Wk3b - Channels: Interactions and Psychophysics

Question 1

T/F: Electrodes have an independent effect on neurons

Answer 1

False - there is overlap

Question 2

____ _____ results in significant Inferior Colliculus activity when 2 monopolar electrodes are stimulated at sub-threshold levels simultaneously

Answer 2

Channel Interaction

Question 3

T/F: Having 2 electrodes active at the same time means the response is the sum of response 1 and response 2 (linear assumption)

Answer 3

False - because of this, it is difficult to generate the desired Spiral Ganglion stimulation with simultaneous stimulation

Question 4

In present CI systems, electrodes are stimulated _____ (simultaneously/in trains of current pulses)

Answer 4

In trains of current pulses (meaning they are interleaved in time/non-simultaneous) which reduces channel interactions

Question 5

What pulse parameters in CIs influence loudness perception?

Answer 5

Current Amplitude
Pulse Period
Positive Duration

Question 6

What is the typical phase duration of a biphasic pulse train?

Answer 6

20-60 microseconds

Question 7

What is a typical pulse rate?

Answer 7

1/T, typically 250-1000 Hz (on each channel)

Question 8

What 2 terms did Ioan not want us to mix up?

Answer 8

Bipolar and biphasic

Question 9

Why are biphasic pulses a critical safety feature?

Answer 9

Monophasic would result in a buildup of charge on the electrodes

• it is important to “sink” as much charge as was “sourced” to avoid corrosion and production of toxins through electrochemical reactions
• each pulse should be charge-balanced to reverse any local reactions

Question 10

Why do we use biphasic pulses?

Answer 10

• to charge-balance pulses to reverse local reactions

- to increase chances of stimulating nearby SG neurons

Question 11

Detection is equivalent to _____

Answer 11

Threshold

Question 12

Discrimination is usually tested at _______ levels

Answer 12

Suprathreshold

Question 13

Scaling has to do with categorization. What information does the patient use to categorize information?

Answer 13

Previous experience

Question 14

What is the ACOUSTIC threshold limited by?

Answer 14

The minimal mechanical vibration that can be picked up by the hair cells and nerve via an active mechanical mechanism (or amplifier)

Question 15

What 4 things does the electric threshold reflect? (I.e what 4 electrical and anatomical aspects will influence threshold?) (NOT current)

Answer 15

• the type of electrode used
• the electrode-tissue interface
• the distance b/w the electrode and the nerve
• the degree and pattern of the nerve survival

Question 16

What is the electric threshold?

Answer 16

The minimal amount of current that can stimulate the AN

Question 17

What two things were shown in the 70’s to influence electric threshold?

Answer 17

Phase duration and pulse rate

- the lowest thresholds were obtained with the longest phase duration and highest pulse rate

Question 18

How does polarity affect electric threshold?

Answer 18

Monopolar has significantly lower thresholds compared to bipolar and especially tripolar
- also shown to be higher threshold variability b/w channels of bi- and tri-polar

Question 19

Since CI electrodes do not stimulate independently, how does this change perception (compared to acoustic hearing)?

Answer 19

The electric fields add up - stimulating 2 separate electrodes simultaneously below threshold may cause a perception, whereas this would not be the case with acoustic hearing

Question 20

When comparing thresholds for simultaneous stimulation, a larger negative shift (lower threshold) indicates a ____ (lower/higher) channel interaction

Answer 20

Higher

Question 21

Which array had the highest channel interaction: mono, bi, or tri-polar?

Answer 21

Monopolar

Question 22

Which had a greater threshold shift: simultaneous interaction or temporally adjacent presentation?

Answer 22

Simultaneous

Question 23

When a time gap was introduced (symmetrically interleaved), there was a(n) _____ (increased/decreased) interaction compared to simultaneous presentation

Answer 23

Decreased

Question 24

What are two ways to reduce channel interaction?

Answer 24

Choose bipolar or tripolar
Temporal Offset (signal)

Question 25

The dynamic range for electric hearing is defined as what?

Answer 25

The difference b/w the threshold current (T) and current producing max comfortable loudness (C)

Question 26

What does the electric dynamic range depend on?

Answer 26

• electrode configuration
• anatomy
• simulation parameters

Question 27

Which type has a smaller dynamic range: monopolar or bipolar?

Answer 27

Monopolar

Question 28

Which type requires more current to reach threshold levels: monopolar or bipolar?

Answer 28

Bipolar

Question 29

As pulse rates increase, thresholds _______ (decrease/increase) and dynamic range ______ (decreases/increases)

Answer 29

Decrease; Increases

Question 30

How does electric dynamic range compare to acoustic dynamic range (according to the pulse train study)?

Answer 30

Electric: 10-30 dB (3x - 30x current)
Acoustic: 100-120 dB

Question 31

Non-impaired acoustic hearing loudness doubles every ___ dB over a wide range

Answer 31

10 dB - almost linear

Question 32

How does electric loudness growth increase compared to acoustic hearing?

Answer 32

Very rapid increase (small dynamic range)

- even more rapidly than hearing-impaired

Question 33

Does the steep loudness growth mean CI patients have better intensity discrimination than NH listeners?

Answer 33

No - JNDs are similar (~0.5-2 dB)

Question 34

How many steps of intensity increase can CI users generally hear? NH listeners?

Answer 34

7-30 steps (CI)

50-150 steps (NH)

Question 35

What did the JND study conclude regarding those with limited number of steps?

Answer 35

Those with limited # of steps showed lower abs. thresholds, wider Dynamic Ranges, and poorer electrode speech ranking (associated with more gradual neural rate intensity functions and sparse dendritic survival)

Question 36

What did the JND study conclude regarding those with higher # of steps?

Answer 36

Those with higher # of steps showed high abs thresholds, small dynamic ranges, and excellent electrode speech ranking - associated c/ steep neuro rate intensity functions and dense axonal survival