The Auditory System

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					The Auditory System
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)
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,
Physics of Sound
Frequency –
   Number of cycles completed by a wave in
   a given amount of time

     Low Frequency        High Frequency
Physics of Sound

 Amplitude –
   Physical strength of a wave

   High Amplitude        Low Amplitude
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 wave’s complexity
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.
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
   MGN
   Auditory cortex
   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.
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
Sound Localization
   Interaural time delay
       Detected at superior olive
       Works at low frequencies
   Interaural intensity difference (sound
       Works at high frequencies
   Pinna localizes sounds vertically (from above
    and below)

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