1B_Adaptation and Loudness Encoding

Question 1

What is auditory adaptation (using tone decay) used to assess?

Answer 1

Neural dysfunction at or above the auditory nerve

Question 2

What are two clinical decay tests?

Answer 2

Acoustic reflex decay (physiological test)
Tone decay (behavioural test)

Question 3

What happens to the firing rate of a Type I Auditory Nerve Fibre during abnormal adaptation?

Answer 3

There is an initial increase in neural firing at the onset of the stimulus, followed by rapid adaptation (very few neural spikes).

Question 4

Describe how an Acoustic Reflex Decay test works

Answer 4

• PT stimulus of 500 or 1000 Hz is presented for 10 seconds at 10 dB above ART
• 50% decay = abnormal (+) and indicates a retrocochlear pathology

Question 5

Describe the Tone Decay Test

Answer 5

• present tone for 1 minute at 5 dB SL
• if tone become inaudible, increase by 5 dB
• subtract beginning intensity level from final level
• 0 - 5 dB = normal
• 10 - 15 dB = mild
• 20 - 25 dB = moderate
• 30 dB or more = marked

Question 6

What is recruitment?

Answer 6

A more rapid growth of loudness exhibited by individuals with cochlear dysfunction

• reduced dynamic range
• LDL may be lower than normal
• associated with loss of OHC cochlear amplifier

Question 7

What kind of things might cause cochlear damage resulting in recruitment?

Answer 7

Damage due to noise, ototoxic substances, aging

Question 8

Name 2 loudness perception clinical tests

Answer 8

MCL and UCL - speech or non-speech stimuli
ABLB and AMLB - PT stimuli loudness balance tests
SISI - detection of small intensity increments

Question 9

What is dynamic range?

Answer 9

The difference between threshold and uncomfortable loudness level

Question 10

What kind of stimuli can be used to test dynamic range?

Answer 10

Words, phrases, ascending PT

- rated using scaling methods

Question 11

Describe ABLB

Answer 11

• Alternate Binaural Loudness Balance
• compares loudness growth in impaired ear to normal reference ear
• only works for unilateral SNHL
• patient reports when tones are of equal loudness

Question 12

What is decruitment?

Answer 12

When individuals with neural dysfunction exhibit reduced growth of loudness with increasing sound intensity

• occurs d/t impairment of neural loudness growth encoding and increased neural adaptation
• may indicate retro-cochlear pathology (e.g. acoustic tumour)

Question 13

Give a brief overview of how Type 1 auditory neurons respond to sound intensity

Answer 13

• 10 - 30 auditory neurons attach to the same IHC
• all have same CF but different thresholds and different intensity growth curves
• each has different dynamic range and spontaneous rate
• discharge rate increases as stimulus level increases
• small dynamic ranges of each neuron overlap to create our larger perceptual dynamic range

Question 14

True or False: at higher intensities, a single neuron will respond to a broader range of frequencies

Answer 14

True - greater spread of vibration along the basilar membrane causes auditory neurons throughout more of the cochlea being activated

Question 15

How is loudness encoded?

Answer 15

By single auditory neurons:

• increased firing rate as level increased
• different thresholds and dynamic ranges
• different spontaneous firing rates
• respond differently to increase in sound level
• improved phase locking to the signal as intensity increases

By cochlear neural population:

• wider regions of cochlea respond at higher intensities (recruits adjacent neurons)
• summation of neural activity across frequency channels
• more neurons phase lock to a signal as intensity increases