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OAEs and the cochlear function

Arturo Moleti Physics Dept Roma Tor Vergata University

and

Renata SistoOccupationa Hygiene Dept, INAIL, Monte Porzio Catone

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Inner ear (cochlea)

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The outer hair cells (OHC)The outer hair cells (OHC)

MT

OHC

IHC

BM

Pure mechano-electrical transduction

Active non linear filter:

Electro mechanical transduction: low

level signal amplification

http://147.162.36.50/cochlea/cochleapages/theory/main.htm

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Cochlear tonotopicityCochlear tonotopicity

■ Each frequency f is amplified in a particular place x(f) , which is a function of f, along the basilar membrane

■ The relation between f and the position of the tonotopically resonant site is given by the Greenwood map (Greenwood, 1980). This map is a quasi-logarithmic function:

x(f ) =

1

kωloge

fmax + f0

f + f0

kω = 0.1382 mm-1

fmax = 20655 Hzf0 = 145 Hz

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 THE MODEL

The differential pressure P(x,t) and the transverse velocity of the BM dξ/dt propagate longitudinally, such as the voltage difference V and the current I along an electric transmission line (P⇔V and dξ/dt ⇔I)

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ICTCA 2007, Heraklion, 1-6 July, 2007

Progressive movement of the BMProgressive movement of the BM

BaseHF

Apex LF

Cochlear models are in agreement in representing the acoustic propagation along the basilar membrane (BM) as a traveling wave (TW). Due to the tonotopicity of the BM each Fourier component of the stimulus propagates up to its resonant place, where it produces the maximum transverse displacement of the BM (the activity pattern peak) and then it is locally absorbed.

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ICTCA 2007, Heraklion, 1-6 July, 2007

Progressive movement of the BMProgressive movement of the BM

BaseHF

Apex LF

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BM response to a single tone

Base ≈ 20 kHz

Apex ≈ 20 Hz

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OAEs

-OAEs are small amplitude backward waves reaching the cochlear base. To detect them, in a model or in an experiment:

- The displacement of the stapes associated with OAEs must be separated from the stimulus; this is easily done for TEOAEs (in the time domain) and for DPOAEs (in the frequency domain)

- Cochlear roughness must be introduced to generate reflections

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Roughness and backward TWs

According to classical analytical computations (de Boer and McKay, 1971), a smooth cochlea does not produce reflections of the forward TW.Two mechanisms are assumed (Shera and Guinan, 1999) to produce backward TWs:- Nonlinear Distortion, wherever a cochlear place is excited above some displacement threshold level- Linear Reflection, due to coherent backscattering from randomly distributed fluctuations of the cochlear mechanical parameters- The first source is automatically present in any nonlinear model- The second one requires a schematization of roughness

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OAE generation

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TEOAEs

TEOAEs are generated by a click stimulus, and recorded in the ear canal after the end of the stimulus.

The mid frequency (1-2 kHz) response onset appears only 5-10 ms after the click, which permits good time domain separation between stimulus and response.

Linear ringing in the ear canal, lasting several ms, may be a serious experimental problem for hf TEOAEs, whose latency may be as short as 2 ms at 6-10 kHz.

Nonlinear acquisition techniques have been used, which exploit the fact that the ringing response is linear, whereas the cochlear response is nonlinear, to cancel the ringing component (ILO nonlinear mode).

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TEOAEs

In the ILO nl mode, an elementary acquisition frame is the average of four responses, three to identical clicks, and the fourth to a triple amplitude opposite polarity click.

Cancellation of the ringing component is not perfectly achieved with this nl mode, thus long onset time acquisition windows have to be used (2.5-5ms), which cut off the short latency hf OAE response.

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TEOAEs

Recently, Keefe et al. proposed a clever nonlinear acquisition paradigm, in which the elementary acquisition frame is the combination of three responses, the first to a p1 stimulus from loudspeaker l1, plus the second to p2=p1+15dB from l2, minus the third to p1+p2, with p1 always coming from l1 an p2 from l2.

By doing so, linearity of each loudspeaker is not necessary to effectively cancel the linear ringing component of the response.

Using this method, Goodman et al., (2009) were able to measure hf TEOAEs up to 12 kHz.

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SIMULATION RESULTS

Traveling wave produced by a click stimulus (80 dBpSPL)

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 RESULTS (model with roughness)

TEOAEs are observed as displacement at the stapes. No TEOAEs are generated in a smooth cochlea, without roughness.

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0-6

-4

-2

0

2

4

6x 1 0- 3

t im e (m s )

Dis

plac

emen

t at s

tape

s(nm

)

N =1000 stimulus level = 80 dB

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 RESULTS (model with roughness)

TEOAEs are observed with the expected time-frequency relation.

0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 00

5

1 0

1 5

2 0

2 5

3 0

3 5

4 0

F req ue ncy (H z )

Tim

e D

elay

(mse

c)

Frequency (H z )

Tim

e D

elay

(ms)

5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0 3 5 0 0 4 0 0 0 4 5 0 0 5 0 0 00

5

1 0

1 5

2 0

2 5

3 0

3 5

4 0

N =1000 stimulus level = 80 dB

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BM response to two tones and DPOAEs

The standard view of the DPOAE generation mechanism is (Shera and Guinan, 1999):- Feeding two nearby tones in the ear canal generates two traveling waves on the BM that interact nonlinearly around the x(f2) place, generating a localized DP source

at the frequency fDP=2f1-f2. - The DP wave propagates both backward and forward. The forward component reaches, and is amplified at, its cochlear resonant place, and part of this wave is backscattered by roughness. - The vector sum of the two backward DP waves reachesthe base, and is observed in the ear canal as a DPOAE.

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DPOAE generation

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SIMULATION RESULTS

Distortion products at the stapes: primary frequencies are 2000 and 2440 Hz, main DP is 1560 Hz

0 5 00 1 00 0 1 50 0 2 00 0 2 50 0 3 00 0 3 50 0 4 00 0

1 0-4

1 0-3

1 0-2

1 0-1

1 00

1 01

F re qu en c y (H z)

Spe

ctra

l den

sity

(mic

ron)

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Modeling basic DPOAE phenomenology

- Focusing our attention on the 2f1-f2 DP, which is the strongest one, we have:

- first generation by nonlinear distortion around the x(f2) cochlear place (of forward and backward DP waves).- generation of a secondary backward wave by linear reflection of the forward DP wave near the x(fDP ) place. - DPOAE spectral fine structure due to the vector sum of the two backward waves at the base, because the first source has flat phase (due to scale invariance) whereas the second one (place-fixed) has rapidly rotating phase.

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DPOAE fine-structure

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DPOAE f-s and DP-gramsA typical DP-gram is sampled at 1/3 octave intervals. As a consequence, if one of these frequencies corresponds (by chance) to a minimum of the f-s the DP level may be 10 dB lower than for a nearby frequency corresponding to a maximum. This is a cause of inter-subject variability of the DP level across subjects with the same hearing threshold which is not intrinsic (i.e. due to poor correlation between DP level and cochlear functionality) but extrinsic (due to the inaccuracy of the DP recording technique). Recently Long and Talmadge (2008) proposed a reliable and fast method to separate the two DP components, getting a measurement that is not affected by the inteference between them.

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OAE-audiometry correlation

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OAE-audiometry correlation

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Conclusion

The diagnostic power of OAEs is not fully exploited by standard data acquisition and analysis methods, due to the complexity of the OAE generation mechanisms

The theoretical knowledge of these mechanisms helps designing advanced acquisition techniques, capable of isolating OAE measurable quantities that are univocally related to the hearing threshold

This task requires collaboration among experts in: cochlear modeling, data acquisition and signal analysis, ORL and audiology