Central auditory processing

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Central auditory processing

• Chapter 11 sensation and perception

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assessment

• Auditory and visual system
• Tuesday 23rd of October

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Beyond the auditory nerve

• From the cochlea to the auditory cortex
• Subcortical structures - Cochlear nucleus
  Superior olivary nucleus inferior colliculus
  medial geniculate nucleus
• They are bilateral on the right and left sides
  of the body
• Primary auditory receiving area (A1) located in
  the temporal lobe

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Auditory areas in the cortex

• Hierarchical processing signals are first
  processed in the core and then travel to the belt
  and then to the parabelt
• Other areas outside of the temporal lobe are also
  believed to be involved in hearing parietal and
  frontal lobes are activated by visual and
  auditory stimuli

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What and Where streams for hearing

• Identifying and location sounds
• What from anterior of core and belt to the
  prefrontal cortex identifying sounds
• Where from the posterior of the core and belt
  to the parietal and prefrontal cortex locating
  sounds
• Evidence from neuron recording, the effects of
  brain damage and brain scanning that support the
  idea that different areas of the brain are
  involved in identifying and locating sounds.

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Auditory cortex and perception

• Important for localising sounds
• May also be important for perceiving pitch and
  discriminating between sounds of different
  pitches
• Tonotopic organisation is maintained
• Phenomenon of the missing fundamental

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Cochlear implant

• Example of the application of hearing research
• Damaged hair cells
• Electrodes are inserted in the cochlea to create
  hearing by electrically stimulating the auditory
  nerve fibres.

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Sound localisation

• How can you tell where a sound is coming from?
• Auditory localisation locating objects based on
  their sound
• Locating sounds in the azimuth, elevation and
  distance
• Which directions are we most accurate at locating
  sounds?

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Auditory localisation

• We are most accurate at localising sounds in
  front and least when they are off to the side and
  behind the head
• How does the auditory system determine the
  location of a sound?
• Location information in vision vs. hearing (Fig.
  12.3)

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Cues for sound location

• 2 Binaural cues - interaural time difference
  (ITD) and interaural level difference (ILD)
• Monaural cues

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Monaural cue

• Before sound enters the auditory canal it is
  reflected from the head and the folds of the
  pinnae
• The head and pinnae act to decrease or increase
  the intensities of some frequencies over others
• Head-related transfer function (HRTF) a
  spectral cue

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HRTF

• Spectral cue as information for location is
  provided by the spectrum of frequencies that
  occurs at each location

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Sound location

• ITD and ILD - used for judging the location of
  sounds along the azimuth ITD best at low
  frequencies and ILD best at high frequencies
• ITD and ILD not good for judging elevation
• ITD and ILD are zero for sounds at positions A, B
  and C
• How do we locate sounds along the elevation
  coordinate?

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Sound location

• Spectral cues are used to judge elevation - also
  head movement
• Summary of cues to locate sounds ITD, ILD, HRTF
  (spectral cues pinnae and head), head movement,
  vision.

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Physiological representation of auditory space

• Identify neurons responsible for ITD and ILD
  superior olivary nucleus the first nucleus in
  the auditory system to receive inputs from both
  ears.
• Auditory cortex is necessary for localisation
  where stream
• Panoramic neurons signal location by their
  pattern of firing

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