alessandro altoè the mechanics of hearing. about today’s lecture many methodological mistakes...
TRANSCRIPT
Alessandro Altoè
The Mechanics of Hearing
About today’s lecture
• Many methodological mistakes when dealing with hearing:• Oversimplifications (engineers):- The cochlea is a filter bank tuned accordingly to a simple model
• Deliberating ignoring the laws of mechanics (psychologists):- The cochlea is a filter bank tuned accordingly to psychoacoustics
• Ignoring the “known unknown” (almost everybody):- The prevailing theory is not always the correct one
• An overview of the physics essential to avoid this errors
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Overview of the hearing system
• The hearing system is composed by 4 sub-system:• The outer ear • The middle ear• The internal ear• The auditory pathway inside the brain
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Overview of the hearing system
• The hearing system is composed by 4 sub-system:• The outer ear • The middle ear
• The internal ear• The auditory pathway inside the brain
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Today!
Outer ear
• The sound, before reaching the ear drum encounters the pinna and travels through the ear canal
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Pinna
• What is its function?• Change of resonances depending on the direction of arrival
of sound (mainly on elevation)
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Ear canal
• “Pipe” delivering sound from the external world to the eardrum
• 2.5 cm long in average, not constant cross-section (conical)• Outer ear act as a “band-pass filter”
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Middle ear
• Sound transmission from the ear canal to the cochlea
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Middle ear
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What are the three ossicles doing there?
• Avoid damages due to loud sound exposure (middle-ear reflex)
• Impedance matcher from air to fluid transmission (in a certain frequency range)
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Middle-ear transfer functions measured on human cadavers. From Puria (2003)
How well does the ear deliver sound to the cochlea?
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Comparison of the relative power spectra of impulses produced by a large cannon and the power that reaches the cochlea of a cat (from Rosowski, 1991) .
Cochlear mechanics
The cochlea
• Most studied “sensor” ever• A lot of open questions about its functioning
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Overview of the cochlea
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From Geisler (1998)
Wave propagation in the cochlea
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Tonotopic organization in the cochlea
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Center frequency=√(stiffnes/mass)
The basilar membrane motion
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Cochlear nonlinearity
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Normalized displacement in response to a click in the cochlear partition with BF 1 kHz
Center Frequency (kHz)
Mag
nitu
de o
f dis
plac
emen
t (dB
)
Cochlear excitation patterns for a 1 kHz tone with varying level
• In vivo measurements show more complex responses
• mechanical properties cannot be represented by simple mass-spring systems
• A closer look to the anatomy of a single cochlear partition will reveal interesting aspects of cochlear mechanics
Figures from Verhulst et al (2012)
Cochlear micro mechanics
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Outer hair cells
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Outer hair cells
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Cochlea amplifier?
• outer hair cells (OHC) motor is so fast that can follow the amplitude of tones above 20 kHz
• Plausible amplification of Basilar Membrane motion
• It allows to model accurately the cochlear nonlinearities
• A number of scientist disagree
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Summarizing
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Additionally…
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The cochlea emits sound!
Otoacoustic emissions
• Various reasons (reflections at the stapes, reflections at the apex, OHC activities, small irregularities in the cochlea)
• Can be evoked or spontaneous (tone-like)
• Essential for neonatal hearing screening
• Otoacoustic emissions can be modulated by e.g. attention
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Electrical transduction in the cochlea
Electrical transduction in the cochlea
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Inner hair cells
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Gating spring mechanism
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Adapted from Cheatham and Dallos (2000)
An engineer view of the inner hair cell
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Low frequency input
Half-wave rectified output
An engineer view of the inner hair cell
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High frequency input Envelope detector
What moves the stereocilia?• Open question, difficult to measure and to model• However, good correlation with BM velocity up to 60 dB
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Neural transduction
Auditory nerve fibers• A.k.a. spiral ganglion• From 10 to 20 connected to a single IHC• ~30.000 of them
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From Kiang (1991) From Meddis (1986)
Auditory nerve fibers action potential• Action potential = spike = all or none signal
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IHC potential (mV)
Nerve fiberspotential
Summingthe spikes
“Spike” histogram
Sp
ike
s
• The spiking of an auditory fiber is a stochastic process• After a fiber “fires” needs time to “reload”
How can they deliver useful information to the brain?
Volley principle!
The spike rate of auditory fibers
• Averaging the spikes of a fiber over many repetition of a stimulus -> post stimulus time histogram (PSTH)
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PSTH
time time
Input signal
From Zhang and Carney (2001)
The spike rate of auditory fibers
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What drives the spike rate of nerve fibers?
Spike rate (t) ~ flow ~ v(t)*q(t)
Neurotransmitter “pool”Neurotransmitter “factory”
Valve controlled by ihc potential v(t)
From Geisler (1998)
Temporal Information
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Neurotransmitter “pool”Neurotransmitter
“factory”
Valve controlled by ihc potential v(t)
Auditory fibers spike rateLow frequency tone
At low frequency the fibers (below 3-4 kHz) firing can phase-lock to the signal
At high frequency not…
High frequency toneAuditory fibers spike rate
From Zhang et al. (2001)
Dynamic range
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Low spontaneous rate large dynamic range
High spontaneous ratehigh sensibility
Leaky “valve”!
From Zagaeski et al. (1994)
Adaptation
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From Geisler (1998)
Inner ear, the big picture
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NON LINEAR!
HIGHLY NON LINEAR!
NON LINEAR STOCHASTIC
Two open questions
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Dynamic range open question
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Many
A few
Very few
Are a few low spontaneous rate fibers enough for the entire dynamic range?
Plus, they saturate at 100 dB, while the human dynamic range is 120 dB.
From Winter and Palmer (1991)
Dynamic range
• The volley principle might answer the question• The cochlear “filters” would have a strong reason to be so
non-linear!
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Center Frequency (kHz)
Mag
nitu
de o
f dis
plac
emen
t (dB
)
Increasing the spl,more and more cochlear partitions (and relative ihc)get “excited”!
How about pitch information?
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Center Frequency (kHz)
Mag
nitu
de o
f dis
plac
emen
t (dB
)
We can exclude that it depends on which partition is more active…
How about pitch information?
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Auditory fibers spike rateLow frequency tone
More likely, is the period between spikes that encodes it
But what about high frequency?
High frequency tone
Pitch encoding at high frequency
• Human performance poor in pitch detection of pure tones at high frequency
• For certain harmonic tones high pass filtered we still perceive pitch.
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To IHC
Frequency
Cochlear filter
Spectral component of high-pass filtered harmonic tone
0 5 kHz
F0
Open question
• Theoretical upper bound on the fundamental frequency
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Frequency
Cochlear filter
Spectral component of high-pass filtered “unresolved” harmonic tone
0 5 kHz
• A “faint” pitch sensation for F0>>upper bound
• The “pitch” information is not completely lost in the transduction process
• How? Distortion Products?
f1 f2
f2 – f1 2f1 – f2
Distortion Products
References• Zagaeski, Mark, et al. "Transfer characteristic of the inner hair cell synapse: Steady‐state analysis." The Journal of the Acoustical
Society of America 95.6 (1994): 3430-3434.
• Geisler, C. Daniel. From sound to synapse: physiology of the mammalian ear. Oxford University Press, 1998.
• Verhulst, Sarah, Torsten Dau, and Christopher A. Shera. "Nonlinear time-domain cochlear model for transient stimulation and human
otoacoustic emission." The Journal of the Acoustical Society of America 132.6 (2012): 3842-3848.
• Meddis, Ray. "Simulation of mechanical to neural transduction in the auditory receptor." The Journal of the Acoustical Society of America
79.3 (1986): 702-711.
• Rosowski, John J. "The effects of external‐and middle‐ear filtering on auditory threshold and noise‐induced hearing loss." The Journal of
the Acoustical Society of America 90.1 (1991): 124-135.
• Kiang, Nelson Yuan-sheng. "Curious oddments of auditory-nerve studies." Hearing research 49.1 (1990): 1-16.
• Puria, Sunil. "Measurements of human middle ear forward and reverse acoustics: implications for otoacoustic emissions." The Journal of
the Acoustical Society of America 113.5 (2003): 2773-2789.
• Winter, Ian M., and Alan R. Palmer. "Intensity coding in low‐frequency auditory‐nerve fibers of the guinea pig." The Journal of the
Acoustical Society of America 90.4 (1991): 1958-1967.
• Cheatham, M. A., and P. Dallos. "The dynamic range of inner hair cell and organ of Corti responses." The Journal of the Acoustical
Society of America 107.3 (2000): 1508-1520.
• Cheatham, M. A., and P. Dallos. "The level dependence of response phase: Observations from cochlear hair cells." The Journal of the
Acoustical Society of America 104.1 (1998): 356-369.
• Zhang, Xuedong, et al. "A phenomenological model for the responses of auditory-nerve fibers: I. Nonlinear tuning with compression and
suppression." The Journal of the Acoustical Society of America 109.2 (2001): 648-670.
Questions?
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Questions for you!
• Why a single cochlear partition cannot be thought neither as linear filter nor as a filter with variable center-frequency?
• What is the volley principle? How can it be used to explain the wide dynamic range of human hearing?
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