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Sound Transduction 2
Or how my phase got all locked up
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Inner ear -> Cochlea -> Scala media
Scala media
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Endolymph filled chamber
Surrounded by perilymph in the scala vestibuli
& scala tympani
Scala Media: where it all happens
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‘Organ of Corti’
Organ o’ Corti contains
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Basilar Membrane (base)
Tectorial Membrane
Inner Hair Cells
Outer Hair Cells
Hair cells embedded in Bas.
Membrane
Outer Hair Cells contact
Tect. Membrane
Basilar Membrane
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Properties of the Basilar
Membrane
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Apex thin and stiff, Base
broad and flexible
Standing Waves
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Upward spread of
masking
Why do higher
Frequencies get masked
by lower frequencies?
Why does it matter that the Basilar
Membrane moves?
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Hair cell magic
Outer Hair Cells
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~ 12,000 in three rows
Afferent and Efferent
connections
Muscle fiber & passive
motion
Inner Hair Cells
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~ 3,500 in single row
Afferent connection
Passive Motion
Actual Transduction!
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Wave along Basilar
Membrane
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Causes inner hair cell
shearing
Shearing opens channel
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Endolymph in Scala
Media attracts perilymph
in Scala Tympani
Change in electrostatic
gradient within nerve
cell
Mechanical – Electrical
energy
What are the outer hair cells
doing?
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Outer Hair cells motile &
embedded in Tectorial
Membrane
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Theory 1. Change motion of
Basilar Membrane
Theory 2. Change position of
Tectorial Membrane
Opposite predictions for
behavioral response of outer
hair cells
What does the ear do with the
Electrical Energy
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Theory 1: Place coding of
frequency
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Why is this a good theory?
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Position along basilar membrane
Tonotopic map cochlea -> cortex
Basilar membrane as tuned resonator
(Bekesy, 1960)
Why is this an insufficient theory?
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Missing Fundamental Demo
Coding Sound (con’t)
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Frequency Theory
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Why is this a good theory?
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Synchronous firing rate relative to
acoustic vibration
Phase Locking (Wever, 1970; Seeback,
1941)
What is the problem?
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Firing rate of a neuron ≈ 1 – 1.5 kHz
max
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Hearing 0.2 – 20 kHz
But Volley Principle?
The Rest of the Auditory Pathway
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Cochlea <-> Cochlear Nerve
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Coch. Nerve -> Olivocochlear bundle x2
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Inferior Collic. Tightens tonotopic map
MGN -> Auditory cortex
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Lemniscus detects binaural activity, interaural
processing
Inferior Colliculus -> Medial Geniculate
Nucleus
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Efferent & Afferent projections
Lemniscus ->Inferior Colliculus
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Ipsi/contralateral projection
Olivocochlear bundle -> lateral lemniscus
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Cochlear Nerve has tonotopic organization
MGN tonotopic/binaural/initial multisensory
input
Auditory Cortex -> Everywhere
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Primary auditory processing, speech center,
etc.
Big Picture
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Outer Ear: Spectral filter for direction
Middle Ear: Attacks impedance problem
Inner Ear: Converts sound to nerve
impulse
Midbrain (olivocochlear bundle): Bilateral
effects
Cortex: Higher-level auditory processor