plasticity in auditory system/ cochlear implant

Question 1

Distinguish the causes of hearing loss

Answer 1

conductive hearing loss:
due to physical obstruction that keeps sound from reaching the inner ear, affecting all frequencies equally

sensorineural hearing loss:
due to damage to auditory nerve afferents or (outer) hair cells

Question 2

describe and name the different parts of the cochlear implants including the external and internal components

Answer 2

external component:

-microphone
(captures sound from the environment.)
-speech processor
(converts these sound signals into an electrical signal; these electrical signals are then transmitted to an internal component, which is surgically placed inside the ear. The speech processor also contains a band-pass filter that allows a specific range of frequencies to pass through while blocking or attenuating frequencies outside this range. This then allows it to pass through to the corresponding electrode along the cochlea to stimulate the correct AN afferents for decoding in the higher centres in the brain.

internal component;

-receiver-stimulator
(its fitted under the scalp and receives the converted electrical signals from the speech processor and now converts them into electrical impulses.)

-cochlear electrode array
(contains tiny electrodes along the length of the cochlea and is in direct contact with AN afferents; they receive the electrical impulses from the receiver-stimulator.

Question 3

what is frequency channels also referred to

Answer 3

electrodes

Question 4

what is a cochlear implant and electrode array?

Answer 4

a cochlear implant is a small device designed for individuals with severe hearing loss or deafness

electrode array is positioned close to the spiral ganglia, which allows for different AN afferents to get stimulated by different electrodes, which are sending information from different frequency-sensitive locations along the basement membrane of the cochlea

Question 5

Discuss coding strategies for cochlear implants to the place coding and temporal coding in the auditory nerve

Answer 5

band pass filter for place coding

For temporal coding, the amplitude envelope determines the strength and intensity of impulses sent to each electrode. There is a continuous train of stimulating impulses sent to each electrode, corresponding to the temporal coding.

Question 6

how to prevent overlapping signals of electrodes

Answer 6

to get them well insulated from each other, allowing stimulation to be selective and allowing for discriminatory hearing

Question 7

what are the limitations of cochlear implants?

Answer 7

• The electrode array does not reach the most apical turn of the cochlea; it is unable to stimulate AN afferents tuned to lowest frequencies
• Although the technology is available to make very small electrodes, in practice the number is limited because of short-circuiting of electrodes by the perilymph and cross talk
• Effective number of frequency channels may be limited to 8-9 reducing selectivity may overlap with a broader range of frequencies due to limited number of electrodes

Question 8

why is pitch perception poor with a cochlear implant?

Answer 8

There is no temporal structure or periodicity due to the continuous train of stimulating pulses sent to the electrode, which isn’t natural; therefore, you cannot add any other timing information; otherwise, it will get to complicated to code, so it either responds to periodicity and doesn’t get stimulated enough or you have a constant train of impulses that increase the likelihood of AN afferent stimulation good enough and no periodicity.

and it can’t resolve full stack of harmonics as majority of these implants can’t resolve AN afferents tuned to low frequencies at the apical end of the cochlea, so reduces pitch perception