Lecture 2.1- Perception of electrical stimuli

Question 1

What creates the AP in the hair cells?

Answer 1

Influx of calcium/potassium into the nerve

Question 2

What happens to hair cells following deafness?

Answer 2

Hair cells that are lacking stimulation shrivel up and die.

Question 3

How is the cochlea tonotopically organized?

Answer 3

High frequencies at base

Low frequencies at apex

Question 4

What is the difference between acoustic stim vs. stim from a CI?

Answer 4

• Go from thousands of hair cels to finite number of channels
• Electrode array only focuses on speech frequencies
• CIs will not transmit certain frequencies that are specific to music

Question 5

What is maxima selection?

Answer 5

o Once the signal is filtered, the device determines which electrodes have the most information
o Default maxima is 8-10
o Maxima selection is the information that is ultimately sent to the electrode array
o That determines which channels fire at threshold level (T-level)

Question 6

What is the charge?

Answer 6

o	The charge of a signal is the accumulation of amplitude over time
o	The area under the biphasic curve
o	Charge (q)= amplitude (i) x pulse width (T)
	To achieve same amount of charge without stimulating the facial nerve is to broaden the pulse width

Question 7

What are action potentials?

Answer 7

o The charge from the CI causes an action potential to initiate on the auditory nerve
o Due to the spread of current, less resolution multiple Aps occur with incoming charge

Question 8

What are the parameters of loudness?

Answer 8

• Amplitude
• Pulse Width (PW)
• Rate

Question 9

Describe loudness growth with amplitude.

Answer 9

o Neural tissue is charge sensitive
o As long as there is sufficient voltage available, current amp can be increased and the percept of loudness growth will result
o If sufficient voltage is not available to meet the current requirement, the channel will be out of compliance and there will be no further loudness growth

o How do we remedy this?
 Increase pulse width

Question 10

Describe loudness growth with PW.

Answer 10

o Shorter pulse durations (narrow PWs) produce higher thresholds and steeper loudness growth curves
 Rate dependent
o Longer pulse durations (wider PWs) produce lower thresholds and larger dynamic ranges, due to slower loudness growth near threshold

Question 11

Describe loudness growth with rate.

Answer 11

o Channel rate x number of maxima = Total stimulation rate (TSR)
o Pitch changes due to rate saturate at 500pps
o Fast rate  fine timing
 Due to recovery period from nerve
o Higher stim rates require narrow PW which consumes significant power
 Reduces battery life
o For short pulses, at low rates, threshold is not affected by PW
o Above 100 pps, threshold falls ~3 dB/octave in terms of how much stimulation is needed
 Most pulse rates are 500-1200 pps
o For longer pulses, thresholds 1st decrease (until ~100pps), then increase with rate increase