Wk4a - Loudness, pitch, & modulation Flashcards
In CIs, what 2 things influence loudness perception?
Current level
Pulse Duration
1 Coulomb of charge = __ amperes/sec of current
1 coulomb = 1 ampere/sec
Explain the equation:
Q = I x T
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
How is intensity encoded with acoustic hearing?
- Increased discharge rate
- Increased spread of activation along BM with higher intensity levels
Which of the 2 methods for encoding intensity acoustically can be used in CIs and why?
Firing rate - spread of activation along the BM is not possible b/c the BM is not involved
Where (anatomically) can we see the spread of activation in electric hearing?
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
How can we increase loudness in CIs?
-Increasing current intensity (amplitude of each pulse or pulse rate)
Increasing duration of stimulation by increasing the phase duration of each pulse
How do we encode pitch with acoustic hearing?
- Place of excitation (place coding)
- Temporal analysis (stimulus rate)
- *remember volley theory/phase locking up to 4 kHz
Give the equation for determining frequency based on timing (i.e. Interspike Interval (ISI) Histogram)
f = 1 / T
What is the theoretical limit of a neurones ability to fire every cycle (due to the refractory period)?
1 kHz
What is temporal jitter?
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)
In an ISI Histogram, we can expect to see ______ (higher/lower) spike numbers with increased stimulus duration
Lower - the onset of a stimulus is marked by the highest amount of neuronal firing, followed by lower and lower peaks
Using a cat, it was shown that there is more temporal jitter with _____ (acoustic/electric) hearing than with _______ (acoustic/electric).
More temporal jitter with acoustic hearing than electric (i.e. steeper spikes) so electric has better phase-locking
Since CI’s have better phase-locking than acoustic hearing, does this mean they have better temporal frequency perception?
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.
How can we simulate the single-place pulsatile stimulation for normal hearing listeners? What would we use this for?
Transposed tones - we use a filtered tone and a high frequency carrier
- we do this to test modulation rate stimulation
Modulation rate discrimination in NH listeners is ____ (more/less/similarly) limited compared to pulse rate stimulation in CI users
Similarly - in the range of 250-400 Hz (pulses per second)
- we currently use amplitude modulation to help with pitch discrimination in CIs
What are 2 methods that have been used to examine Place Pitch perception in CI users?
- 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.
In general, pitch perception is better at the ____ (basal/apical) portion of the electrode
Basal (high pitches)
Characteristic frequencies on the BM are ______ (linearly/logarithmically) spaced
Logarithmic
What sort of patient is needed to participate in pitch-matching experiments?
Ideally they would have a CI in one ear and normal hearing in the other
What is the “Greenwood Function”?
A function that relates frequency to place on the BM based on average electrode position across subjects
Why don’t the pitch-matching results go down to the very low frequencies with CI users?
It’s very hard to reach the apex (low frequency region) of the cochlea with the electrode array
How does modulation perception help with pitch perception in acoustic hearing?
It increases temporal resolution
In acoustic hearing, modulation perception acts as a ____-pass filter
Low
- when modulation frequencies increase, the modulation is increasingly difficult to detect (cut-off ~70 Hz)
A modulation index of 1 means that the lowest and highest pulse amplitudes will be __ and __ respectively
0 and 1
A modulation index of 0.25 means that the lowest and highest pulse amplitudes will be __ and __ respectively
0.75 and 1
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
b) 5 pulses/cycle (the modulation rate would be twice as fast)
What happens to pitch perception if the pulse rate is too low and the modulation rate is too high?
- 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)
Maximum modulation rate is 1/___ of the pulse rate
1/4
1/6 would be better
f(mod) x 4 (or 6) = ____
minimum pulses per sec (pps)
Will a signal with an amplitude modulation of 50 Hz be well represented using 200 pps?
Yes:
f(mod) x 4 = min pps
50 x 4 = 200 pps
Will a signal with an amplitude modulation of 50 Hz be well represented using 100 pps?
No:
f(mod) x 4 = min pps
50 x 4 = 200 pps, therefore 100 pps is too little
How are amplitude modulated signals transformed by the auditory system?
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
T/F: A study found that CI users have better modulation depth perception at lower and moderate frequencies compared to normal hearing
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
According to the study by Shannon et al, what is the ideal pulse rate for CIs?
150 Hz x 6 = 900 pps or higher
Electric thresholds are very _____ (similar/different) compared to acoustic thresholds
Different - depend on many aspects
The dynamic range for electric hearing is ______ (bigger/smaller) than for acoustic hearing
Smaller - keep in mind for signal processing: compression
What 2 things limit pitch placement in CIs?
Electrode spacing
Spread of excitation
CI users have a steep roll-off near ___ Hz for modulation detection
150
