Cochlear Physiology V: Efferent part 2 Flashcards
What do we know about the cochlear gain by compression (efferent)? (4)
How do we test cochlear compression (efference) in humans?
- Using forward masking
- Masker (rather than signal) level as the dependent variable
- Compare off-fre and on-fre masking at any level (On-frequency = the CF)
How is gain expressed when we measure cochlear compression (efference) in humans using forward masking?
- Gain = the difference between on and off the masking
- Compression is shown as the reduced gain at a higher sound level
This graph shows the result of forward masking in humans: the gain is the difference between on-frequency and off-frequency curves, what else can we see from this graph?
The left graph shows the signal at 6 kHz we can use a masker on signal 6khz or off 3khz, the masking effect will be produced by the active component and will not be seen at the 6khz location Masking threshold increases with the signal level
When you use off masker you won’t see the masking effect
The level between two measurement show gain, the difference becomes smaller = the gain becomes smaller
Right: No difference between on and off masker, No active gain in hearing impaired
What does this graph show?
The feature of OHC amplification is frequency and level selective.
The CF tone increases when Off CF. If you use a lower frequency compared to the CF, only the passive component that is part of the CF (the tail of low frequency in solid lines) will show the same increase
What does this graph show?
Unmasking effect of cochlear efferent, seen in all SR groups, more for low SR units
C = if we present background noise without stimulation,
Background noise can produce total suppression
D = If we apply efferent stimulation, Line shift, and decrease of response to background noise, suppression upon plateau by background noise is reduced (Unmasking effect)
What are the LOC functions?
- The excitability of the afferent ANFs is related to the protection of afferent IHC-SGN synapses
84 dB SPL noise was applied over 168 hours, showing no synapse loss in control mice where the efferent system is intact but 40% loss in mice with deefferentation (destroyed cochlear efferent control) surgery larger synaptical loss
- Most important effect of Cochlear efferent occurs through MOC not LOC
What are two other effects of Efferent control?
- Efferent control at higher intensity: provides 20-30 dB attenuation, especially for low SR fibers, protects afferent synapses (IHC-SGN), fine rate decrease
- Suppression of cochlear responses via Acoustic Reflex to the cochlea (not ME)
What are the effects of the efferent acoustic reflex? (5)
- natural activation of cochlear efferents
- Activation of cochlear efferents by electrical stimulation is not natural, the result is much stronger than what can be reached in real hearing
- Efferent AR can be evaluated by natural activation of the circuit: acoustic stimulation
- Suppression of OAE on the CAP
- Masking release in OAE, CAP, and speech
How to can we observe the frequency selective depression?
- A DPOAE is evoked by two tones of f1 and f2 at the teste ear.
- Contralateral stimulation depresses DPOAE.
- The frequency selectivity is examined by changing the frequency of the contralateral signal. The largest depression occurs at the region of f1 and f2 (the two tones to produce DPOAE in the tested ear.
What is the first way to evaluate Efferentiaton functions? (4)
- There is a contralateral suppression of OAE
- Contralateral suppression signals: can be noise or tones at below moderate level (to avoid saturation and AR through the middle ear)
- Both TEOAE and DPOAE can be used, but TEOAE is better for low frequency and DPOAE for high frequency
- Suppression variable— generally the effect is small - up to 3 dB
What is the second way to evaluate Efferention functions? (5)
- Suppression on CAP
- Record CAP in ECochG
- CAP amplitude reduced by contralateral signal
- Lager reduction (up to 10 dB)
- Frequency selective
What effect do see from the ipsilateral MOC and DPOAE?
Ipsilateral MOC suppression on DPOAE and the Suppression indicated by adaption
What effect do we see from the contralateral signal MOC on DPOAE?
- DPOAE suppression by contralateral signal (CS), the adaptation occurs after onset, the adaptation after onset.
- Long latency for suppression to reach plateau (2-3 min); Larger effect requires longer stimulation
- Overall magnitude of suppression <3 dB
What effects do we see from the Contralateral signal on CAP?
Larger suppression at the low sound level
Larger suppression than that in DPOAE: 10 dB in CAP versus 3 dB in DPOAE
A = suppression is larger on CAP
A – C = Low suppression occurs at lower sound level
What results do we see from the Masking release in CAP?
- A: Tone pips only
- B: tone pips + ipsilateral noise (Noise reduce the CAP)
- C. B + contralateral noise
Explain the Fluctuation profiles in ANF response to speech:
Formants interaction makes speech temporal fluctuation
ANFs response should also fluctuate in time
However, the fluctuation is less at formant peaks because of saturation
Therefore, the fluctuation profile mirrors the speech spectrum
This profile may activate efferent differentially across channels
What is the role of efferent control in signal processing?
- Anti-masking effect by MOC
- Reset the ANFs to work in different level range
- Gain control and enhance the fluctuation profile
- Attention selection
