Transcript Chapter 11:

Chapter 11: Perception of Sound
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
– The diaphragm also moves in, pulling the
air molecules apart
– The cycle of this process creates
alternating high- and low-pressure regions
that travel through the air
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 energy in
the moving air molecules
• 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 Sine-wave pressure changes for a pure tone.
Sound Waves - continued
• Frequency - number of cycles within a given
time period
– Measured in Hertz (Hz) - 1 Hz is 1 cycle
per second
– Perception of pitch is related to frequency
– Tone height is the increase in pitch that
happens when frequency is increased
Musical Scales and Frequency
• Letters in the musical scale repeat
• Notes with the same letter name (separated
by octaves) sound similar - called tone
chroma
• Notes separated by octaves have frequencies
that are multiples of each other
Range of Human Hearing
Sound Quality: Timbre
• All other properties of sound except for
loudness and pitch constitute timbre
• Timbre is created partially by the multiple
frequencies that make up complex tones
– Fundamental frequency is the first harmonic
– Musical tones have additional harmonics that
are multiples of the fundamental frequency
Sound Quality: Timbre
• 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
Frequency Spectrum The heights of the lines
indicate the amplitude of
each of the frequencies
that make up the tone.
Figure 11.10
Frequency spectra for
a guitar, a bassoon,
and an alto
saxophone playing a
tone with a
fundamental
frequency of 196 Hz.
(Note is a G)
• The Human Ear
M
S
Oval
Window
Round
Window
Figure 11.12 The middle ear.
Function of Ossicles
• Outer and inner ear are filled with air
• Inner ear 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
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.
Mechanical amplification of the sound message
The Inner Ear
• Main structure is the cochlea
– Fluid-filled snail-like structure 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
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
• Transduction at the hair cells takes place due
to the interaction of these structures
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
Figure 11.18 Hair cells all along the cochlea send signals to nerve fibers that combine to form the auditory
nerve. This figure indicates that these fibers can signal a tone’s frequency: (a) by which fibers fire in
response to the tone; and (b) by the pattern of nerve impulses in response to the tone. See text for details.
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 a cadaver
– Building a model of the cochlea using the
physical properties of the basilar
membrane
Figure 11.21 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 black
dashed line. P indicates the peak of the basilar membrane vibration. (From Experiments in Hearing, by G.
von Békésy, 1960, figures 11.43. Copyright © 1960 by McGraw-Hill Book Company, New York. Reprinted
by permission.)
Figure 11.22 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 high frequencies
• Base responds best to low frequencies
Figure 11.23 Tonotopic map of the guinea pig
cochlea.
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
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 11.25 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).
Figure 11.26 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. (Adapted from “On the Masking Pattern of a Simple Auditory Stimulus,” by J. P. Egan and
H. W. Hake, 1950, Journal of the Acoustical Society of America, 22, 622-630. Copyright © 1950 by the
American Institute of Physics. Adapted by permission.)
Response of Basilar Membrane to Complex
Tones
• Fourier analysis - mathematic process that
separates complex waveforms into a number
of sine waves
• Research on the response of the basilar
membrane shows the highest response in
auditory nerve fibers with characteristic
frequencies that correspond to the sine-wave
components of complex tones
• Thus the cochlea is called a frequency
analyzer
Figure 11.32 A complex sound wave. Applying Fourier analysis to this wave indicates that it is made up of
the three components in (b), (c), and (d).
Figure 11.33 The cochlea is called a frequency analyzer because it analyzes incoming sound into its
frequency components and translates the components into separated areas of excitation along its length.
In this example, the complex tone, which consists of frequency components at 440, 880, and 1,320 Hz,
enters the outer ear. The shaded areas on the basilar membrane represent places of peak vibration for the
tone’s three components, and the darkened hair cells represent the hair cells that will be most active in
response to this tone.
Timing of Neural Firing 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 11.34 The response of three nerve fibers (a, b, and c) that are phase-locked to the stimulus. Notice
that the fibers don’t fire on every cycle of the stimulus, but when they do fire, they fire only when the
stimulus is at or near its peak.
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 temporal lobe is pulled back to show areas that would not be visible
from the surface. (Adapted from J. H. Kaas, T. A. Hackett, and M. J. Trano, (1999). “Auditory processing in
primate cerebral cortex,” Current Opinion in Neurobiology, 9, 164-170.)
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
– However, early research did not show a
direct relationship between pitch
perception and the tonotopic map
Figure 11.41 The outline of the core area of the monkey auditory cortex, showing the tonotopic map in 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).
Effect of the Missing Fundamental
• The fundamental frequency is the lowest
frequency in a complex tone
• When the fundamental and other lower
harmonics are removed, the perceived pitch
is the same, but the timbre changes
• The pitch perceived in such tones is called
periodicity pitch
Figure 11.44 (a) Frequency spectra for a tone with a fundamental frequency of 400 Hz. (b) First harmonic
(the fundamental) removed. (c) First three harmonics removed. All three of these tones are perceived to
have the same pitch.
Periodicity Pitch
• Pattern of stimulation on the basilar
membrane cannot explain this phenomenon
since removing the fundamental and
harmonics creates different patterns
• Periodicity pitch is perceived even when the
tones are presented to two ears
• Thus, pitch must be processed in an area
where the signal from both ears are
combined -- a central pitch processor
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
– 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