Cochlear Physiology l

Question 1

What are the two classical theories of frequency analysis in the cochlea?

Answer 1

Traveling Wave
HC Physiology

Question 2

What are some traveling wave details? (4)

Answer 2

Bekesy’s early finding and current understanding of how sound travels in the cochlea
Modern tech and active component
Effect of sound level on tuning and peak shifting
Behavior test for level-related peak shifting

Question 3

What are determining factors for BM? (3)

Answer 3

GRADIENTS: Stiffness determined by BM widths, thickness Mass determined by the size of organ of Corti

EFFECT: phase/time delay of different value

RESULTS: : BM vibration as traveling wave from base to apex

Question 4

What is the major determining factor of BM vibration?

Answer 4

MASS

Question 5

What device did Bekesy use to measure the traveling waves from the BM displacement?

Answer 5

Stroboscopic device

Question 6

What are the characteristics of traveling waves? (4)

Answer 6

Direction: base to apex and broader at low frequencies
Peak location: F
Asymmetric Envelope and speeds up toward peak then fades away
Traveling free and speed does not sound frequency and sound speed

Question 7

What makes the vibration travel along the BM?

Answer 7

Mass gradient OC gets denser and less stiff which cause phase delay varied along the cochlear duct

Question 8

What did Bekesy missed in his study?

Answer 8

Bekesy missed the active component because he studied on a dead cochlea

Question 9

What do we know about the active vs. the passive components in the cochlea? (3)

Answer 9

1. Dominant at low sound levels. Passive one is dominated at high levels
2. Overall amplitude of active component is smaller than the passive one at higher sound levels.
3. Active measures require healthy cochlea 2 and 3 make the active component more difficult to record

Question 10

What are the requirements to measure the active component of the cochlea? (3)

Answer 10

Living cochlea, healthy OHCs
Low Sound Level
Highly Sensitive Equipment

Question 11

What can we see in this image related to BM response in dead and living cochlea?

Answer 11

Black lines: responses of dead cochlea
Red lines: responses of living (healthy cochlea)

At low sound level, cochlea with healthy OHCs shows sharper tuning, larger amplitude in its BM vibration than the dead/passive cochlea

At high sound levels, the BM vibration is similar between healthy/active cochlea and dead/passive one

Question 12

What are the Methods of BM vibration observation? (3)

Answer 12

Capacity probes
Mossbauer technique
Laser Doppler interferometry

Question 13

What is the capacity probe method of BM vibration observation?

Answer 13

Capacitor: two conductive plates separated by a non-conductive layer
The probe serves as one plate, BM the other one
Dry air in between is the insolation.
BM vibration change the distance between the two plate and therefore the capacitance.
Require water to be drained, bad for cochlea
Sensitivity is poor

Question 14

What is the Mossbauer method of BM vibration observation? (3)

Answer 14

• Doppler effect
• Radioactive material applied to BM to produce signal of known frequency
• Receive test vibration of BM by measuring the frequency change

Question 15

What is the Laser Doppler Interferometry method of BM vibration observation?

Answer 15

Doppler shifting from light reflection (from applied glass balls, or from cellular fat)

Question 16

What is the Doppler Effect?

Answer 16

Increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other.

Question 17

What are the limitations of using the light beam for BM vibration measurement?

Answer 17

Is mostly used at basal and apical ends

Question 18

Why do we use point test to test BM vibration instead?

Answer 18

Point test requires smaller surgical opening, therefore smaller damage
More practical
Overall pattern of vibration can be elicited by test at multiple points, from multiple subjects.
In point test, signal frequency is changed, and the probe location is fixed
In pattern test, probe location is changed, not signal

Question 19

Which test should we use to compare the displacement vs speed along the BM? Why? (3)

Answer 19

Velocity Test

Vibration at high frequency cannot have large displacement; therefore, the amplitude is small.

Velocity test is fair to compare across different frequency regions.

Question 20

Which test should we use to compare the displacement vs speed along the BM? Why?

Answer 20

Velocity Test

Vibration at high frequency cannot have large displacement; therefore, the amplitude is small.

Velocity test is fair to compare across different frequency regions.

Question 21

What is a point to remember on BM frequency selectivity in point test?

Answer 21

The test is done at a fixed location, so the results are vibration that can be seen at this point. Stimulation varies in intensity and frequency.

Question 22

What is a point to remember on BM frequency selectivity in point test?

Answer 22

The test is done at a fixed location (point test meaning at one point), so the results are vibration that can be seen at this point. Stimulation varies in intensity and frequency.

Question 23

What can we see from Basilar Frequency in Frequency selectivity of receptor potential?

Answer 23

BF changes seen in receptor potential
BF shifts to low fre at higher intensity

Question 24

What can we conclude from this point test? (3)

Answer 24

Left: Vibration change with sound level (test at a fixed point)

Right: Gain changes with sound level

Gain reduces with sound level, although vibration amplitude (here is measured as speed) increases with intensity. “Best frequency” shifted downward to low frequencies in both graphs.

Question 25

What is the reference of 1 Pa is 94 dB SPL?

Answer 25

The reference is the speed of vibration (mm/s) produced by 94 dB SPL. Assuming that this number is o, and the vibration speed by a sound of p dB SPL is q (mm/s), the gain is q/o in dB + 94-p.

Question 26

With high SPL, what happens to the tunning?

Answer 26

Tuning becomes wider w/ high SPL

Question 27

With high SPL (at fixed point), what happens to the BF?

Answer 27

Shifts to low frequency

Question 28

What happens to the spread of vibrations of low-frequency sounds on the Basilar membrane at a high sound level?

Answer 28

A low-frequency signal produces a vibration that is spread to the higher-frequency regions at the high sound level

Question 29

Why do we use a single point (frequency) when measuring the vibration across the cochlea?

Answer 29

The overall vibration across cochlea can’t be directly measured

Question 30

Why happens to the cochlea if we open the shell too much?

Answer 30

Cochlear integrity will be lost if we open the shell too much

Question 31

What occurs in the point test to the BF with sound level increase?

Answer 31

In point test, BF decreases with sound level

Question 32

To which frequency does the peak shift with increasing sound level?

Answer 32

In overall pattern: vibration peak shifts towards basal end (higher frequency)

Question 33

What occurs in this graph?

Answer 33

Peak shifting to HF in overall pattern = BF to low Frequency in point test.

The location is labeled by the peak vibration of CF tones at low SPL.
The vibration peaks to higher frequency at higher sound level.
In the test at a fixed point, when the SPL is higher, the tone of 1.5 kHz produces the largest vibration at 2 kHz location.

Question 34

What occurs in this graph?

Answer 34

BM displacement response to a 15 kHz tone measured at 15 positions along the BM from 12.5-27 kHz, over a range of intensities from 15-100 dB SPL. A second peak appears at around 17 kHz at higher intensity.

Question 35

What is acoustic trauma?

Answer 35

Acoustic trauma is a common cause of sensory hearing loss. Damage to the hearing mechanisms within the inner ear may be caused by: Explosion near the ear. Firing a gun near the ear. Long-term exposure to loud noises (such as loud music or machinery)

Question 36

What evidence do we have from acoustic trauma? (4)

Answer 36

Applying 4k tone at high intensity
TTS and PTS are maximal at 6 kHz
Hair Cell loss
Half-octave Law

Question 37

Why does acoustic trauma affect primarily high frequencies?

Answer 37

Since high frequencies are processed by Hair Cells at the base, they are the first to die before those towards the apex

Question 38

How does dead/living cochlea respond to low/high stimuli?

Answer 38

Low sound level: Cochlea with healthy OHCs shows sharper tuning and larger amplitude in its BM vibration.

High sound level: BM vibration is similar between healthy/active cochlea and dead/passive ones.

Question 39

What occurs when a probe send low level stimuli to the BM?

Answer 39

Probes at low levels cause BM vibration with sharp peak correctly located at limited frequency region

When interval = 0, probes are heard at a high level, and produce its own shifting (red line), so the masked threshold peaks at 6 k

Question 40

What does the time interval between the tone masker and the sound from a probe cause on BM?

Answer 40

The time interval allows a reduction of vibrations along the BM and therefore the use of a low-level probe

Question 41

What does this graph show?

Answer 41

The curves with long delays show peaks at higher frequency regions.