Download November 3

The Auditory System
Audition (Hearing)
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Transduction of physical sound waves into
brain activity via the ear.
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Sound is perceptual and subjective.
Structure of the ear.
Pathways from the ear to the cortex.
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MGN (medial geniculate nucleus of the thalamus)
The Nature of Sound
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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).
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
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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.
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Eustachian tube – equalizes pressure and protects
ear from loud noise
Inner ear – cochlea transduces sound waves
into neural signals.
Auditory Pathways
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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
Tonotopy
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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
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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.
Sound Localization
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Interaural time delay
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Interaural intensity difference (sound
shadows)
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Detected at superior olive
Works at low frequencies
Works at high frequencies
Pinna localizes sounds vertically (from above
and below)