CS 551651:

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CS 551/651 Structure of Spoken Language Lecture
10 Overview of Sound Perception John-Paul
Hosom Fall 2008
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Anatomy
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Anatomy

• The Outer Ear
• composed of pinna (auricle) and external
  auditory meatus (ear canal)
• functions
• irregular shape of pinna directs high-frequency
  sounds into ear canal
• shape of pinna helps with determining location of
  sound
• ear canal acts as resonator (2.7 cm long), with
  broad resonance between 3 to 5 kHz
• implications
• smaller animals better at hearing high-frequency
  sounds

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Anatomy

• The Middle Ear
• composed of
• chamber (tympanic cavity) containing ossicular
  chain malleus (hammer), incus (anvil), stapes
  (stirrup)
• middle ear also contains epitympanic recess. (The
  ossicles are lodged in the epitympanic recess.)
• tympanic membrane (ear drum) is partition between
  ear canal (outer ear) and middle ear
• sound transferred from tympanic membrane to
  cochlea (inner ear) via ossicular chain
• stapes connects to footplate, which connects to
  oval window,which is membrane of inner ear
• functions
• matches acoustic vibration of air to that of
  fluid in cochlea(if air directly hits oval
  window (water), theres a 30 dB drop in energy)
• has low-pass filter effect

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Anatomy

• The Inner Ear
• composed of
• cochlea, semi-circular canals
• function
• simply speaking, the inner ear performs a
  frequency analysis
• of the incoming sound, which is transmitted via
  VIIIth cranial
• nerve to CNS.
• cochlea
• spiral in shape
• 35 mm long, wound in 2 ¾ turns
• filled with incompressible water-like fluid
• separated into 3 parts by two membranes, the
  Basilar Membrane (BM) and Vestibular (Reissners)
  Membrane
• thousands of hair cells are attached to BM these
  cells are connected to neurons that fire in
  response to sound

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Anatomy

• The Inner Ear
• cochlea
• Sound from ear canal is amplified by middle ear
• Vibration of bone against oval window is received
  by cochlea (not just at oval window, but by
  entire cochlea no standing waves)
• Entire cochlea vibrates at the same frequency as
  the stimulus
• Different locations of the cochlea respond better
  to particular frequencies
• Higher frequencies respond more near base of
  cochlea.
• Cochlea and VIIIth nerve have tonotopic
  organization direct mapping between place and
  frequency

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Anatomy
A schematic of the cochlea unrolled (middle)
and basilar membrane (bottom). The top figure
indicates the tonotopic organization. (from J.D.
Durrant and J.H. Lovrinic, Bases of Hearing
Science, 1977, in Daniloff p. 395)
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Anatomy
This figure shows instantaneous displacement of
the BM for two instants in time, in response to
200-Hz sine wave, and the envelope of amplitude
peaks for this wave. Each point on BM vibrates
at a frequency equal to the input frequency (200
Hz).
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Anatomy

• Tonotopic organization
• BM varies in tautness and shape along its length,
  creating different frequency responses
• Tautness at base responds well to high-frequency
  sounds compliance at apex (tip) responds well to
  low-frequency sounds.
• Each point in BM has a characteristic frequency
  (CF) at which the frequency response is maximum
• The bandpass shape of a CF filter has constant
  ratio of frequency to bandwidth, implying better
  resolution (lower bandwidth) at lower frequencies

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Anatomy

• Transduction
• Between BM and tectorial membrane (A thin,
  responsive, gelatinous membrane) are hair cells
  about 25,000-30,000 outer hair cells, 3500-5000
  inner hair cells in humans. (Tunnel of Corti
  separates outer from inner). Each hair cell has
  30-300 hair-like projections called stereocilia
  protruding from the surface into the fluid-filled
  cavity in a bundle.
• When BM vibrates up and down, it creates a
  shearing motion between tectorial membrane and
  stereocilia. This motion causes tips of
  stereocilia to be displaced, causing electrical
  action potentials in a hair cell the electrical
  signal is then transmitted down auditory nerve.

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Anatomy

• Transduction
• Most (95) neurons connecting cochlea to higher
  levels in auditory system connect to inner hair
  cells
• Function of outer hair cells less clear provides
  amplification, sharp tuning (partially under the
  control of higher levels).
• Hair cells connect to neurons about 30,000
  neurons in one human auditory nerve .

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Anatomy
Organ of Corti contains hair cells and neurons
three parallel rows
1-Inner hair cell2-Outer hair cells3-Tunnel of
Corti4-Basilar membrane5-Habenula perforata
6-Tectorial membrane7-Deiters' cells8-Space of
Nuel9-Hensen's cells10-Inner spiral sulcus
11- nerves
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from http//www.iurc.montp.inserm.fr/cric/auditio
n/english/corti/corti.htm
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Anatomy
The same picture from Grays Anatomy (1918)
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Neural Response
Each neuron in the auditory nerve responds to
certain frequencies the response to each
frequency can be plotted by stimulating a neuron
with a particular frequency and measuring the
response rate (firing rate) of the neuron
The most sensitive frequency is the
Characteristic Frequency (CF)
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Neural Response

• Auditory firings processed by two types of
  neurons
• ones extracting precise temporal features (onset
  chopper units),
• others for spectral features (transient chopper
  units). (OShaughnessy p. 113)
• Each neuron has spontaneous rate of firing this
  rate depends on the sensitivity of the neuron
  (high spontaneous rates associated with low
  threshold of firing).
• 3 groups of spontaneous rates
• high rate (61, 18 to 250 spikes/sec),
• medium rate (23, 0.5 to 18 spikes/sec),
• low rate (16, lt0.5 spikes/sec).

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Neural Response
The firing rate of a neuron to a given stimulus
can be plotted
audio visual detection level threshold

• Firing rate has a dynamic range if intensity is
  below or above this range, firing rate wont
  change.
• Typical range of 20 dB for low-threshold fibers,
  40-50 dB for high-threshold fibers

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Neural Response
With three groups of neurons with different
thresholds and firing rates, can cover wide range
of signal levels at a given frequency
low rate
high rate
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Neural Response

• Phase Locking
• In addition to encoding frequency according to
  place along the BM, information is encoded in the
  rate of neuron firing
• Upper limit of 4 to 5 kHz for phase locking

tone
firings
1.0 msec/group 1000 Hz
1.18 msec/group 850 Hz
2.45 msec/group 408 Hz
count
msec
msec
msec
This figure shows the number of neuron firings
over time in response to three different tones
the timing of the firings is related to the
frequency of the tone
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Neural Response
Neural Recruitment Another method for
increasing dynamic range is for multiple neurons
to fire in response to the same stimulus If
stimulus is low in energy, a small number of
neurons, located near the CF, fire More intense
stimuli cause more neurons, located farther
from the CF, to fire
strongstimulus
weak stimulus
1 line 50100 fibers
(same frequency)
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Neural Response

• Adaptation
• If stimulus remains, neurons adapt to it,
  decreasing the firing rate with an exponential
  rate of decay (time constant ? ? 40 msec).
• Most of decay occurs within 15-20 msec of
  stimulus onset.
• When stimulus removed, firing rate falls to near
  zero andthen exponentially increases back to
  spontaneous rate.
• There may be two classes of neurons
• neurons that respond to steady-state sounds,
• neurons that respond to changes in frequency,
  with frequencysensitivity greatest at levels
  near human speech (OShaugnessy p. 119)

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Hearing Threshold
This figure shows the absolute thresholds of
hearing, as a function of frequency
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JND

• Just Noticeable Difference measure of ability
  to perceive a difference
• JND tests
• Two stimuli differ along one dimension, otherwise
  identical
• Subjects asked if two sounds are the same or
  different (AX test, is XA?)
• Or subjects are asked which of two sounds most
  resembles third (ABX or AXB test, is X A or
  B?)
• The JND occurs when 75 of responses are
  different (AX) or correctly identified (ABX)
• People are able to discriminate between 100 Hz
  and 101 Hz,
• but cant identify if a tone is 100, 101, , 109
  Hz without
• making pairwise comparisons

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JND
JND Trivia JND is greater for louder sounds,
sounds with duration ? 250 msec Sounds of equal
intensity increase in loudness up to 200
msec Below 1 kHz, two tones must be different by
1 to 3 Hz to be perceived as different At higher
frequencies, JND is larger (approx. 8 kHz tones
require a 100 Hz separation to be heard as
different) Entire frequency range has 1600
distinguishable frequencies and 350 intensities,
or about 300,000 tones of frequency and intensity
that can be identified in pairwise combination
(for durations gt 200 msec) For duration lt 250
msec, there are 850 frequency levels for
duration lt 10 msec only 120 levels and 170
intensities Identification of frequencies in
isolation yields much fewer tones.
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JND
JND Trivia, Timing Information Onsets of two
signals must differ by at least 2 msec to be
heard as separate sounds To identify order of
two signals, about 17 msec gap is requiredand
sounds must be 125-200 msec long However, people
use rise and fall of amplitude to segment speech
can not identify order of 4 vowels of 200 msec
duration in repeating sequence, but can identify
much shorter vowels if there are amplitude onsets
and offsets as well as 50 msec gap between
vowels. Sounds with energy onset lt 20 msec heard
as plucks otherwise,heard as bow