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The Ear

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How is sound processed by the auditory system? ... Tectorial membrane. Here is where the BM vibrations are transduced into electrical signals ... – PowerPoint PPT presentation

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Title: The Ear


1
The Ear
  • Chapter 11 in Sensation and Perception

2
recap
  • No lecture tomorrow
  • Sound
  • Frequency, pitch, amplitude, loudness, timbre
  • Pure tones harmonic sounds

3
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4
Auditory system
  • How is sound processed by the auditory system?
  • Before we hear the auditory system must deliver
    the sound stimulus to the receptors, transduce
    pressure changes into electrical signals, process
    the electrical signals to give information such
    as pitch, loudness, timbre, location.
  • Outer, middle and inner ear

5
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6
The outer ear
  • Pinnae determining the location of sounds
  • Auditory canal
  • Tympanic membrane / ear drum
  • Function middle ear protection, boost the
    intensities of some frequencies its resonance
    amplifies frequencies in the region of 2-5kHz

7
The middle ear
  • Vibration of tympanic membrane
  • Ossicles malleus (hammer), incus (anvil), stapes
    (stirrup)
  • Vibrations transmitted to the inner ear via the
    oval window
  • Outer and middle ear air inner ear is fluid
    filled (denser than air)
  • Function of the ossicles amplify vibration to
    allow movement of the fluid in the inner ear 2
    ways -

8
The middle ear
  • 1. Concentrating the vibration of the large
    tympanic membrane onto the much smaller stapes
  • 2. Being hinged to create a lever action
  • Middle ear muscles attached to the ossicles
    contract at very high sound intensities protect
    the inner ear against damage from loud sounds

9
The inner ear
  • Fluid filled cochlea
  • Set into vibration by the movement of the stapes
    against the oval window
  • Cochlea contains the basilar membrane (BM) and
    Organ of Corti

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12
Organ of Corti
  • Basilar membrane (BM) vibrates in response to
    sound
  • Inner and outer hair cells, with cilia on top
    bending of these in response to sound
  • Tectorial membrane
  • Here is where the BM vibrations are transduced
    into electrical signals
  • The electrical signals occur in the fibres of the
    auditory nerve
  • The bending of the cilia of the inner hair cells
    generates the electrical signal that is
    transmitted to fibres in the auditory nerve.

13
Basilar membrane response to sound
  • Vibrates in a travelling wave in response to
    pressure changes
  • Basal end is narrower and stiffer than the apical
    end
  • Point of maximum displacement on the BM indicates
    the frequency of the pure tone frequency
    selective
  • This maximum displacement is important because it
    tells us which hair cells and nerve fibres along
    the BM will be affected the most by the
    membranes vibration.

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15
Basilar membrane
  • The amount the cilia move depends on the amount
    that the BM is displaced
  • Low frequencies cause max vibration near the apex
  • High frequencies cause max vibration near the
    base
  • Basis for the place theory of pitch perception
  • Tonotopic organisation
  • the frequency of a tone is represented by the
    firing of fibres located at specific places along
    the cochlea

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18
Basilar membrane (BM)
  • BM response to complex tones i.e. with more
    than one frequency component, e.g. speech and
    musical instrument sounds and most of what we
    hear
  • Performs an analysis of the sound into its
    frequency components each of the frequency
    components activate a different area of the
    membrane and hence different nerve fibres
  • Basilar membrane and amplitude / loudness

19
Auditory nerve
  • Frequency also represented in the timing of the
    electrical signals from the auditory nerve
    fibres.
  • Phase locking - neurons always fire at the same
    point, or phase, of the sound stimulus.
  • Timing of the firing (electrical signals) depends
    on the frequency of the sound.

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21
Auditory nerve
  • How are the sound vibrations represented in the
    firing of neurons in the auditory nerve?
  • Investigated mainly using pure tones
  • How is the frequency of the tone represented?
  • Two possible ways
  • 1. Place - i.e. which neurons are firing in
    response to a particular frequency place coding
    von Bekesy
  • 2. Temporal coding - The pattern of firing from
    these neurons
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