Impacts of signal processing factors on perceptual restoration in cochlear-implant users.

Cochlear-implant (CI) users have previously demonstrated perceptual restoration, or successful repair of noise-interrupted speech, using the interrupted sentences paradigm [Bhargava, Gaudrain, and Baskent (2014). "Top-down restoration of speech in cochlear-implant users," Hear. Res. 309, 113-123]. The perceptual restoration effect was defined experimentally as higher speech understanding scores with noise-burst interrupted sentences compared to silent-gap interrupted sentences. For the perceptual restoration illusion to occur, it is often necessary for the masking or interrupting noise bursts to have a higher intensity than the adjacent speech signal to be perceived as a plausible masker. Thus, signal processing factors like noise reduction algorithms and automatic gain control could have a negative impact on speech repair in this population. Surprisingly, evidence that participants with cochlear implants experienced the perceptual restoration illusion was not observed across the two planned experiments. A separate experiment, which aimed to provide a close replication of previous work on perceptual restoration in CI users, also found no consistent evidence of perceptual restoration, contrasting the original study's previously reported findings. Typical speech repair of interrupted sentences was not observed in the present work's sample of CI users, and signal-processing factors did not appear to affect speech repair.

Accuracy and cue use in word segmentation for cochlear-implant listeners and normal-hearing listeners presented vocoded speech.

Cochlear-implant (CI) listeners experience signal degradation, which leads to poorer speech perception than normal-hearing (NH) listeners. In the present study, difficulty with word segmentation, the process of perceptually parsing the speech stream into separate words, is considered as a possible contributor to this decrease in performance. CI listeners were compared to a group of NH listeners (presented with unprocessed speech and eight-channel noise-vocoded speech) in their ability to segment phrases with word segmentation ambiguities (e.g., "an iceman" vs "a nice man"). The results showed that CI listeners and NH listeners were worse at segmenting words when hearing processed speech than NH listeners were when presented with unprocessed speech. When viewed at a broad level, all of the groups used cues to word segmentation in similar ways. Detailed analyses, however, indicated that the two processed speech groups weighted top-down knowledge cues to word boundaries more and weighted acoustic cues to word boundaries less relative to NH listeners presented with unprocessed speech.

Access to semantic cues does not lead to perceptual restoration of interrupted speech in cochlear-implant users.

Cochlear-implant (CI) users experience less success in understanding speech in noisy, real-world listening environments than normal-hearing (NH) listeners. Perceptual restoration is one method NH listeners use to repair noise-interrupted speech. Whereas previous work has reported that CI users can use perceptual restoration in certain cases, they failed to do so under listening conditions in which NH listeners can successfully restore. Providing increased opportunities to use top-down linguistic knowledge is one possible method to increase perceptual restoration use in CI users. This work tested perceptual restoration abilities in 18 CI users and varied whether a semantic cue (presented visually) was available prior to the target sentence (presented auditorily). Results showed that whereas access to a semantic cue generally improved performance with interrupted speech, CI users failed to perceptually restore speech regardless of the semantic cue availability. The lack of restoration in this population directly contradicts previous work in this field and raises questions of whether restoration is possible in CI users. One reason for speech-in-noise understanding difficulty in CI users could be that they are unable to use tools like restoration to process noise-interrupted speech effectively.

Recognition of Accented Speech by Cochlear-Implant Listeners: Benefit of Audiovisual Cues.

OBJECTIVES: When auditory and visual speech information are presented together, listeners obtain an audiovisual (AV) benefit or a speech understanding improvement compared with auditory-only (AO) or visual-only (VO) presentations. Cochlear-implant (CI) listeners, who receive degraded speech input and therefore understand speech using primarily temporal information, seem to readily use visual cues and can achieve a larger AV benefit than normal-hearing (NH) listeners. It is unclear, however, if the AV benefit remains relatively large for CI listeners when trying to understand foreign-accented speech when compared with unaccented speech. Accented speech can introduce changes to temporal auditory cues and visual cues, which could decrease the usefulness of AV information. Furthermore, we sought to determine if the AV benefit was relatively larger in CI compared with NH listeners for both unaccented and accented speech. DESIGN: AV benefit was investigated for unaccented and Spanish-accented speech by presenting English sentences in AO, VO, and AV conditions to 15 CI and 15 age- and performance-matched NH listeners. Performance matching between NH and CI listeners was achieved by varying the number of channels of a noise vocoder for the NH listeners. Because of the differences in age and hearing history of the CI listeners, the effects of listener-related variables on speech understanding performance and AV benefit were also examined. RESULTS: AV benefit was observed for both unaccented and accented conditions and for both CI and NH listeners. The two groups showed similar performance for the AO and AV conditions, and the normalized AV benefit was relatively smaller for the accented than the unaccented conditions. In the CI listeners, older age was associated with significantly poorer performance with the accented speaker compared with the unaccented speaker. The negative impact of age was somewhat reduced by a significant improvement in performance with access to AV information. CONCLUSIONS: When auditory speech information is degraded by CI sound processing, visual cues can be used to improve speech understanding, even in the presence of a Spanish accent. The AV benefit of the CI listeners closely matched that of the NH listeners presented with vocoded speech, which was unexpected given that CI listeners appear to rely more on visual information to communicate. This result is perhaps due to the one-to-one age and performance matching of the listeners. While aging decreased CI listener performance with the accented speaker, access to visual cues boosted performance and could partially overcome the age-related speech understanding deficits for the older CI listeners.

Effects of Aging on Perceptual and Electrophysiological Responses to Acoustic Pulse Trains as a Function of Rate.

Purpose As pulse rate increases beyond a few hundred Hertz, younger normal-hearing (NH) participants' ability to encode temporal information in band-limited acoustic pulse trains decreases, demonstrating a rate limitation in processing rapid temporal information. Rate discrimination abilities, however, have yet to be investigated in older NH participants-a population that experiences age-related temporal processing deficits. It was hypothesized that age-related temporal processing deficits lead to decreased temporal rate discrimination abilities in older compared with younger NH participants, which could be observed in both perceptual and electrophysiological measurements. Method Fifteen younger and 15 older NH participants were presented acoustic pulse trains with a 4-kHz center frequency and 1-kHz bandwidth at 75 dB SPL monaurally. The pulse rate was 80, 200, or 400 Hz. Just noticeable differences were obtained using an adaptive procedure that instructed the participants to identify the pulse train with the highest pitch. Auditory steady-state responses (ASSRs) were recorded to the same pulse trains with 2 additional rates-20 and 40 Hz. The Digit Symbol Coding and Digit Symbol Search subtests of the Wechsler Adult Intelligence Scale ( Wechsler, 1997 ) were measured as correlates to domain-general cognitive processing speed. Results As rate increased from 80 to 400 Hz, performance on the perceptual rate discrimination task worsened in both groups. ASSR spectral energy also decreased, but only in the older group. Perceptual performance was equivalent between groups across rates. The older group had lower ASSR spectral energy (lower signal-to-noise ratios) at the 400-Hz rate than the younger group, but there were no group differences for the other rates. The overall strength of neural rate representation, along with speed of processing performance, predicted perceptual performance for the 400-Hz rate. Conclusion These results suggest that neural representation at early levels of the auditory system and processing speed are factors in perceptual auditory temporal processing performance, especially in older adults.

Age effects on perceptual restoration of degraded interrupted sentences.

Adult cochlear-implant (CI) users show small or non-existent perceptual restoration effects when listening to interrupted speech. Perceptual restoration is believed to be a top-down mechanism that enhances speech perception in adverse listening conditions, and appears to be particularly utilized by older normal-hearing participants. Whether older normal-hearing participants can derive any restoration benefits from degraded speech (as would be presented through a CI speech processor) is the focus of this study. Two groups of normal-hearing participants (younger: age ≤30 yrs; older: age ≥60 yrs) were tested for perceptual restoration effects in the context of interrupted sentences. Speech signal degradations were controlled by manipulating parameters of a noise vocoder and were used to analyze effects of spectral resolution and noise burst spectral content on perceptual restoration. Older normal-hearing participants generally showed larger and more consistent perceptual restoration benefits for vocoded speech than did younger normal-hearing participants, even in the lowest spectral resolution conditions. Reduced restoration in CI users thus may be caused by factors like noise reduction strategies or small dynamic ranges rather than an interaction of aging effects and low spectral resolution.

Speech Rate Normalization and Phonemic Boundary Perception in Cochlear-Implant Users.

Purpose: Normal-hearing (NH) listeners rate normalize, temporarily remapping phonemic category boundaries to account for a talker's speech rate. It is unknown if adults who use auditory prostheses called cochlear implants (CI) can rate normalize, as CIs transmit degraded speech signals to the auditory nerve. Ineffective adjustment to rate information could explain some of the variability in this population's speech perception outcomes. Method: Phonemes with manipulated voice-onset-time (VOT) durations were embedded in sentences with different speech rates. Twenty-three CI and 29 NH participants performed a phoneme identification task. NH participants heard the same unprocessed stimuli as the CI participants or stimuli degraded by a sine vocoder, simulating aspects of CI processing. Results: CI participants showed larger rate normalization effects (6.6 ms) than the NH participants (3.7 ms) and had shallower (less reliable) category boundary slopes. NH participants showed similarly shallow slopes when presented acoustically degraded vocoded signals, but an equal or smaller rate effect in response to reductions in available spectral and temporal information. Conclusion: CI participants can rate normalize, despite their degraded speech input, and show a larger rate effect compared to NH participants. CI participants may particularly rely on rate normalization to better maintain perceptual constancy of the speech signal.