The Auditory System

1
The Auditory System
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Audition (Hearing)

• Transduction of physical sound waves into brain
  activity via the ear.
• Sound is perceptual and subjective.
• Structure of the ear.
• Pathways from the ear to the cortex.
• MGN (medial geniculate nucleus of the thalamus)

3
The Nature of Sound

• When displaced by a moving object, air becomes
  compressed.
• Vibrations produce periodic patches of compressed
  air.
• Frequency is the number of such patches per
  second (Hz).
• Intensity is the amount of air pressure (dB,
  decibels).

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Physics of Sound

• Frequency Number of cycles completed by a wave
  in a given amount of time

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Physics of Sound
Amplitude Physical strength of a wave
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Pitch, Loudness, and Timbre

• Pitch Sensory characteristic of sound produced
  by the frequency of the sound wave
• Loudness Sensory characteristic of sound
  produced by the amplitude (intensity) of the
  sound wave
• Timbre Quality of a sound wave that derives
  from the waves complexity

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Anatomy of Auditory Perception

• Outer ear directs sound waves to tympanic
  membrane, pinna localizes sound.
• Middle ear amplifies the wave and transmits it
  to the fluid-filled inner ear.
• Eustachian tube equalizes pressure and protects
  ear from loud noise
• Inner ear cochlea transduces sound waves into
  neural signals.

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Auditory Pathways

• Auditory receptors in cochlea exit via auditory
  nerve.
• Brain stem neurons at superior olive permit sound
  localization.
• Separate pathways for each ear up the brain stem.
• MGN
• Auditory cortex

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Tonotopy

• Portions of the basilar membrane and frequency
  selective.
• Frequencies maintain their relation to each other
  in the MGN and auditory cortex.
• Phase locking represents low frequencies,
  tonotopy and phase locking both represent mid
  level frequencies, tonotopy alone is useful at
  highest levels.

10
Attenuation Reflex

• Muscles contract to make the ossicles more rigid,
  reducing sound transmission to the inner ear
  protects the ear.
• Operates more at low frequencies.
• Prevents saturation, making high frequency sounds
  more discernible.
• Makes speech easier to understand in a noisy
  environment.

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Sound Localization

• Interaural time delay
• Detected at superior olive
• Works at low frequencies
• Interaural intensity difference (sound shadows)
• Works at high frequencies
• Pinna localizes sounds vertically (from above and
  below)