Download Sound frequency (pitch, tone) measured in hertz (cycles per sec)

10/18/15
Psych 2200, Class 15
Hearing and the Auditory System
Oct 20, 2015
**ONLINE LECTURE – See webpage or email for link**
Sam office hours this week – wed 11-12p, thurs 1-4p, Friday 1-2p.
Sound frequency (pitch, tone) measured in
hertz (cycles per sec)
If 2 msec (0.002 sec) is the interval, then the
stimulus frequency is 500 Hz (1/ 0.002 sec = 500 Hz)
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Sound amplitude (loudness, sound pressure (SPL)),
measured in decibels (dB)
soft
loud
Sound Intensity (db)
Ticking of Watch (20)
Whisper (30)
Normal Speech (50-60)
Car Traffic (70)
Chain Saw (110)
Jackhammer (120)
Jet Engine (130)
AMPLITUDE
A plot of frequency X minimum dB audible = audiogram
FREQUENCY
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EXTERNAL
(skin & air)
MIDDLE
(bones & air)
INNER
(cochlea & fluid)
Semicircular canals code vestibular info,
and send axons to the vesibular nerve
which joins with the auditory nerve to
become the vestibulocochlear cranial
nerve
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Scala vestibuli,
perilymph
Reissner’s
membrane
Scala media,
endolymph
Tectorial
membrane
Spiral axons
Organ of Corti
Basilar
membrane
Scala tympani, perilymph
Vibrations in the bony stapes (connects to oval window) cause vibrations in the perilymph (like squeezing
and bulging in a water balloon). The fluid motion induces corresponding vibrations in underlying Basilar
(and Reissner) membranes. Since the endolymph of the Scala media is in a separate compartment, the
Tectorial membrane does not pick up fluid vibrations. The Organ of Corti contains inner and outer hair
cells which have their base in the Basilar (vibrating) membrane and whose tips (bundles of serteocilia
filled with actin) project into the Tectorial (non-vibrating) membrane. This mismatch leads to
transduction.
TRANSDUCTION FROM VIBRATION TO NEURAL IMPULSE
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Tectorial membrane
hair
cells
(cilia)
Basilar membrane
2
hair
cells
(cilia)
Tectorial membrane
Basilar membrane
Motion causes ion gates of hair cells open to K+, leading to graded depolarization
of the hair cell
Hair Cell
Bi-polar spiral neuron
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A side note on sensory transduction and action potentials
For touch/pain & prioprioceptive (deep tissue) receptors, the receptor is part of the
sensory neuron -- a special modification in the sensory neurons dendritic ending. This
leads to direct action potentials in the sensory neuron when stimulated.
Unipolar
sensory neuron
Sensory dendrite
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For smell, (taste), hearing, and vision, the sensory receptors are
NOT neurons. They produce graded depolarization only, but release
transmitter when stimulated that leads to an action potential
in an associated (secondary) neuron:
Smell – olfactory hair cell -> mitral neuron
Hearing – hair cell -> spiral neuron
Vision – photoreceptor/bipolar -> ganglion neuron
Hair Cell, graded potential
(or depolarization)
Spiral dendrite contacts the hair cell.
Bi-polar spiral neuron
can fire action potential
Tectorial
membrane
Basilar
membrane
Spiral axon runs in the cochlear and then vestibulocochlear (cranial) nerve.
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Type I spiral neurons (95%)
ennervate a single inner hair
cell. Therefore each Type I
neuron exhibits the
“prefered frequency” of its
hair cell.
Type II small, unmyelinated
spiral neurons branch to
connect multiple outer hair
cells, generally in the same
row.
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Outer hair cells don’t detect sound -- they fine-tune the Organ of Corti
by changing their sterocilia legth based on feedback from the brain.
Individual type I axons in the auditory nerve are often called
"auditory nerve fibers.” Since each connects to only 1 hair cell,
Each has its own characteristic frequency (CF)
Auditory nerve fiber tuning curves
The prefered frequency (most sensitive) is the “characteristic frequency” (CF),
which is the tip of the above curve (each color=unique axon)
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Apex=
low
Base=
High
The deformation (“ripple”) of the
basilar membrane is a traveling wave.
When motion of the stapes creates a
sound wave in the fluid of the inner
ear, each region of the basilar
membrane “ripples” in response to this
pressure. The part near the base, with
its high resonance frequency, moves
first --- followed by lower frequency
(more apical) segments.
Apex=Low
Apex=
low
Base=
High
Base=High
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SOUND WAVES AND THE BASILAR MEMBRANE -- MOVIE CLIP
Hair cells with preferred frequency or characteristic frequency (CF)
are organized in a low-to-high format (called TONOTOPY).
This starts at level of hair cells in cochlea and is conserved all the
way up primary divisions of structures in the auditory system.
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ENCODING FREQUENCY (tone, pitch)
measured in hertz or cycles per sec
Single spiral neurons CANT fire fast enough to
encode high frequencies. The frequency encoding problem
is solved by:
1) SPATIAL SEGREGATION (TONOTOPY).
2)  PHASE LOCKING (collective firing of sub-groups of neurons in
sync with frequency rate).
ENCODING AMPLITUDE
(intensity, loudness, sound pressure)
measured in decibels
soft
loud
Intensity encoding starts at the auditory nerve with:
2) SPATIAL “SPREAD”
Louder
1)  NEURAL FIRING RATE
Louder
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YIPES -- a lot of terms!! Which are the most important …
1. Divisions of the ear -- outer (ear & ear-drum), middle (bones),
inner (cochlea).
2. Parts of the cochlea -- oval window, perilymph, basilar & tectorial
membranes, organ of corti, hair cells (inner & outer), spiral neurons.
3. Transduction at the hair cell -- stereocilia bend due to vibrations in
the basilar membrane while tectorial membrane stays still. Bending
causes depolarization, spiral neuron fires.
4. Tonotopy -- the basilar membrane is organized so that the base vibrates
preferentially to high frequencies, and the tip to low frequencies. The hair
cells for each region thus have a preferred or characteristic frequency (&
so does their spiral neuron).
5. Encoding frequency -- tonotopy, phase-locking.
6. Encoding amplitude -- firing rate, spatial spread.
7. Auditory structures -- cochlea, cochlear nucleus, olivary nucleus,
inferior colliculus, medial geniculate nucleus, A1, A2.
AMPLITUDE MODULATION (AM)
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FREQUENCY MODULATION (FM)
FM vs AM signals
/da/
Frequency (Hertz)
/ba/
F4
4,000
3,000
F3
2,000
F2
1,000
F1
0
40
250 0
40
250
TIME (ms)
Spectrograph for consonant-vowel (CV)
syllables /ba/ and /da/.
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DOLPHINS USE SOUND TO “SEE” -- MOVIE CLIP
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Classification of hearing loss relates to severity (complete deafness vs
hearing-impaired); mechanism (peripheral vs central); and timing.
Congenital - Peripheral (abnormalities of the hearing apparatus)
- Central (abnormalities in auditory nerve
or brain regions)
Progressive -- Due to degeneration of hair cells or nerves
Injury-induced -- birth trauma (hypoxia); jet engine engineer
Age-related hearing loss -- loss of tympanic membrane flexibility,
hair-cell loss
Mechanical (peripheral) abnormalities can sometimes be surgically fixed,
or treated with cochlear implant (candidacy is best for congenitally
deaf or low residual hearing children, or adults deafened post-lingually).
Partial/progressive loss can be ameliorated with hearing aids.
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A hearing aid can be used to “boost” a signal, if there is only a
partial hearing loss (“hard of hearing”).
A hearing aid cannot help if there is no sound processing to begin
with.
A cochlear implant can be used if hearing loss is profound and cannot
be boosted by hearing aids.
The cochlear implant has severe limitations including:
-loss of any residual hearing due to destruction of hair cells;
-effective mainly for recent loss (deaf children, and
adults with hearing loss)
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For the profoundly deaf, a viable alternative is visual sign language
(ASL, BSL, JSL – many dialects).
In fact, within the Deaf Community, many view deafness as a
non-medical -- but rather, “cultural” -- affiliation, and celebrate the
birth of deaf children.
Many technological innovations (blinking alarms, text-phones) make
this possible. Similar adaptations and cultural affiliation are not seen
in the blind community.
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For those with congenital or very-early acquired deafness,
a re-organization of auditory cortex occurs.
Remember -- Cortical plasticity is greatest in children
Although the auditory area of the brain is not being used, deaf
individuals do rely heavily on vision, for sign language and other
information.
In fact, in the congenitally deaf, Wernicke’s area is re-allocated to
“visual speech” (American Sign Language or other languages).
Thus a stroke to Wernicke’s in a congenitally deaf individual will
cause difficulty interpreting ASL (or other sign languages)
Wernicke’s
Broca’s
JSL = Japanese Sign Language (study done in Japan)
Age-dependent plasticity in the superior temporal sulcus in deaf humans: a functional MRI study, BMC Neuroscience
Norihiro Sadato1,2*, Hiroki Yamada2, Tomohisa Okada1, Masaki Yoshida4, Takehiro Hasegawa4, Ken-Ichi Matsuki4, Yoshiharu Yonekura5 and Harumi Itoh3
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