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Investigating benefits of current focusing on complex pitch perception in cochlear implants

Fielden, Claire Alexandra

[Thesis]. Manchester, UK: The University of Manchester; 2014.

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Abstract

Cochlear implants are a recognised treatment for severe and profound hearing losses, and can greatly improve speech discrimination in a quiet listening environment. However, poor specificity of neural excitation resulting from wide current spread within the cochlea leads to inadequate complex pitch perception, affecting speech discrimination in noisy environments and music perception. Tripolar is a stimulation mode with a greater degree of current focusing than the monopolar mode currently used in the clinic. The overall aim of the experiments in this thesis was to evaluate the potential of tripolar stimulation mode to improve complex pitch perception compared to monopolar stimulation mode in a group of Advanced Bionics cochlear implant listeners.First, the place specificity of tripolar and monopolar stimulation was compared using a psychophysical forward masking task with a dual-electrode masking stimulus to limit off-site listening. An overall improvement in the place specificity of tripolar compared to monopolar stimulation indicated that current focusing may provide more independent transmission of temporal information from different electrode places. This conclusion was unaffected by the degree of residual masking which, although measurable, was similar in both modes. Second, the effect of current focusing on delivery of independent temporal patterns was evaluated, specifically whether transmission of fine temporal information about interpulse intervals was improved using tripolar over monopolar stimulation. No advantage was found for current focusing in the delivery of temporal patterns on either a single electrode site, or across-electrodes. Third, the effect of mode on complex pitch perception was investigated by comparing a tripolar with a monopolar listening program. Results showed no improvement in pitch ranking ability from current focusing using sung vowel stimuli, and further suggested that a cue related to the centroid of excitation was providing a stronger cue to pitch than the temporal modulations available in both modes in this group. In conclusion, tripolar stimulation, on average, improved place specificity using forward masking, but not temporal cues to pitch using the methods chosen to evaluate this. Furthermore, current focusing did not improve the ability to rank sung vowels based on pitch over monopolar mode. Current focusing, therefore, was not found to enhance perception of complex harmonic pitch in this group. However, a degree of inter-participant variability in amount of benefit from current focusing observed in this series of experiments suggests that it may be useful for some cochlear implant listeners if a predictive measure of benefit were developed.

Layman's Abstract

Cochlear implants are a recognised treatment for severe and profound hearing losses. They work by using current generated by an electrode array implanted within the cochlea to stimulate the auditory nerve and produce a sensation of sound. Many cochlear implant users are able to hear speech well if there are no competing sounds. However, listening through a cochlear implant can be adversely affected by background noise, and music perception is generally poor. It is thought that wide spread of current throughout the cochlea is responsible for some of these listening difficulties. This thesis investigated the potential benefits of reducing the spread of current in the cochlea. Results did not, on average, show a benefit of current focusing on pitch perception in cochlear implant listeners, although some variability was noted between listeners in the amount of benefit obtained.

Bibliographic metadata

Type of resource:
Content type:
Form of thesis:
Type of submission:
Degree type:
Doctor of Philosophy
Degree programme:
PhD Audiology
Publication date:
Location:
Manchester, UK
Total pages:
194
Abstract:
Cochlear implants are a recognised treatment for severe and profound hearing losses, and can greatly improve speech discrimination in a quiet listening environment. However, poor specificity of neural excitation resulting from wide current spread within the cochlea leads to inadequate complex pitch perception, affecting speech discrimination in noisy environments and music perception. Tripolar is a stimulation mode with a greater degree of current focusing than the monopolar mode currently used in the clinic. The overall aim of the experiments in this thesis was to evaluate the potential of tripolar stimulation mode to improve complex pitch perception compared to monopolar stimulation mode in a group of Advanced Bionics cochlear implant listeners.First, the place specificity of tripolar and monopolar stimulation was compared using a psychophysical forward masking task with a dual-electrode masking stimulus to limit off-site listening. An overall improvement in the place specificity of tripolar compared to monopolar stimulation indicated that current focusing may provide more independent transmission of temporal information from different electrode places. This conclusion was unaffected by the degree of residual masking which, although measurable, was similar in both modes. Second, the effect of current focusing on delivery of independent temporal patterns was evaluated, specifically whether transmission of fine temporal information about interpulse intervals was improved using tripolar over monopolar stimulation. No advantage was found for current focusing in the delivery of temporal patterns on either a single electrode site, or across-electrodes. Third, the effect of mode on complex pitch perception was investigated by comparing a tripolar with a monopolar listening program. Results showed no improvement in pitch ranking ability from current focusing using sung vowel stimuli, and further suggested that a cue related to the centroid of excitation was providing a stronger cue to pitch than the temporal modulations available in both modes in this group. In conclusion, tripolar stimulation, on average, improved place specificity using forward masking, but not temporal cues to pitch using the methods chosen to evaluate this. Furthermore, current focusing did not improve the ability to rank sung vowels based on pitch over monopolar mode. Current focusing, therefore, was not found to enhance perception of complex harmonic pitch in this group. However, a degree of inter-participant variability in amount of benefit from current focusing observed in this series of experiments suggests that it may be useful for some cochlear implant listeners if a predictive measure of benefit were developed.
Layman's abstract:
Cochlear implants are a recognised treatment for severe and profound hearing losses. They work by using current generated by an electrode array implanted within the cochlea to stimulate the auditory nerve and produce a sensation of sound. Many cochlear implant users are able to hear speech well if there are no competing sounds. However, listening through a cochlear implant can be adversely affected by background noise, and music perception is generally poor. It is thought that wide spread of current throughout the cochlea is responsible for some of these listening difficulties. This thesis investigated the potential benefits of reducing the spread of current in the cochlea. Results did not, on average, show a benefit of current focusing on pitch perception in cochlear implant listeners, although some variability was noted between listeners in the amount of benefit obtained.
Thesis main supervisor(s):
Language:
en

Institutional metadata

University researcher(s):

Record metadata

Manchester eScholar ID:
uk-ac-man-scw:240891
Created by:
Fielden, Claire
Created:
25th November, 2014, 19:07:43
Last modified by:
Fielden, Claire
Last modified:
9th September, 2016, 12:56:20

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