Wk4a - Loudness, pitch, & modulation

Question 1

In CIs, what 2 things influence loudness perception?

Answer 1

Current level

Pulse Duration

Question 2

1 Coulomb of charge = __ amperes/sec of current

Answer 2

1 coulomb = 1 ampere/sec

Question 3

Explain the equation:

Q = I x T

Answer 3

Q = charge
I = intensity
T = time (pulse duration)
so, as we increase current or pulse duration, we increase charge
- more charge = more neurons => increase loudness

Question 4

How is intensity encoded with acoustic hearing?

Answer 4

• Increased discharge rate

- Increased spread of activation along BM with higher intensity levels

Question 5

Which of the 2 methods for encoding intensity acoustically can be used in CIs and why?

Answer 5

Firing rate - spread of activation along the BM is not possible b/c the BM is not involved

Question 6

Where (anatomically) can we see the spread of activation in electric hearing?

Answer 6

The inferior colliculus - like acoustic hearing, the firing rate and number of neurons activated encodes intensity (more neurons with a larger current) BUT over a much smaller range

Question 7

How can we increase loudness in CIs?

Answer 7

-Increasing current intensity (amplitude of each pulse or pulse rate)
Increasing duration of stimulation by increasing the phase duration of each pulse

Question 8

How do we encode pitch with acoustic hearing?

Answer 8

• Place of excitation (place coding)
• Temporal analysis (stimulus rate)
• *remember volley theory/phase locking up to 4 kHz

Question 9

Give the equation for determining frequency based on timing (i.e. Interspike Interval (ISI) Histogram)

Answer 9

f = 1 / T

Question 10

What is the theoretical limit of a neurones ability to fire every cycle (due to the refractory period)?

Answer 10

1 kHz

Question 11

What is temporal jitter?

Answer 11

The circumstance where some spikes occur off-peak (e.g. in an ISI Histogram, most neurons will fire at the same time, but others will be slightly off, causing the peak to have a slope on either side)

Question 12

In an ISI Histogram, we can expect to see ______ (higher/lower) spike numbers with increased stimulus duration

Answer 12

Lower - the onset of a stimulus is marked by the highest amount of neuronal firing, followed by lower and lower peaks

Question 13

Using a cat, it was shown that there is more temporal jitter with _____ (acoustic/electric) hearing than with _______ (acoustic/electric).

Answer 13

More temporal jitter with acoustic hearing than electric (i.e. steeper spikes) so electric has better phase-locking

Question 14

Since CI’s have better phase-locking than acoustic hearing, does this mean they have better temporal frequency perception?

Answer 14

An experiment showed CI users have poorer JNDs with increased frequency with a max JND at 300 Hz, so CI users cannot discriminate above 300 Hz.

Question 15

How can we simulate the single-place pulsatile stimulation for normal hearing listeners? What would we use this for?

Answer 15

Transposed tones - we use a filtered tone and a high frequency carrier
- we do this to test modulation rate stimulation

Question 16

Modulation rate discrimination in NH listeners is ____ (more/less/similarly) limited compared to pulse rate stimulation in CI users

Answer 16

Similarly - in the range of 250-400 Hz (pulses per second)

- we currently use amplitude modulation to help with pitch discrimination in CIs

Question 17

What are 2 methods that have been used to examine Place Pitch perception in CI users?

Answer 17

• Pitch ranking - judging the higher of two pitches associated with stimulation of 2 electrodes and converting it to a measure of discriminability
• Pitch scaling - subjects assigned a number b/w 1 and 100 to describe a stimulus delivered through one of the electrodes.

Question 18

In general, pitch perception is better at the ____ (basal/apical) portion of the electrode

Answer 18

Basal (high pitches)

Question 19

Characteristic frequencies on the BM are ______ (linearly/logarithmically) spaced

Answer 19

Logarithmic

Question 20

What sort of patient is needed to participate in pitch-matching experiments?

Answer 20

Ideally they would have a CI in one ear and normal hearing in the other

Question 21

What is the “Greenwood Function”?

Answer 21

A function that relates frequency to place on the BM based on average electrode position across subjects

Question 22

Why don’t the pitch-matching results go down to the very low frequencies with CI users?

Answer 22

It’s very hard to reach the apex (low frequency region) of the cochlea with the electrode array

Question 23

How does modulation perception help with pitch perception in acoustic hearing?

Answer 23

It increases temporal resolution

Question 24

In acoustic hearing, modulation perception acts as a ____-pass filter

Answer 24

Low

- when modulation frequencies increase, the modulation is increasingly difficult to detect (cut-off ~70 Hz)

Question 25

A modulation index of 1 means that the lowest and highest pulse amplitudes will be __ and __ respectively

Answer 25

0 and 1

Question 26

A modulation index of 0.25 means that the lowest and highest pulse amplitudes will be __ and __ respectively

Answer 26

0.75 and 1

Question 27

Assuming the same pulse rate, which of the following would have a higher modulation rate:

a) a signal with 10 pulses/cycle
b) a signal with 5 pulses/cycle

Answer 27

b) 5 pulses/cycle (the modulation rate would be twice as fast)

Question 28

What happens to pitch perception if the pulse rate is too low and the modulation rate is too high?

Answer 28

• the modulation index will not be represented in the modulation amplitude
• the pulse rate is not fast enough to follow the modulation
  (we need enough pulses in each modulation cycle that at least one of them is close to the peak, or at least 4X THE MODULATION RATE)

Question 29

Maximum modulation rate is 1/___ of the pulse rate

Answer 29

1/4

1/6 would be better

Question 30

f(mod) x 4 (or 6) = ____

Answer 30

minimum pulses per sec (pps)

Question 31

Will a signal with an amplitude modulation of 50 Hz be well represented using 200 pps?

Answer 31

Yes:
f(mod) x 4 = min pps
50 x 4 = 200 pps

Question 32

Will a signal with an amplitude modulation of 50 Hz be well represented using 100 pps?

Answer 32

No:
f(mod) x 4 = min pps
50 x 4 = 200 pps, therefore 100 pps is too little

Question 33

How are amplitude modulated signals transformed by the auditory system?

Answer 33

More depth of modulation is needed to perceive a JND as modulation rate (Hz) increases
- therefore, the amplitude modulated signals are gradually low-pass filtered with a cut-off near 50 Hz

Question 34

T/F: A study found that CI users have better modulation depth perception at lower and moderate frequencies compared to normal hearing

Answer 34

True: CI users can often detect smaller amounts of modulation than NH listeners when considering the electrical stimulus
- cutoff frequencies for CI users in their temporal modulation transfer functions are later but steeper ~100-200 Hz

Question 35

According to the study by Shannon et al, what is the ideal pulse rate for CIs?

Answer 35

150 Hz x 6 = 900 pps or higher

Question 36

Electric thresholds are very _____ (similar/different) compared to acoustic thresholds

Answer 36

Different - depend on many aspects

Question 37

The dynamic range for electric hearing is ______ (bigger/smaller) than for acoustic hearing

Answer 37

Smaller - keep in mind for signal processing: compression

Question 38

What 2 things limit pitch placement in CIs?

Answer 38

Electrode spacing

Spread of excitation

Question 39

CI users have a steep roll-off near ___ Hz for modulation detection

Answer 39

150