L13 - Hearing Loss

Question 1

Why can’t the ear be examined in detail easily?

Answer 1

• Info tends to be limited

Question 2

What can go wrong with the auditory system?

Answer 2

Conductive hearing loss: sound does not get from outside to cochlea
- Ear canal can be blocked
- Ear drum perforated
- Ossicles dislocated - due to blow to head
- Ossicles immobilised (things start sticking together - old age)
- Middle ear filled with fluid
Sensorineural hearing loss: hard to work out
- Loss of perilymph: hear nothing at all - happens to deep sea divers
- Loss/absence of outer hair cells - same ones you have when you die are same as born, not replaced
- Loss/absence of inner hair cells
- Loss of endocochlear potential: can’t drive ion movement if no potential - occurs when older
- Loss of auditory nerve fibres
Unsure of cause
- Meniere’s disease - issues with balance and hearing
- Tinnitus

Question 3

What devices do audiometrists do?

Answer 3

• Detect the faintest sound you can hear - absolute threshold
• Headphones: for testing air-conducted thresholds
• Bone vibrator: testing bone conducted threshold by passing the middle ear - can be placed on skill and send sound through the bone to send to cochlea
• If threshold in bone vibrator is normal but not in air = conductive hearing loss, BUT if threshold is the same = sensorineural hearing loss = amplifier is needed via cochlea implant or hearing aids

Question 4

How do the loss of outer hair cells lead to hearing loss? (+study)

Answer 4

• Physiologically vulnerable - harder to maintain in older people = responsible for more problems than IHC
• Researchers used furosemide that makes outer hair cells stop working: and made the basilar membrane response drop only 40 mins after taking it
• Took an hour to recover and vibrating at the original tone again = outer hair cells stop providing the amplification
• When tuning is needed, amplification is lowered when furosemide is used as OHC are impaired, but they bounce back after.
• Results show a broken stick shape instead of a straight line = response of basilar membrane responsible for perception for loudness as seen by usage of furosemide
• Shutting down OHC does not affect the linear line at high dB (bc no need to be amplified)

Question 5

What do hearing impairment people have? OHC

Answer 5

• Can measure loudness using magnitude estimation
• Data collected from people with various degrees of hearing impairments: impaired hearing = slopes are shifting across = thresholds are elevated by impairment, but when they can hear = have a rapid growth of loudness and then a slower rate of loudness = OHC not working very well & means no compressive response for some loudness and goes up rapidly straight away
• Hearing impaired listeners lack normal compressive growth in loudness, and experience loudness recruitment instead

Question 6

What is frequency sensitivity in OHC? (+notch study)

Answer 6

• How people differentiate one frequency from another detected by notch to notch noise
• High frequency, another frequency, and tone in middle of that gap, can you detect that sound. Larger the gap = easier to detect
• Found ppt who have one deaf ear and compared their normal ear to impaired
• Normal ear, threshold drops steeply as you increase notch width
• Impaired ear, threshold falls more slowly and widening of notch does not help participants as much as in their normal ear
• Data is turned into filter shapes = caused by a degree of frequency sensitivity on the basilar membrane showing max sensitivity in a spot, and where you cannot hear

Question 7

How to detect that someone has no inner hair cells?

Answer 7

• Dead regions = no inner hair cells = cannot hear at that frequency
• Some frequencies cause a movement at a place on the basilar membrane
• No OHC tuned to one place on BM, but it will vibrate, so weaker activation of displaced OHC
• Hearing tones only because vibration carries
• Present tone in dead region, increase sound level of tone where curve gets above threshold in the area where you can hear
• Add artificial noise to increase threshold as IHC are more tuned to noise (noise irl even when looking at absolute threshold)
• Threshold equalising noise: different levels of noise = same threshold of noise so participant can hear in a dead region

Question 8

What is the loss of endocochlear potential

Answer 8

• The stria vascularis degrades with age
• Some individuals with known hearing loss have intact hair cells at post-mortem
• Endocochlear potential is known to fall with age in animal models - hard to measure

Question 9

What does EP look like?

Answer 9

• Hard to distinguish from IHC and OHC loss because loss of EP affects both IHC/OHC function
• Affects whole cochlea, so effects are probably fairly uniform across frequency
• Most age-related hearing loss mainly affects high frequencies
• BUT we do not know how much OHCs amplify at low frequencies

Question 10

What is Cochlear synaptopathy? (Hidden Hearing Loss)

Answer 10

• All data about this comes from 1 lab in Harvard and whole aspect is controversial
• Exposed mice to loud noise on one occasion, and measured their thresholds over a few days. At first they were elevated and then got better - closer to normal after a couple of days = temporary threshold shift
• DPOAE and ABR meant to measure hearing levels objectively (because can’t ask them if they heard it)
• In healthy system, both should be strong = DPOAE is like an echo = Play two tones into ear, several harmonically related tones come back out = ears can make sound due to OHC activity = response makes its way back and be emitted = used to test newborn hearing
• ABR is like an EEG only measured in one position, and play sounds into ear and get an electrical response when auditory system processes the sound
• At post-mortem: mice had lost huge numbers of auditory nerve fibres - synapses on IHC have permanently disappeared

Question 11

Why is synaptopathy called hidden hearing loss?

Answer 11

• Thresholds go back to normal so hidden as we measure thresholds as an indication of hearing loss
• Only takes a few synapses to detect a sound in auditory nerves, and so may be invisible audiologically
• Thresholds might be normal but there might be some suprathreshold tasks that may be affected more
• Idea chimes with people who say they can hear sounds but not clearly

Question 12

What is the evidence from human histology?

Answer 12

• The number of IHC synapses in human cadaver temporal bones declines with age at death
• Psychophysics: if there was a problem with auditory nerve you would start to fail them
  1) Interaural phase difference detection: using different units for tone detection - looking at difference between timing of waveforms = data shows there is not much change a decline reflecting deterioration in fibres among normal hearing population
  2) Amplitude modulation detection: have a steady sound and another going up and down, and trying to differentiate between both = can be affected by loss of auditory fibres = performance with age does not seem to vary
• Not sure if cochlear synaptopathy is a huge deal with hearing loss