Sound Transduction 2 Or how my phase got all locked up Announcements: Now Online. Get assignments, lecture notes and other goodies here. CCIT Building now has phone lines. Call me at (905)

Previous lecture Inner ear -> Cochlea -> Scala media Inner ear -> Cochlea -> Scala media Scala media Scala media Endolymph filled chamber Endolymph filled chamber Surrounded by perilymph in the scala vestibuli & scala tympani Surrounded by perilymph in the scala vestibuli & scala tympani

Scala Media: where it all happens ‘Organ of Corti’ ‘Organ of Corti’ Organ o’ Corti contains Organ o’ Corti contains Basilar Membrane (base) Basilar Membrane (base) Tectorial Membrane Tectorial Membrane Inner Hair Cells Inner Hair Cells Outer Hair Cells Outer Hair Cells Hair cells embedded in Bas. Membrane Hair cells embedded in Bas. Membrane Outer Hair Cells contact Tect. Membrane Outer Hair Cells contact Tect. Membrane

Basilar Membrane Properties of the Basilar Membrane Properties of the Basilar Membrane Apex thin and stiff, Base broad and flexible Apex thin and stiff, Base broad and flexible Standing Waves Standing Waves Upward spread of masking Upward spread of masking Why do higher Frequencies get masked by lower frequencies? Why do higher Frequencies get masked by lower frequencies?

Why does it matter that the Basilar Membrane moves? Hair cell magic Hair cell magic Outer Hair Cells Outer Hair Cells ~ 12,000 in three rows ~ 12,000 in three rows Afferent and Efferent connections Afferent and Efferent connections Muscle fiber & passive motion Muscle fiber & passive motion Inner Hair Cells Inner Hair Cells ~ 3,500 in single row ~ 3,500 in single row Afferent connection Afferent connection Passive Motion Passive Motion

Actual Transduction! Wave along Basilar Membrane Wave along Basilar Membrane Causes inner hair cell shearing Causes inner hair cell shearing Shearing opens channel Shearing opens channel Endolymph in Scala Media attracts perilymph in Scala Tympani Endolymph in Scala Media attracts perilymph in Scala Tympani Change in electrostatic gradient within nerve cell Change in electrostatic gradient within nerve cell Mechanical – Electrical energy Mechanical – Electrical energy

What are the outer hair cells doing? Outer Hair cells motile & embedded in Tectorial Membrane Outer Hair cells motile & embedded in Tectorial Membrane Theory 1. Change motion of Basilar Membrane Theory 1. Change motion of Basilar Membrane Theory 2. Change position of Tectorial Membrane Theory 2. Change position of Tectorial Membrane Opposite predictions for behavioral response of outer hair cells Opposite predictions for behavioral response of outer hair cells

What does the ear do with the Electrical Energy Theory 1: Place coding of frequency Theory 1: Place coding of frequency Position along basilar membrane Position along basilar membrane Tonotopic map cochlea -> cortex Tonotopic map cochlea -> cortex Why is this a good theory? Why is this a good theory? Basilar membrane as tuned resonator (Bekesy, 1960) Basilar membrane as tuned resonator (Bekesy, 1960) Why is this an insufficient theory? Why is this an insufficient theory? Missing Fundamental Demo Missing Fundamental Demo

Coding Sound (con’t) Frequency Theory Frequency Theory Synchronous firing rate relative to acoustic vibration Synchronous firing rate relative to acoustic vibration Why is this a good theory? Why is this a good theory? Phase Locking (Wever, 1970; Seeback, 1941) Phase Locking (Wever, 1970; Seeback, 1941) What is the problem? What is the problem? Firing rate of a neuron ≈ 1 – 1.5 kHz max Firing rate of a neuron ≈ 1 – 1.5 kHz max Hearing 0.2 – 20 kHz Hearing 0.2 – 20 kHz But Volley Principle? But Volley Principle?

The Rest of the Auditory Pathway Cochlea Cochlear Nerve Cochlea Cochlear Nerve Cochlear Nerve has tonotopic organization Cochlear Nerve has tonotopic organization Coch. Nerve -> Olivocochlear bundle x2 Coch. Nerve -> Olivocochlear bundle x2 Ipsi/contralateral projection Ipsi/contralateral projection Olivocochlear bundle -> lateral lemniscus Olivocochlear bundle -> lateral lemniscus Efferent & Afferent projections Efferent & Afferent projections Lemniscus ->Inferior Colliculus Lemniscus ->Inferior Colliculus Lemniscus detects binaural activity, interaural processing Lemniscus detects binaural activity, interaural processing Inferior Colliculus -> Medial Geniculate Nucleus Inferior Colliculus -> Medial Geniculate Nucleus Inferior Collic. Tightens tonotopic map Inferior Collic. Tightens tonotopic map MGN -> Auditory cortex MGN -> Auditory cortex MGN tonotopic/binaural/initial multisensory input MGN tonotopic/binaural/initial multisensory input Auditory Cortex -> Everywhere Auditory Cortex -> Everywhere Primary auditory processing, speech center, etc. Primary auditory processing, speech center, etc.

Big Picture Outer Ear: Spectral filter for direction Outer Ear: Spectral filter for direction Middle Ear: Attacks impedance problem Middle Ear: Attacks impedance problem Inner Ear: Converts sound to nerve impulse Inner Ear: Converts sound to nerve impulse Midbrain (olivocochlear bundle): Bilateral effects Midbrain (olivocochlear bundle): Bilateral effects Cortex: Higher-level auditory processor Cortex: Higher-level auditory processor