Cochlear implants provide hearing to people who are deaf, by electrically stimulating the auditory nerve. Children with a single cochlear implant suffer deficiencies inherent to unilateral hearing, including inability to locate sounds. A second cochlear implant may improve sound localization, which normally requires interpretation of differences in sound intensity and time of arrival between two ears. Currently, it is unknown whether these cues are available to children who were provided with a second cochlear implant after a period of using one implant alone. We asked whether such children could interpret inter-implant level and timing cues. Results indicated that children using two cochlear implants detected level cues but had difficulty interpreting timing cues. Further, children rarely reported that sounds were perceived to come from the middle. Children receiving bilateral cochlear implants sequentially do not process bilateral auditory cues normally but can use inter-implant level cues to make judgments about where sound is coming from.

Cochlear implants provide hearing to people who are deaf, by electrically stimulating the auditory nerve. Children with a single cochlear implant suffer deficiencies inherent to unilateral hearing, including inability to locate sounds. A second cochlear implant may improve sound localization, which normally requires interpretation of differences in sound intensity and time of arrival between two ears. Currently, it is unknown whether these cues are available to children who were provided with a second cochlear implant after a period of using one implant alone. We asked whether such children could interpret inter-implant level and timing cues. Results indicated that children using two cochlear implants detected level cues but had difficulty interpreting timing cues. Further, children rarely reported that sounds were perceived to come from the middle. Children receiving bilateral cochlear implants sequentially do not process bilateral auditory cues normally but can use inter-implant level cues to make judgments about where sound is coming from.

Cochlear implants (CIs) provide coarse representations of pitch, which are adequate for speech but not for music. Despite increasing interest in music processing by CI users, the available information is fragmentary. The present experiment attempted to fill this void by conducting a comprehensive assessment of music processing in adult CI users. CI users (n =6) and normally hearing (NH) controls (n = 12) were tested on several tasks involving melody and rhythm perception, recognition of familiar music, and emotion of recognition in speech and music. CI performance was substantially poorer than NH performance and at chance levels on pitch processing tasks. Performance was highly variable, however, with one individual achieving NH performance levels on some tasks, probably because of low-frequency residual hearing in his unimplanted ear. Future research with a larger sample of CI users can shed light on factors associated with good and poor music processing in this population.

Cochlear implants (CIs) provide coarse representations of pitch, which are adequate for speech but not for music. Despite increasing interest in music processing by CI users, the available information is fragmentary. The present experiment attempted to fill this void by conducting a comprehensive assessment of music processing in adult CI users. CI users (n =6) and normally hearing (NH) controls (n = 12) were tested on several tasks involving melody and rhythm perception, recognition of familiar music, and emotion of recognition in speech and music. CI performance was substantially poorer than NH performance and at chance levels on pitch processing tasks. Performance was highly variable, however, with one individual achieving NH performance levels on some tasks, probably because of low-frequency residual hearing in his unimplanted ear. Future research with a larger sample of CI users can shed light on factors associated with good and poor music processing in this population.

Listening to speech with competing speech in the background is challenging and becomes harder with age. Three experiments examined the auditory and cognitive aspects of auditory spatial attention in conditions in which the location of the target was uncertain. In all experiments, word identification was measured for target sentences presented with two competitor sentences. On each trial, the three sentences were presented with one from each of three spatially separated loudspeakers. A priori cues specified the location and identity callsign of the target. In Experiments I and II, sentences were also presented in conditions of simulated spatial separation achieved with the precedence effect. Participants were younger and older adults with normal hearing sensitivity below 4 kHz. For both age groups, the contributions of richer acoustic cues (those present when there was real spatial separation, but absent when there was simulated spatial separation) were most pronounced when the target occurred at “unlikely” spatial listening locations, suggesting that both age groups benefit similarly from richer acoustical cues. In Experiment II, the effect of time between the callsign cue and target word on word identification was investigated. Four timing conditions were tested: the original sentences (which contained about 300 ms of filler speech between the callsign cue and the onset of the target words), or modified sentences with silent pauses of 0, 150, or 300 ms replacing the filler speech. For targets presented from unlikely locations, word identification was better for all listeners when there was more time between the callsign cue and key words, suggesting that time is needed to switch spatial attention. In Experiment III, the effects of single and multiple switches of attention were investigated. The key finding was that, whereas both age groups performed similarly in conditions requiring a single switch of attention, the performance of older, but not younger listeners, was reduced when multiple switches of spatial attention were required. This finding suggests that difficulties disengaging attention may contribute to the listening difficulties of older adults. In conclusion, cognitive and auditory factors contributing to auditory spatial attention appear to operate similarly for all listeners in relatively simple situations, and age-related differences are observed in more complex situations.

Electrophysiologic responses are used to estimate hearing thresholds and fit hearing aids in young infants, but these estimates are not exact. An objective test of speech encoding could be used to validate infant fittings by showing that speech has been registered in the central auditory system. Such a test could also show the effects of auditory processing problems on the neural representation of speech. This thesis describes techniques for recording electrophysiologic responses to natural speech stimuli from the brainstem and auditory cortex. The first technique uses a Fourier analyzer to measure steady-state brainstem responses to periodicities and envelope changes in vowels, and the second uses a windowed cross-correlation procedure to measure cortical responses to the envelopes of sentences. Two studies were conducted with the Fourier analyzer. The first measured responses to natural vowels with steady and changing fundamentals, and changing formants. Significant responses to the fundamental were detected for all of the vowels, in all of the subjects, in 19 – 73 s (on average). The second study recorded responses to a vowel fundamental and harmonics. Vowels were presented in opposite polarities to distinguish envelope responses from responses to the spectrum. Significant envelope responses were detected in all subjects at the fundamental. Significant spectral responses were detected in most subjects at harmonics near formant peaks. The third study used cross-correlation to measure cortical responses to sentences. Significant envelope responses were detected to all sentences, at delays of roughly 180 ms. Responses were localized to the posterior auditory cortices. A model based on a series of overlapping transient responses to envelope changes could also account for the results, suggesting that the cortex either directly follows the speech envelope or consistently reacts to changes in this envelope. The strengths and weaknesses of both techniques are discussed in relation to their potential clinical applications.

Listening to speech with competing speech in the background is challenging and becomes harder with age. Three experiments examined the auditory and cognitive aspects of auditory spatial attention in conditions in which the location of the target was uncertain. In all experiments, word identification was measured for target sentences presented with two competitor sentences. On each trial, the three sentences were presented with one from each of three spatially separated loudspeakers. A priori cues specified the location and identity callsign of the target. In Experiments I and II, sentences were also presented in conditions of simulated spatial separation achieved with the precedence effect. Participants were younger and older adults with normal hearing sensitivity below 4 kHz. For both age groups, the contributions of richer acoustic cues (those present when there was real spatial separation, but absent when there was simulated spatial separation) were most pronounced when the target occurred at “unlikely” spatial listening locations, suggesting that both age groups benefit similarly from richer acoustical cues. In Experiment II, the effect of time between the callsign cue and target word on word identification was investigated. Four timing conditions were tested: the original sentences (which contained about 300 ms of filler speech between the callsign cue and the onset of the target words), or modified sentences with silent pauses of 0, 150, or 300 ms replacing the filler speech. For targets presented from unlikely locations, word identification was better for all listeners when there was more time between the callsign cue and key words, suggesting that time is needed to switch spatial attention. In Experiment III, the effects of single and multiple switches of attention were investigated. The key finding was that, whereas both age groups performed similarly in conditions requiring a single switch of attention, the performance of older, but not younger listeners, was reduced when multiple switches of spatial attention were required. This finding suggests that difficulties disengaging attention may contribute to the listening difficulties of older adults. In conclusion, cognitive and auditory factors contributing to auditory spatial attention appear to operate similarly for all listeners in relatively simple situations, and age-related differences are observed in more complex situations.

Electrophysiologic responses are used to estimate hearing thresholds and fit hearing aids in young infants, but these estimates are not exact. An objective test of speech encoding could be used to validate infant fittings by showing that speech has been registered in the central auditory system. Such a test could also show the effects of auditory processing problems on the neural representation of speech. This thesis describes techniques for recording electrophysiologic responses to natural speech stimuli from the brainstem and auditory cortex. The first technique uses a Fourier analyzer to measure steady-state brainstem responses to periodicities and envelope changes in vowels, and the second uses a windowed cross-correlation procedure to measure cortical responses to the envelopes of sentences. Two studies were conducted with the Fourier analyzer. The first measured responses to natural vowels with steady and changing fundamentals, and changing formants. Significant responses to the fundamental were detected for all of the vowels, in all of the subjects, in 19 – 73 s (on average). The second study recorded responses to a vowel fundamental and harmonics. Vowels were presented in opposite polarities to distinguish envelope responses from responses to the spectrum. Significant envelope responses were detected in all subjects at the fundamental. Significant spectral responses were detected in most subjects at harmonics near formant peaks. The third study used cross-correlation to measure cortical responses to sentences. Significant envelope responses were detected to all sentences, at delays of roughly 180 ms. Responses were localized to the posterior auditory cortices. A model based on a series of overlapping transient responses to envelope changes could also account for the results, suggesting that the cortex either directly follows the speech envelope or consistently reacts to changes in this envelope. The strengths and weaknesses of both techniques are discussed in relation to their potential clinical applications.

Thèse : Lettres : Neuchâtel : 1980. / Bibliogr. p. 185-194. Index.

Age-related declines in hearing abilities are common and can be attributed to changes in the peripheral and central levels of the auditory system. Although central auditory processing is enhanced in younger musicians, the influence of lifelong musicianship on age-related decline in central auditory processing has not yet been investigated. Therefore, the purpose of this dissertation was to investigate whether lifelong musicianship can mitigate age-related decline in central auditory processing. In the first experiment, age-related declines on four hearing assessments were compared between musicians and non-musicians. Speech-in-noise and gap-detection thresholds were found to decline at a slower rate in musicians, providing an increasing advantage with age. Furthermore, musicians had a lifelong advantage in detecting a mistuned harmonic, although the rate of age-related decline was similar for both musicians and non-musicians. Importantly, there was no significant effect of musicianship on pure-tone thresholds, suggesting that lifelong musicianship can mitigate age-related decline in central but not peripheral auditory processing. To test this hypothesis, a second experiment compared auditory evoked responses (AERs) between groups of older and younger musicians and non-musicians. Results indicated that exogenous neural activity was enhanced in musicians, but that age-related changes were similar between musicians and nonmusicians. Furthermore, endogenous, attention-dependent neural activity was enhanced in older adults, suggesting a compensatory cognitive strategy. Importantly, endogenous activity was preferentially enhanced in older musicians, suggesting that lifelong musicianship enhanced cognitive processes related to auditory perception. In the final experiment, the ability to segregate simultaneous sounds was tested in older and younger musicians and non-musicians by using a mistuned harmonic paradigm, where AERs to harmonic complexes were compared to AERs when one of the harmonics was mistuned. Results indicated that musical training in older adults has little effect on early automatic registration of the mistuned harmonic. In contrast, late attention-dependent activity, associated with the perception of the mistuned harmonic as a separate sound, was influenced by musical training in older adults, suggesting that lifelong musicianship preserves or enhances cognitive components of concurrent sound segregation. In summary, musical training was found to reduce age-related decline in hearing abilities due to enhanced central processing of auditory information.