The Role of the Efferent System in Auditory Performance in Background Noise

The Role of the Efferent System in Auditory Performance in Background Noise

Utah Speech-Language Hearing Association, 2015

Skyler G. Jennings Ph.D., Au.D. CCC-A

Outline

• Hearing in a noisy background

• Normal vs. hearing impaired listeners

• The medial olivocochlear (MOC) system

• Effects on basilar membrane responses

• Time course of the MOC reflex

• The effects of the MOC reflex on perception

• Improved signal-to-noise ratio

• Improved peak-valley contrast

• Perceptual evidence for the role of the MOC reflex

• Auditory masking

• Auditory intensity discrimination

• Speech-in-noise performance

• Clinical applications

• Conclusions

Learning objectives

• Understand the anatomy and physiology of the auditory efferent system

• Understand the role of the medial olivocochlear reflex in regulating outer hair cell amplification

• Understand recent research on how the auditory efferent system may be dysfunctional in listeners with cochlear hearing loss, leading to altered

auditory perception.

Hearing in a noisy background

Normal vs. hearing impaired listeners

Listening in background noise is difficult for

hearing-impaired individuals

Normal-hearing listeners do quite well

understanding speech in a noisy background

Many factors may contribute to effectively understanding speech in a noisy background

• Peripheral factors

• Audibility

• Clear signal (healthy OHCs/IHCs)

• Redundancy among neural pathways

• Efferent function

Many factors may contribute to effectively understanding speech in a noisy background

• Central factors

• Auditory stream segregation

• Pitch

• Common fate

• Location

• Decoding the speech message from an

impoverished signal

• Brain noise may

increase with age

and HI

Medial olivocochlear anatomy / physiology

A brief review

Medial olivocochlear neurons innervate the outer hair cells

10

from Maison and Liberman (2000)

Innervation of OHCs is primarily efferent

OHC

Medial olivocochlear reflex

pathway

Detour: review of

cochlear mechanics

The basilar membrane exhibits tonotopic traveling motion

Traveling wave demo from Rick Rabbitt in bioengineering

The response of the cochlea depends on the state of the outer hair cells

• When OHCs are impaired, the cochlear response is:

• Insensitive (high thresholds)

• Broadly tuned

• Linear

• When OHCs are healthy , the cochlear response is:

• Sensitive

• Sharply tuned

• Compressive

*Outer hair cells amplify BM motion and sharpen the peak of the traveling wave.

Active and passive cochlear processes

• Cochlear motion is the sum of passive and active processes-

• Passive process: motion due to the physical

properties of the basilar membrane (e.g., stiffness and mass)

• Active process:

amplification introduced by the OHCs to motion of the basilar membrane.

*The active process is decreased with sensorineural hearing loss and absent after death.

The interaction of passive and active processes results in “cochlear compression.”

17

This graph of the basilar membrane output as a function of stimulus level is called a cochlear

“input/output”

function.

Signal going in to the cochlea Signal coming

out of cochlea

Compression facilitates a large

perceptual dynamic range (but…)

Modified from Ruggero et al (1997)

Large Range of Input Levels Compressed

Output

Byproducts of compression: reduced SNR

N S

Favorable input SNR Poorer post-cochlear SNR

Noise Speech Noise

Speech

*Cochlear compression may limit speech understanding in

background noise.

The speech envelope is important for understanding speech

Speech envelope: has gross fluctuations characterized by peaks and valleys

*Early cochlear implant studies showed that the speech envelope must be faithfully coded for speech understanding (e.g., Shannon, 1995).

Byproducts of compression:

reduced peak-valley contrast

Valley Peak

V P

Favorable input peak-valley contrast Poorer post-cochlear

peak-valley contrast

Valley Peak

*Cochlear compression may limit the contrast between speech peaks and valleys of the speech envelope. This may negatively influence speech understanding.

Summary of cochlear mechanics

• Cochlear motion is determined by active and passive processes

• The interaction of active and passive processes results in cochlear compression

• Compression may lead to a reduced post-cochlear SNR

• Compression may lead to a reduced post-cochlear

peak-valley contrast

Back to MOC efferents…

Nature’s solution to avoiding the byproducts of cochlear

compression

Efferent (MOC) Effects on Basilar Membrane Responses (Cooper and Guinan, 2006)

Brainstem slice where MOC

neurons originate

MOC neurons connect to OHCs

Basilar membrane motion measured

MOC neurons stimulated by electric shock

Efferent (MOC) Effects on Basilar Membrane Responses (Cooper and Guinan, 2006)

The influence of

stimulating the MOC bundle is to reduce the amplification provided by OHCs.

*MOC stimulation results in decreased basilar membrane displacement, especially at low sound levels

(no shocks)

(shocks)

The MOC reflex is elicited by sound and

its response grows with sound level

The MOC timecourse includes onset, delay, build-up, offset delay, and decay stages

Based on Backus and Guinan (2006)

Onset delay: ~25 ms Build-up: ~200 ms Offset delay: ~25 ms Decay: ~100-200 ms

*This suggests that there is a slight lag between when the stimulus is turned on and off and when the MOC system effects

MOC stimulation decompresses the cochlear response and may improve SNR

Without the MOC (MOC-):

• Post-cochlear SNR is small

• Post-cochlear contrast is poor

With the MOC (MOC+):

• Post-cochlear SNR is improved

• Post-cochlear contrast is greater

Signal coming out of the cochlea

Signal going in to the cochlea

Potential roles of the MOC reflex in perception

• Reduction in masking by improving SNR (Guinan, 2006)

• Facilitate selective attention (Scharf et al, 1997)

• Protection against noise-induced hearing loss (Maison and Liberman, 2000)

• Protection against loss of spiral ganglion neurons

(Liberman and Maison, 2014)

Listeners with sensorineural hearing loss may have reduced MOC function

• Damage to cochlear structures disrupts the MOC pathway

• This may reduce the ability of the MOC

reflex to regulate OHC amplification

• This suggests that the hearing-impaired

system may have

difficulty adapting to a

noisy background.

Interim conclusions

• Sensorineural hearing loss results in difficulty listening in background noise

• Hearing aids currently do not address this issue

• The MOC efferent system regulates the amplification of the outer hair cells

• Regulation of OHC amplification can improve SNR and peak-valley contrast

• Listeners with sensorineural hearing loss may lose

partial function of the MOC reflex, resulting in the

inability to adapt to a noisy background

MOC effects in human

perception

Perceptual studies on the MOC reflex

• Simple stimuli

• Masking

• Intensity increment/decrement detection

• Complex stimuli

• Speech in noise

Framework for masking experiments

• Assumption #1: The probe is detected through an auditory filter centered on the cochlear location that responds best to the probe

• Assumption #2: The probe energy through the auditory filter must be greater than the masker energy through the same filter by some criterion amount (criterion post-

cochlear SNR).

34

Auditory Filterbank

…

probe masker

criterion post-cochlear SNR

A perceptual task that takes advantage of the sluggish start of the MOC reflex: Masking

Masking Stimulus Basilar Membrane Response

MOC Time course onset

Temporal center

onset

temporal center

Detection threshold improves by up to 20 dB when the probe is moved from the onset to the temporal center

onset

Temporal center

0 10 20 30 40 50 60 70 80

60 dB SPL Masker

Probe Level (dB SPL)

Onset Temporal Center

Improvement called

“overshoot”

Normal Hearing Listeners

Hearing impaired listeners show little or no improvement in threshold when the probe is moved (i.e, small

overshoot)

onset

Temporal center

0 10 20 30 40 50 60 70 80

60 dB SPL Masker

Probe Level (dB SPL)

Onset Temporal Center

Small overshoot

Hearing-Impaired Listeners

Overshoot in NH listeners who ingest aspirin show very little improvement in threshold

• High aspirin ingestion produces a temporary hearing loss by

temporarily impairing the OHCs

• Overshoot decreases during an aspirin

regimen, while

temporary hearing loss increases

McFadden and Champlin (1990)

Before aspirin

Weeks after aspirin Days during

aspirin regimen

Overshoot can be explained with an

auditory model that includes MOC effects

OHC component of the model was used to manually regulated OHC amplification

Jennings et al (2011)

Model predicts reduced overshoot in HI listeners

Normal hearing simulations Hearing impaired simulations

Jennings et al (2011)

MOC reflex magnitude and growth rate may be reduced in HI listeners

• OHC amplification is reduced at a rate of -1 dB/dB with increasing masker level in YNH and ONH subjects.

• OHC amplification is reduced changes at a slower rate in OHI

subjects (-0.56 dB/dB).

• On average the largest reduction in OHC

amplification was roughly -45 dB in normal-hearing listeners and about -25 dB in hearing-impaired

listeners.

Framework for decrement detection experiments

• Assumption #1: The

decrement is detected through an auditory filter centered on the cochlear location that responds best to the pedestal

• Assumption #2: The peak- valley contrast through the auditory filter must reach some criterion amount (criterion PV contrast).

42

Auditory Filterbank

…

decrement

criterion peak-valley ratio

A perceptual task that takes advantage of the sluggish start of the MOC reflex: Decrement Detection

Decrement Stimulus Basilar Membrane Response

MOC Time course onset

Temporal center

Decrement detection improves as the decrement is move to the temporal center of the pedestal

Lower numbers = better performance

Chen and Jennings, ARO 2015

Summary of simple perceptual experiments

• Detection thresholds improve as a tone is moved to a center of a noise

• This improvement only occurs in NH listeners

• This improvement can be explained by an increase in SNR during the course of the masker

• Decrement detection threshold improve as the decrement is moved to the center of the pedestal

• This improvement can be explained by a relatively better

peak-valley contrast.

MOC effects in speech perception

• Contralateral suppression of OAEs is a technique for

assessing MOC strength

• Kumar and Vanaja (2004) found a significant

correlation between speech identification and

contralateral suppression of OAEs.

• They speculated that the relatively better speech

identification in individuals

with stronger OAE inhibition

was due to the MOC reflex

regulating OHC amplification

MOC effects in speech perception (cont…)

• ABR latency shift in the presence of noise can predict the degree of masking

• Longer latencies  more masking

• ABR latency shift and consonant discrimination was measured in the same listeners

• Consonant were discriminated on a continuum from “ba” to “ga”

• The consonant discrimination thresholds is the distance along the continuum

before “ba” is clearly distinguished from

• Lower scores are better.“ga”

• Consonant discrimination is correlated with ABR latency shift

• This latency shift was also correlated with contralateral suppression of OAEs

• The authors concluded that the

unmasking effect may be due to the

MOC regulation of OHC amplification de Boer et al. (2012)

Potential applications

and conclusions

Potential applications: MOC inspired noise reduction

• Speech in noise with linear compression

• Speech in noise with

MOC-inspired adjustment in OHC amplification

Speech in noise sound clip from:

http://www.ece.rochester.edu/~zduan/is2012/examples.html

Conclusions

• Robust speech perception in noise may be facilitated by the MOC reflex in NH listeners

• The MOC reflex improves speech understanding in noise by improving the SNR and increasing peak-valley contrast

• HI listener may suffer from a partially dysfunctional MOC reflex

• Behavioral studies in masking, decrement detection, and consonant identification support the potential role of the MOC reflex in speech perception

• An MOC inspired processing algorithm may improve the

benefit derived from hearing aids and cochlear implants

when listening to speech in a noisy background

references

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• Cooper, N. P., & Guinan, J. J., Jr. (2006). Efferent-mediated control of basilar membrane motion. J Physiol, 576(Pt 1), 49-54.

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