Chapter 10: !
Sound, the Auditory System,
and Pitch Perception!
Auditory
Stimuli!
distal stimulus: object that
vibrates!
proximal stimulus:
fluctuations of air pressure"
Speed of sound (in air) =
340 m/s!
The Physical/Perceptual
Characteristics of Sound!
“pure” tone"
Frequency: 1 Hertz = one cycle/second (abbreviated Hz)"
How do we get from soundwave to hearing?!
The Physical/
Perceptual
Characteristics of
Sound (cont.)!
Pressure (p/p0)!
1"
10"
100"
1000"
10000"
100000"
1000000"
10000000"
100000000"
dB!
0"
20"
40"
60"
80"
100"
120"
140"
160"
sound!
threshold of hearing"
soft whisper"
quiet office"
normal conversation"
loud music/heavy traffic"
subway"
propeller plane"
jet engine (pain threshold)"
rocket launch"
Frequency: how quickly the pressure changes in a sound wave "
" (i.e., how quickly the cycle repeats itself)"
" " measured in Hertz (Hz), or cycles/second (1 Hz = 1 cycle/second)"
" " for pure tones (i.e., having only a single frequency),"
" "
frequency is equivalent to "
" " "
pitch (the perception of sound most related to its freq.)"
" " can hear frequencies between 20 and 20,000 Hz"
Amplitude: magnitude of the difference between the low and high pressure
portions of the sound wave."
" " measured in decibels (dB)"
" " 1 dB = 20 log (p/p0)!
! ! !
p0 = 20 micropascals, threshold for 1000 Hz sound"
" " "
log (10n) = n"
Complex Waveform!
Simple sine waves can be
added together to create any
sound."
Logarithms"
Amplitude: magnitude of the difference between the low and high pressure
portions of the sound wave."
"
"
measured in decibels (dB)"
"
"
1 dB = 20 log (p/p0)!
!
!
!
p0 = 20 micropascals, threshold for 1000 Hz sound"
"
"
"
log (10n) = n"
So … " log 100 = log 102 = 2, etc…"
"
log 1000 = log 103 = 3, etc…"
Note that equation is multiplied by 20 … so a pressure 10 times as much as
threshold = 20 dB, 100 times as much as threshold = 40 dB, etc…"
Logarithms are just a simple way to compress a wide range of sounds
(from threshold of hearing all the way to a jet engine taking off!) into a
more manageable scale.!
Auditory Transduction!
Outer" Middle"
Ear"
Ear"
Harmonic: sine wave with
frequency that is a multiple of
the fundamental frequency."
•#If fundamental frequency =
440Hz, then harmonics will be
found at 880Hz, 1320Hz, etc…"
A “C” played on a clarinet
sounds different from a “C”
played on a piano because of
their differing harmonics."
(plural: Pinnae)!
(eardrum)!
(More on this next time…)"
Inner"
Ear"
(hammer)!
(anvil)!
(stirrup)!
Outer Ear!
Outer" Middle"
Ear"
Ear"
Middle Ear!
Pinna (plural = pinnae): funnel
sound into the auditory canal!
Auditory canal: 3 cm tube that
caries sound from the pinnae to
the tympanic membrane!
" • Protects middle/inner ear by
allowing them to be located
further inside the head, wax acts
as bug repellent"
" • Acts as "echo chamber" to
amplify sounds with frequencies
of 2,000 - 5,000 Hz"
Inner"
Ear"
Middle"
Ear"
(hammer)!
(anvil)!
(stirrup)!
amplify vibrations by:"
Tympanic membrane (eardrum):
membrane that transmits
vibrations of the sound waves in
the auditory canal to the bones of
the middle ear "
(plural: Pinnae)!
(eardrum)!
Ossicles: bones (smallest in
the body) that transmit and
amplify the vibrations from the
tympanic membrane to the
oval window of the inner ear."
" • malleus ("hammer"):
connected to the tympanic
membrane and incus"
" • incus ("anvil"): connected
to the malleus and stapes"
" • stapes (stirrup):
connected to the incus and
the oval window"
" • lever principle (amplify by 1.3x)"
" • concentrate vibrations from
large eardrum to small oval
window (amplify 17x)"
" "
" • 22x total"
(eardrum)!
Inner Ear (Cochlea)!
Inner hair cells don$t actually
touch tectorial membrane"
~12,000 "
in 3 rows"
~3500
in 1 row"
Embedded directly
in membrane"
Scala vestibuli"
Reissner's "
membrane"
Basilar "
membrane"
Scala tympani"
Deiter$s cells!
Cochlea!
Auditory
Pathways!
Cilia only have to move 1 trillionth
of a cm! (Imagine top of Eiffel
tower moving 10 mm…)"
Cochlear
Hair Cells!
Efferent Projections!
Efferent = “away from”!
•#nerve fibers that project from the brain back to the cochlea!
" regulate auditory sensitivity by:"
" " • tightening muscles attached to the ossicles"
" " • directly regulating excitability of the inner hair cells"
" " • tightening/retracting cilia of the outer hair cells"
Why?"
" • prevent loud noises from overwhelming the auditory system"
" • eliminate background noise"
" • help focus attention by reducing response to ignored stimuli"
" • help focus the envelope of the cochlear traveling wave."
Place Code for Pitch Perception!
different frequencies are encoded by hair cells in different parts of
the cochlea"
" " Originally proposed by H. von Helmholtz (1800's)"
" " " Resonance Theory (later disproven)"
" " Georg von Bekesy (1920's, Nobel Prize in 1961)"
" " " Traveling Wave!
High Hz"
Low Hz"
Characteristics of basilar membrane change over
length of the cochlea:"
" "
• More narrow at base than at apex"
" "
• 100x more stiff at base than at apex"
Place Code
(cont.)!
These changing characteristics cause:"
"
• a peak in the envelope of the traveling wave"
"
• location of the peak differs for different freqs."
"
"
low freq. = apex"
"
"
high freq = base"
Place Code: Frequency Tuning
Curves!
Place code
continued
from inner
hair cells up
to A1!"
Inner hair cell"
Auditory nerve fibers"
Place Code: Tonotopic Maps in
Auditory Cortex!
Place Code:
Outer Hair Cells
Fine-tune the
Traveling Wave!
The outer hair cells help focus
the envelope of the traveling
wave by tightening/retracting
their cilia in response to a
sound (motile response)."
(Also psychophysical evidence … see chapter 10)"
Rate Code for Pitch Perception!
frequency is represented by the timing of action
potentials in the neurons that encode the different
sounds."
100
cycles/s
300
cycles/s
100
spikes/s
300
spikes/s
Rate Code: The Volley Principle!
high firing rates can be accomplished in groups of neurons
-- rather than individual neurons -- with only a portion of the
cells firing at any given moment."
Why not just one
cell firing at every
peak?"
Primarily responsible for the perception of low
frequencies (< 500 Hz)"
Rate Code: Phase Locking!
tendency of some
auditory neurons to
fire in synchrony with
the phase of the
auditory stimulus"
Duplicity Theory
of Audition: "
the frequency of an
auditory stimulus is
encoded by both a place
code and a timing code
(at low frequencies)"
What can go wrong…?"
Conduction loss: disorder with outer or
middle ear; mechanical transmission
problem; overall reduction to all freq$s."
•#Build up of ear wax (eventually cause
fluid backup)"
•#Middle ear infection (otitis media) gives
abnormal input; causes permanent
hearing difficulties"
•#otosclerosis gradual immobilization
of stapes; can replace stapes with
plastic; disease also can affect
Intensity of
cochlea (and cannot be fixed); "
hearing loss
•#How to tell difference? Bone
by age"
conduction!!
What can go wrong…?"
Sensory/neural loss: disorder with inner ear or auditory
nerves/cortex; may include some or all frequencies."
•#Presbycusis: due to age; loss of elasticity in cochlea? Or
constant environmental noise? Or genetics or diet??"
•#Exposure to very loud noise: explosions, rock concerts,
headphones, guns, assembly line workers, own home
(dishwasher = 60dB, vacuum = 75dB, garbage disposal =
100dB!), etc…"
•#temporary threshold shift vs. permanent threshold shift"
•#Nicotine: most loss at higher frequencies (due to narrowing
blood vessels in ear)"
•#Asprin: 4-8g/day = 10-40dB shift in hearing!! (1 tablet = 1/4g)"
•#Tinnitus: high-pitched ringing in ears"
Not listening…"
Soundwave"
+"
It$s opposite…"
Cancels out!"
•#Useful for getting rid of annoying sounds
like vacuum, computer fan, engine hum on
airplane, teacher$s voice (kidding!)"
•#Can buy headphones that automatically
block out whatever noise is near you!"
Cochlear Implants!
External"
•#microphone + mini signal processor"
Internal"
•#thin, flexible array of microelectrodes"
•#inserted in cochlea along basilar membrane"
Point is to restore ability to understand speech"
•#speech composed of fairly high frequencies—so can$t
" use rate code"