Word segmentation from noise-band vocoded speech

How to vocode: Using channel vocoders for cochlear-implant research

The channel vocoder has become a useful tool to understand the impact of specific forms of auditory degradation-particularly the spectral and temporal degradation that reflect cochlear-implant processing. Vocoders have many parameters that allow researchers to answer questions about cochlear-implant processing in ways that overcome some logistical complications of controlling for factors in individual cochlear implant users. However, there is such a large variety in the implementation of vocoders that the term "vocoder" is not specific enough to describe the signal processing used in these experiments. Misunderstanding vocoder parameters can result in experimental confounds or unexpected stimulus distortions. This paper highlights the signal processing parameters that should be specified when describing vocoder construction. The paper also provides guidance on how to determine vocoder parameters within perception experiments, given the experimenter's goals and research questions, to avoid common signal processing mistakes. Throughout, we will assume that experimenters are interested in vocoders with the specific goal of better understanding cochlear implants.

Strategic perceptual weighting of acoustic cues for word stress in listeners with cochlear implants, acoustic hearing, or simulated bimodal hearing

Perception of word stress is an important aspect of recognizing speech, guiding the listener toward candidate words based on the perceived stress pattern. Cochlear implant (CI) signal processing is likely to disrupt some of the available cues for word stress, particularly vowel quality and pitch contour changes. In this study, we used a cue weighting paradigm to investigate differences in stress cue weighting patterns between participants listening with CIs and those with normal hearing (NH). We found that participants with CIs gave less weight to frequency-based pitch and vowel quality cues than NH listeners but compensated by upweighting vowel duration and intensity cues. Nonetheless, CI listeners' stress judgments were also significantly influenced by vowel quality and pitch, and they modulated their usage of these cues depending on the specific word pair in a manner similar to NH participants. In a series of separate online experiments with NH listeners, we simulated aspects of bimodal hearing by combining low-pass filtered speech with a vocoded signal. In these conditions, participants upweighted pitch and vowel quality cues relative to a fully vocoded control condition, suggesting that bimodal listening holds promise for restoring the stress cue weighting patterns exhibited by listeners with NH.

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.

Prosody-driven syntax learning is robust to impoverished pitch and spectral cues

Across languages, prosodic boundaries tend to align with syntactic boundaries, and both infant and adult language learners capitalize on these correlations to jump-start syntax acquisition. However, it is unclear which prosodic cues-pauses, final-syllable lengthening, and/or pitch resets across boundaries-are necessary for prosodic bootstrapping to occur. It is also unknown how syntax acquisition is impacted when listeners do not have access to the full range of prosodic or spectral information. These questions were addressed using 14-channel noise-vocoded (spectrally degraded) speech. While pre-boundary lengthening and pauses are well-transmitted through noise-vocoded speech, pitch is not; overall intelligibility is also decreased. In two artificial grammar experiments, adult native English speakers showed a similar ability to use English-like prosody to bootstrap unfamiliar syntactic structures from degraded speech and natural, unmanipulated speech. Contrary to previous findings that listeners may require pitch resets and final lengthening to co-occur if no pause cue is present, participants in the degraded speech conditions were able to detect prosodic boundaries from lengthening alone. Results suggest that pitch is not necessary for adult English speakers to perceive prosodic boundaries associated with syntactic structures, and that prosodic bootstrapping is robust to degraded spectral information.