Sensitivity to binaural temporal-envelope beats with single-sided deafness and a cochlear implant as a measure of tonotopic match (L).

For cochlear-implant users with near-normal contralateral hearing, a mismatch between the frequency-to-place mapping in the two ears could produce a suboptimal performance. This study assesses tonotopic matches via binaural interactions. Dynamic interaural time-difference sensitivity was measured using bandpass-filtered pulse trains at different rates in the acoustic and implanted ear, creating binaural envelope beats. Sensitivity to beats should peak when the same tonotopic region is stimulated in both ears. All nine participants detected dynamic interaural timing differences and demonstrated some frequency selectivity. This method provides a guide to frequency-to-place mapping without compensation for inherent latency differences between the acoustic and implanted ears.

Pupillometry Reveals That Context Benefit in Speech Perception Can Be Disrupted by Later-Occurring Sounds, Especially in Listeners With Cochlear Implants.

Contextual cues can be used to improve speech recognition, especially for people with hearing impairment. However, previous work has suggested that when the auditory signal is degraded, context might be used more slowly than when the signal is clear. This potentially puts the hearing-impaired listener in a dilemma of continuing to process the last sentence when the next sentence has already begun. This study measured the time course of the benefit of context using pupillary responses to high- and low-context sentences that were followed by silence or various auditory distractors (babble noise, ignored digits, or attended digits). Participants were listeners with cochlear implants or normal hearing using a 12-channel noise vocoder. Context-related differences in pupil dilation were greater for normal hearing than for cochlear implant listeners, even when scaled for differences in pupil reactivity. The benefit of context was systematically reduced for both groups by the presence of the later-occurring sounds, including virtually complete negation when sentences were followed by another attended utterance. These results challenge how we interpret the benefit of context in experiments that present just one utterance at a time. If a listener uses context to "repair" part of a sentence, and later-occurring auditory stimuli interfere with that repair process, the benefit of context might not survive outside the idealized laboratory or clinical environment. Elevated listening effort in hearing-impaired listeners might therefore result not just from poor auditory encoding but also inefficient use of context and prolonged processing of misperceived utterances competing with perception of incoming speech.

Binaural hearing in children using Gaussian enveloped and transposed tones.

Children who use bilateral cochlear implants (BiCIs) show significantly poorer sound localization skills than their normal hearing (NH) peers. This difference has been attributed, in part, to the fact that cochlear implants (CIs) do not faithfully transmit interaural time differences (ITDs) and interaural level differences (ILDs), which are known to be important cues for sound localization. Interestingly, little is known about binaural sensitivity in NH children, in particular, with stimuli that constrain acoustic cues in a manner representative of CI processing. In order to better understand and evaluate binaural hearing in children with BiCIs, the authors first undertook a study on binaural sensitivity in NH children ages 8-10, and in adults. Experiments evaluated sound discrimination and lateralization using ITD and ILD cues, for stimuli with robust envelope cues, but poor representation of temporal fine structure. Stimuli were spondaic words, Gaussian-enveloped tone pulse trains (100 pulse-per-second), and transposed tones. Results showed that discrimination thresholds in children were adult-like (15-389 µs for ITDs and 0.5-6.0 dB for ILDs). However, lateralization based on the same binaural cues showed higher variability than seen in adults. Results are discussed in the context of factors that may be responsible for poor representation of binaural cues in bilaterally implanted children.

Predicting contrast effects following reliable spectral properties in speech perception.

Vowel perception is influenced by precursor sounds that are resynthesized to shift frequency regions [Ladefoged and Broadbent (1957). J. Acoust. Soc. Am. 29(1), 98-104] or filtered to emphasize narrow [Kiefte and Kluender (2008). J. Acoust. Soc. Am. 123(1), 366-376] or broad frequency regions [Watkins (1991). J. Acoust. Soc. Am. 90(6), 2942-2955]. Spectral differences between filtered precursors and vowel targets are perceptually enhanced, producing spectral contrast effects (e.g., emphasizing spectral properties of /ɪ/ in the precursor elicited more /ɛ/ responses to an /ɪ/-/ɛ/ vowel continuum, and vice versa). Historically, precursors have been processed by high-gain filters, resulting in prominent stable long-term spectral properties. Perceptual sensitivity to subtler but equally reliable spectral properties is unknown. Here, precursor sentences were processed by filters of variable bandwidths and different gains, then followed by vowel sounds varying from /ɪ/-/ɛ/. Contrast effects were widely observed, including when filters had only 100-Hz bandwidth or +5 dB gain. Average filter power was a good predictor of the magnitudes of contrast effects, revealing a close linear correspondence between the prominence of a reliable spectral property and the size of shifts in perceptual responses. High sensitivity to subtle spectral regularities suggests contrast effects are not limited to high-power filters, and thus may be more pervasive in speech perception than previously thought.

Using speech sounds to test functional spectral resolution in listeners with cochlear implants.

In this study, spectral properties of speech sounds were used to test functional spectral resolution in people who use cochlear implants (CIs). Specifically, perception of the /ba/-/da/ contrast was tested using two spectral cues: Formant transitions (a fine-resolution cue) and spectral tilt (a coarse-resolution cue). Higher weighting of the formant cues was used as an index of better spectral cue perception. Participants included 19 CI listeners and 10 listeners with normal hearing (NH), for whom spectral resolution was explicitly controlled using a noise vocoder with variable carrier filter widths to simulate electrical current spread. Perceptual weighting of the two cues was modeled with mixed-effects logistic regression, and was found to systematically vary with spectral resolution. The use of formant cues was greatest for NH listeners for unprocessed speech, and declined in the two vocoded conditions. Compared to NH listeners, CI listeners relied less on formant transitions, and more on spectral tilt. Cue-weighting results showed moderately good correspondence with word recognition scores. The current approach to testing functional spectral resolution uses auditory cues that are known to be important for speech categorization, and can thus potentially serve as the basis upon which CI processing strategies and innovations are tested.

The Impact of Auditory Spectral Resolution on Listening Effort Revealed by Pupil Dilation.

OBJECTIVES: This study measured the impact of auditory spectral resolution on listening effort. Systematic degradation in spectral resolution was hypothesized to elicit corresponding systematic increases in pupil dilation, consistent with the notion of pupil dilation as a marker of cognitive load. DESIGN: Spectral resolution of sentences was varied with two different vocoders: (1) a noise-channel vocoder with a variable number of spectral channels; and (2) a vocoder designed to simulate front-end processing of a cochlear implant, including peak-picking channel selection with variable synthesis filter slopes to simulate spread of neural excitation. Pupil dilation was measured after subject-specific luminance adjustment and trial-specific baseline measures. Mixed-effects growth curve analysis was used to model pupillary responses over time. RESULTS: For both types of vocoder, pupil dilation grew with each successive degradation in spectral resolution. Within each condition, pupillary responses were not related to intelligibility scores, and the effect of spectral resolution on pupil dilation persisted even when only analyzing trials in which responses were 100% correct. CONCLUSIONS: Intelligibility scores alone were not sufficient to quantify the effort required to understand speech with poor resolution. Degraded spectral resolution results in increased effort required to understand speech, even when intelligibility is at 100%. Pupillary responses were a sensitive and highly granular measurement to reveal changes in listening effort. Pupillary responses might potentially reveal the benefits of aural prostheses that are not captured by speech intelligibility performance alone as well as the disadvantages that are overcome by increased listening effort.