basilar membrane, showing the position of the membrane at three instants in
time, indicated by the blue, green, and red lines, and the envelope of the vibration, indicated by the yellow
dashed line. P indicates the peak of the basilar membrane vibration. (From von Bekesy, 1960).
Figure 11.24 The envelope of the basilar membrane’s vibration at frequencies ranging from 25 to 1,600 Hz,
as measured by Békésy (1960). These envelopes were based on measurements of damaged cochleas.
The envelopes are more sharply peaked in healthy cochleas.
Evidence for the Place Theory
• Tonotopic map
– Cochlea shows an orderly map of
frequencies along its length
• Apex responds best to low frequencies
• Base responds best to high frequencies
Figure 11.25 Tonotopic map of the guinea pig cochlea. Numbers indicate the location of the maximum
electrical response for each frequency. (From Culler, Coakley, Lowy, & Gross, 1943).
Evidence for the Place Theory - continued
• Neural frequency tuning curves
– Pure tones are used to determine the
threshold for specific frequencies
measured at single neurons.
– Plotting thresholds for frequencies results
in tuning curves.
– Frequency to which the neuron is most
sensitive is the characteristic frequency.
Figure 11.26 Frequency tuning curves of cat auditory nerve fibers. The characteristic frequency of each
fiber is indicated by the arrows along the frequency axis. The frequency scale is in kilohertz (kHz), where
one kHz = 1,000 Hz. (From Palmer, 1987)
Evidence for the Place Theory - continued
• Auditory masking experiments
– First, thresholds for a number of
frequencies are determined.
– Then, a single intense masking frequency
is presented at the same time that the
thresholds for the original frequencies are
re-determined.
– The masking effect is seen at the masking
tone’s frequency and spreads to higher
frequencies more than lower ones.
Figure 11.27 The procedure for a masking experiment. (a) Threshold is determined across a range of
frequencies. Each blue arrow indicates a frequency where the threshold is measured. (b) The threshold is
redetermined at each frequency (blue arrows) in the presence of a masking stimulus (red arrow). The larger
blue arrows indicate that the intensities must be increased to hear these test tones when the masking tone
is present.
Figure 11.28 Results of Egan and Hake’s (1950) masking experiment. The threshold increases the most
near the frequencies of the masking noise and the masking effect spreads more to high frequencies than to
low frequencies. (From Egan and Hake, 1950).
Figure 11.29 Vibration patterns caused by 200- and 800-Hz test tones, and the 400-Hz mask (shaded),
taken from basilar membrane vibration patterns in Figure 11.24. Notice that the vibration caused by the
masking tone overlaps the 800-Hz vibration more than the 200-Hz vibration.
Basilar Membrane Response to Complex
Tones
• Basilar membrane can be described as an
acoustic prism.
– There are peaks in the membrane’s
vibration that correspond to each harmonic
in a complex tone.
– Each peak is associated with the frequency
of a harmonic.
Figure 11.30 (a) Waveform of a complex tone consisting of three harmonics; (b) Basilar membrane. The
shaded areas indicate locations of peak vibration associated with each harmonic in the complex tone.
Updating Békésy’s Place Theory
• Békésy used basilar membranes isolated
from cadavers and his results showed no
difference in response for close frequencies
that people can distinguish.
• New research with live membranes shows
that the entire outer hair cells respond to
sound by slight tilting and a change in length.
– For this reason these cells are called the
cochlear amplifier.
Figure 11.31 The outer hair cells (a) elongate when cilia bend in one direction; (b) contract when the cilia
bend in the other direction. This results in an amplifying effect on the motion of the basilar membrane.
Figure 11.32 Effect of OHC damage on frequency tuning curve. The solid curve is the frequency tuning
curve of a neuron with a characteristic frequency of about 8,000 Hz. The dashed curve is the tuning curve
for the same neuron after the outer hair cells were destroyed by injection of a chemical (Adapted from
Fettipalce & Hackney, 2006).
Temporal Coding and Frequency
• Phase locking
– Nerve fibers fire in bursts.
– Firing bursts happen at or near the peak of
the sine-wave stimulus.
– Thus, they are “locked in phase” with the
wave.
– Groups of fibers fire with periods of silent
intervals creating a pattern of firing.
Figure 11.33 How hair cell activation and auditory nerve fiber firing area synchronized with pressure
changes of the stimulus. The auditory nerve fiber fires when the cilia are bent to the right. This occurs at the
peak of the sine-wave change in pressure.
Place and Temporal Coding for Pitch
• Place coding is effective for the entire range
of hearing.
• Temporal coding with phase locking is
effective up to 4,000 Hz.
• Thus, both codes work for frequencies below
4,000 Hz.
Hearing Loss
• Two types
– Conductive hearing loss
• Blockage of sound from the receptor
cells
– Sensorineural hearing loss
• Damage to hair cells
• Damage to the auditory nerve or brain
• Most common type is prebycusis
Hearing Loss - continued
• Presbycusis
– Greatest loss is at high frequencies
– Affects males more severely than females
– Appears to be caused by exposure to
damaging noises or drugs
• Noise-induced hearing loss
– Loud noise can severely damage the
Organ of Corti
– OSHA standards for noise levels at work
are set to protect workers
– Leisure noise can also cause hearing loss
Figure 11.34 Hearing loss in presbycusis as a function of age. All the curves are plotted relative to the 20year-old curve, which is taken as the standard (Adapted from Dubno, in press).
Figure 11.35 Sound level of game 3 of the 2006 Stanley Cup finals between the Edmunton Oilers (the home
team) and the Carolina Hurricanes. Sound levels were recorded by a small microphone in a spectator’s ear.
The red line indicates a “safe” level for a 3-hour game. (From Hodgetts & Liu, 2006)
Pathway from the Cochlea to the Cortex
• Auditory nerve fibers synapse in a series of
subcortical structures
– Cochlear nucleus
– Superior olivary nucleus (in the brain stem)
– Inferior colliculus (in the midbrain)
– Medial geniculate nucleus (in the
thalamus)
– Auditory receiving area (A1 in the temporal
lobe)
Figure 11.36 Diagram of the auditory pathways. This diagram is greatly simplified, as numerous
connections between the structures are not shown. Note that auditory structures are bilateral -- they exist
on both the left and right sides of the body -- and that messages can cross over between the two sides.
(Adapted from Wever, 1949.)
Auditory Areas in the Cortex
• Hierarchical processing occurs in the cortex
– Neural signals travel through the core, then
belt, followed by the parabelt area.
– Simple sounds cause activation in the core
area.
– Belt and parabelt areas are activated in
response to more complex stimuli made up
of many frequencies.
Figure 11.37 The three main auditory areas in the cortex are the core area, which contains the primary
auditory receiving area (A1), the belt area, and the parabelt area. Signals, indicated by the arrows, travel
from core, to belt, to parabelt. The dark lines show where the temporal lobe is pulled back to show areas
that would not be visible from the surface. (From Kaas, Hackett, & Tramo, 1999).
What and Where Streams for Hearing
• What, or ventral stream, starts in the anterior
portion of the core and belt and extends to
the prefrontal cortex.
– It is responsible for identifying sounds.
• Where, or dorsal stream, starts in the
posterior core and belt and extends to the
parietal and prefrontal cortices.
– It is responsible for locating sounds.
• Evidence from neural recordings, brain
damage, and brain scanning support these
findings.
Figure 11.38 Areas in the monkey cortex that respond to auditory stimuli. The green areas respond to
auditory stimuli, the purple areas to both auditory and visual stimuli. The arrows from the temporal lobe to
the frontal lobe represent the what and where streams in the auditory system. (Adapted from Porembra, et
al., 2003.)
Perceiving Pitch and Complex Sounds
• Tonotopic maps are found in A1
– Neurons that respond better to low
frequencies are on the left and those that
respond best to high frequencies are on
the right.
– Research on stroke patients and
marmosets provided support for the link
between perception and physiological
response in A1.
Figure 11.41 The outline of the core area of the monkey auditory cortex, showing the tonotopic map on the
primary auditory receiving area, A1, which is located within the core. The numbers represent the
characteristic frequencies (CF) of neurons in thousands of Hz. Low CF’s are on the left, and high CF’s are
on the right. (Adapted from Kosaki et al., 1997).
Figure 11.42 Performance of patient A, with auditory cortex damage on four tasks. See text for details.
(Based on data from Tramo et al., 2002).
Figure 11.43 Records from a pitch neuron recorded from the marmoset auditory cortex. (a) Frequency
spectra for tones with fundamental frequency of 182 Hz. Each tone contains three harmonic components of
the 182 Hz fundamental frequency; (b) Response of the neuron to each stimulus. (Adapted from Bendor &
Wang, 2005).
Recent Evidence of Pitch Perception in A1
• Effect of training on tonotopic maps
– Owl monkeys were trained to discriminate
between two frequencies near 2,500 Hz.
– Trained monkeys showed tonotopic maps
with enlarged areas with neurons that
responded to 2,500 Hz compared to
untrained monkeys.
• Cases of humans with brain damage to this
area show perception difficulties with pitch.
Figure 11.44 (a) Tonotopic map of the owl monkey’s primary auditory receiving area (A1), showing areas
that contain neurons with the characteristic frequencies indicated. The blue area contains neurons with CF
= 2,500 Hz. (b) Tonotopic map of an owl monkey that was trained to discriminate between frequencies near
2,500 Hz. The blue areas indicate that after training more of the cortex responds best to 2,500 Hz. (From
Recanzone et al., 1993).
Recent Evidence of Pitch Perception in A1 continued
• Brain scans in humans
– Tasks that require pitch recognition activate
areas equivalent to the core area in
monkeys.
– Tasks that require recognition of complex
stimuli activate areas equivalent to the
parabelt area in monkeys.
– Thus, stimuli that are more complex are
processed farther “downstream” in the
nervous system.
Effect of Experience on the Auditory Cortex
• Musicians show enlarged auditory cortices
that respond to piano tones and stronger
neural responses than non-musicians.
• Experiment by Fritz et al.
– Marmosets were trained to lick a water
spout in response to a pure tone
embedded within a stream of complex
tones.
– Neurons became quickly tuned to the
target frequency and maintained the effect
for hours after the testing session.
Figure 11.45 Response of a neuron in the ferret auditory cortex: (a) before training; (b) after training. See
text for details. (Fritz et al., 2003_
Cochlear Implants
• Electrodes are inserted into the cochlea to
electrically stimulate auditory nerve fibers.
• The device is made up of:
– a microphone worn behind the ear,
– a sound processor,
– a transmitter mounted on the mastoid
bone,
– and a receiver surgically mounted on the
mastoid bone.
Figure 11.46 Cochlear implant device. See text for details.
Cochlear Implants - continued
• Implants stimulate the cochlea at different
places on the tonotopic map according to
specific frequencies in the stimulus.
• These devices help deaf people to hear some
sounds and to understand language.
• They work best for people who receive them
early in life or for those who have lost their
hearing, although they have caused some
controversy in the deaf community.