The Ear

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The Ear

• Chapter 11 in Sensation and Perception

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Sound and the auditory system
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Sound

• Physical Definititon
• Sound is pressure chasnges in the air or other
  medium
• Perceptual Definition
• Sound is the experience we have when we hear

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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

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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

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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 -

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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

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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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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.

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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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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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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

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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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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
• Place - i.e. which neurons are firing in
  response to a particular frequency place coding
  von Bekesy
• Temporal coding - The pattern of firing from
  these neurons