L13: Hearing Impairment

Measuring the inner ear

Extremely difficult and dangerous

• Little improvement

• Only way to measure is the cochlear implant

Menuere’s disease

• Sensorineural hearing loss

• Causes fluctuating hearing loss and balance, not sure why it happens

Conductive hearing loss

Means no sound is getting from the outside to the inner ear

Sensorineural hearing loss

Caused by damage to the inner ear

What is used to test air-conducted thresholds

Headphones

What is used to test bone-conducted thresholds (by-passing any middle-ear problems)

Bone vibrator

Bone vibrator

• Used to test bone-conducted thresholds

• Bypasses outer and middle ear, straight through skull

• By comparing against normal thresholds for air and bone audiometry, can find the air-bone gap

  • The difference in discrepancy with normal hearing

What are the two types of pure-tone audiology

Headphones and Bone vibrator

If there is a larger discrepancy in normal hearing with the headphones than with the bone vibrator, what kind of hearing loss does this suggest?

Conductive loss

If there is a larger discrepancy in normal hearing with the bone vibrator than with the headphones, what kind of hearing loss does this suggest?

Sensori-neural loss

What are the two things outer hair cells do physiologically

1. Amplify sound

2. Improve the frequency selectively (only amplify sounds of the right frequency)

What can temporarily reduce the response of the basilar membrane

Furosemide

Loss of OHCs: Deactivation with furosemide (Ruggero & Rich, 1991)

• Measures motion of the membrane (through shining laser light through round window)

  • Means can ONLY look at very high frequencies

• Furosemide

  • Affects how the ear operates

  • 14 minutes after administration, can see reduction in basilar membrane activity

    • They are being suppressed by this drug

• Results

  • No amplification occurring

  • When given a different frequency that the basilar membrane is not tuned to, there is no change in response due to the furosemide

    • THEREFORE, the outer hair cells don’t make any difference away from the correct frequency

  • Amplification reduces as sound level increases

• Conclusion

  • Support that OHCs used in amplification and are frequency selective

Loss of OHCs: Loudness magnitude estimation

• P’s with hearing impairment

• Magnitude estimation

  • Give someone sound, say it is 100, give them another sound and as how much is that worth?

    • From before, the amount by which loudness goes up is much smaller than the amount the sound has increased in intensity

• Results

  • Don’t hear very low sound levels

  • Loudness curve goes up very fast until it is quite similar to a normal hearing person

    • Very sensitive to small increments in intensity

• Conclusion

  • Hearing impaired individuals lack normal “compressive” growth in loudness, instead experience “loudness recruitment”

Loss of outer hair cells: Frequency selectivity (Glasberg & Moore, 1986)

• Used the noise notch technique

• P’s had hearing impairment on only one side, so within-subject design as compared normal hearing on one side to impaired on the other

• Results

  • Frequency selection was poor regardless of width for hearing impaired side (broad filter)

What is the noise notch technique

• Psychophysical technique

• One low noise and one high noise, in between is a notch

  • Part of the sound spectrum where there’s no noise, just a tone

• Measure the lowest intensity of that tone that people can detect with that noise

  • If tone close to noise, high threshold

  • If far away, lower threshold

• Manipulate the width of the notch

What is a bigger issue: Loss of OHCs or IHCs?

OHCs

• IHCs more physiologically robust

Why do you not go completely deaf in a frequency region from loss of some IHCs

• You don’t go completely deaf in a frequency region from loss of some IHCs

  • BECAUSE each IHC has a range of response, so just less responsive

If you don’t go completely deaf from loss of IHCs, how do you tell you have loss of IHCs?

Dead regions

• Excitation in the dead region would NOT reach of threshold for detectable movement in a basilar membrane

• Even if the tone is increased enough so that the sound is detected outside of its preferred frequency range (despite this being in the dead zone), adding noise prevents the hair cell from picking it up

  • Noise it at preferred tone so trumps the sound

• Conclusion

  • If tone is in dead zone can still be picked up but need really intense frequency

How are dead zones measured?

Threshold Equalising Noise (TEN)

Threshold Equalising Noise (TEN)

• In a normal hearing listener, TEN should mask tones equally well across frequencies

  • Called threshold equalising as tend to get the same threshold regardless of frequency of tone

• TEN and Dead Zone

  • If person has significantly higher masked threshold at a specific frequency this suggests:

    • The tone is only being detected through off-frequency listening, indicating a dead region

  • All the thresholds suddenly become the same regardless of what level of threshold equalising noise being used

Why does amplifying a dead region not help with speech recognition and may even worsen it

• Amplification boosts the sound energy, but there are no functional cells or neurons (IHCs) to transmit it

• Off-frequency listening creates distortion, as neighbouring IHCs will try encode it instead

  • May mask meaningful cues

• i.e. Imagine trying to read a book using a magnifying glass — but part of the page is smudged beyond recognition. If you magnify the smudge, you don't see more detail — you just see a bigger smudge. Similarly, amplifying dead-region frequencies doesn’t restore clarity; it just makes irrelevant or confusing signals louder.

What area pumps loads of K+ ions into the Scala media

Stria vascularis

Loss of endocochlear potential (EP)

• Might suppress both inner and outer hair cells

Why might we think loss of endocochlear potential (EP) is something that happens?

1. The stria vascularis degrades with age

   • Don’t know how much it drops in humans, can only get animal models

2. Some individuals with known hearing loss have intact hair cells at post-mortem

   • Hearing loss can’t be due to hair cells

3. The endocochlear potential is known to fall with age in animal models

Why is loss of EP hard to distinguish from IHC and OHC loss

Loss of EP affects both IHC and OHC function

Does age-related hearing loss mainly affect high or low frequencies?

High frequencies

What might be evidence against loss of EP as an age-related hearing loss?

• As it affects whole cochlea, would expect its effects are fairly uniform across frequency

  • BUT most age-related hearing loss mainly affects high frequencies so not uniform

  • BUT we can’t measure inner ear so knowledge is limited

Cochlear synaptopathy: The evidence from noise-exposed mouse histology

• Measured mouse hearing through two objective measures (DPOAEs and ABRs)

• Exposed mouse to very loud sound

• Found noise threshold increased for a few days after before returning to baseline

• BUT if you take out the cochlears of exposed mice and count the synapses on the IHCs

  • Compared to control mice down about 2/3 rds

  • Does this happen in humans?

Why is cochlear synaptopathy sometimes referred to as “hidden hearing loss”

• It only takes a few synapses to detect a sound, so may be invisible audiologically

Cochlear synaptopathy: The evidence from human histology

• The number of IHC synapses in human cadaver temporal bones declines with age at death

Cochlear synaptopathy: The evidence from human psychophysics

• IPD = Interaural phase difference detection

  • Sound localisation skill, can detect time it takes for sound to hit first ear then the second

  • Found no decline in IPD tasks across ages

• AMD = Amplitude modulation detection

  • Ability to detect variations in sound level, thought to be dependent on the survival of audiotry nerve fibres and synpases

  • BUT again found no significant different in AMD tasks across ages

Summary of things that can go wrong with hearing