Etienne Gaudrain

Gender Categorization Is Abnormal in Cochlear Implant Users

In normal hearing (NH), the perception of the gender of a speaker is strongly affected by two anatomically related vocal characteristics: the fundamental frequency (F0), related to vocal pitch, and the vocal tract length (VTL), related to the height of the speaker. Previous studies on gender categorization in cochlear implant (CI) users found that performance was variable, with few CI users performing at the level of NH listeners. Data collected with recorded speech produced by multiple talkers suggests that CI users might rely more on F0 and less on VTL than NH listeners. However, because VTL cannot be accurately estimated from recordings, it is difficult to know how VTL contributes to gender categorization. In the present study, speech was synthesized to systematically vary F0, VTL, or both. Gender categorization was measured in CI users, as well as in NH participants listening to unprocessed (only synthesized) and vocoded (and synthesized) speech. Perceptual weights for F0 and VTL were derived from the performance data. With unprocessed speech, NH listeners used both cues (normalized perceptual weight: F0 = 3.76, VTL = 5.56). With vocoded speech, NH listeners still made use of both cues but less efficiently (normalized perceptual weight: F0 = 1.68, VTL = 0.63). CI users relied almost exclusively on F0 while VTL perception was profoundly impaired (normalized perceptual weight: F0 = 6.88, VTL = 0.59). As a result, CI users’ gender categorization was abnormal compared to NH listeners. Future CI signal processing should aim to improve the transmission of both F0 cues and VTL cues, as a normal gender categorization may benefit speech understanding in competing talker situations.

Effect of spectral smearing on the perceptual segregation of vowel sequences

Although segregation of both simultaneous and sequential speech items may be involved in the reception of speech in noisy environments, research on the latter is relatively sparse. Further, previous studies examining the ability of hearing-impaired listeners to form distinct auditory streams have produced mixed results. Finally, there is little work investigating streaming in cochlear implant recipients, who also have poor frequency resolution. The present study focused on the mechanisms involved in the segregation of vowel sequences and potential limitations to segregation associated with poor frequency resolution. An objective temporal-order paradigm was employed in which listeners reported the order of constituent vowels within a sequence. In Experiment 1, it was found that fundamental frequency based mechanisms contribute to segregation. In Experiment 2, reduced frequency tuning often associated with hearing impairment was simulated in normal-hearing listeners. In that experiment, it was found that spectral smearing of the vowels increased accurate identification of their order, presumably by reducing the tendency to form separate auditory streams. These experiments suggest that a reduction in spectral resolution may result in a reduced ability to form separate auditory streams, which may contribute to the difficulties of hearing-impaired listeners, and probably cochlear implant recipients as well, in multi-talker cocktail-party situations.

Top–down restoration of speech in cochlear-implant users

In noisy listening conditions, intelligibility of degraded speech can be enhanced by top-down restoration. Cochlear implant (CI) users have difficulty understanding speech in noisy environments. This could partially be due to reduced top-down restoration of speech, which may be related to the changes that the electrical stimulation imposes on the bottom-up cues. We tested this hypothesis using the phonemic restoration (PhR) paradigm in which speech interrupted with periodic silent intervals is perceived illusorily continuous (continuity illusion or CoI) and becomes more intelligible (PhR benefit) when the interruptions are filled with noise bursts. Using meaningful sentences, both CoI and PhR benefit were measured in CI users, and compared with those of normal-hearing (NH) listeners presented with normal speech and 8-channel noise-band vocoded speech, acoustically simulating CIs. CI users showed different patterns in both PhR benefit and CoI, compared to NH results with or without the noise-band vocoding. However, they were able to use top-down restoration under certain test conditions. This observation supports the idea that changes in bottom-up cues can impose changes to the top-down processes needed to enhance intelligibility of degraded speech. The knowledge that CI users seem to be able to do restoration under the right circumstances could be exploited in patient rehabilitation and product development.

Streaming of vowel sequences based on fundamental frequency in a cochlear-implant simulation.

Cochlear-implant (CI) users often have difficulties perceiving speech in noisy environments. Although this problem likely involves auditory scene analysis, few studies have examined sequential segregation in CI listening situations. The present study aims to assess the possible role of fundamental frequency (F(0)) cues for the segregation of vowel sequences, using a noise-excited envelope vocoder that simulates certain aspects of CI stimulation. Obligatory streaming was evaluated using an order-naming task in two experiments involving normal-hearing subjects. In the first experiment, it was found that streaming did not occur based on F(0) cues when natural-duration vowels were processed to reduce spectral cues using the vocoder. In the second experiment, shorter duration vowels were used to enhance streaming. Under these conditions, F(0)-related streaming appeared even when vowels were processed to reduce spectral cues. However, the observed segregation could not be convincingly attributed to temporal periodicity cues. A subsequent analysis of the stimuli revealed that an F(0)-related spectral cue could have elicited the observed segregation. Thus, streaming under conditions of severely reduced spectral cues, such as those associated with CIs, may potentially occur as a result of this particular cue.

Musician advantage for speech-on-speech perception

Evidence for transfer of musical training to better perception of speech in noise has been mixed. Unlike speech-in-noise, speech-on-speech perception utilizes many of the skills that musical training improves, such as better pitch perception and stream segregation, as well as use of higher-level auditory cognitive functions, such as attention. Indeed, despite the few non-musicians who performed as well as musicians, on a group level, there was a strong musician benefit for speech perception in a speech masker. This benefit does not seem to result from better voice processing and could instead be related to better stream segregation or enhanced cognitive functions.

A neural mechanism for recognizing speech spoken by different speakers

Understanding speech from different speakers is a sophisticated process, particularly because the same acoustic parameters convey important information about both the speech message and the person speaking. How the human brain accomplishes speech recognition under such conditions is unknown. One view is that speaker information is discarded at early processing stages and not used for understanding the speech message. An alternative view is that speaker information is exploited to improve speech recognition. Consistent with the latter view, previous research identified functional interactions between the left- and the right-hemispheric superior temporal sulcus/gyrus, which process speech- and speaker-specific vocal tract parameters, respectively. Vocal tract parameters are one of the two major acoustic features that determine both speaker identity and speech message (phonemes). Here, using functional magnetic resonance imaging (fMRI), we show that a similar interaction exists for glottal fold parameters between the left and right Heschls gyri. Glottal fold parameters are the other main acoustic feature that determines speaker identity and speech message (linguistic prosody). The findings suggest that interactions between left- and right-hemispheric areas are specific to the processing of different acoustic features of speech and speaker, and that they represent a general neural mechanism when understanding speech from different speakers.

The Perception of Family and Register in Musical Tones

This chapter is about the sounds made by musical instruments and how we perceive them. It explains the basics of musical note perception, such as why a particular instrument plays a specific range of notes; why instruments come in families; and why we hear distinctive differences between members of a given instrument family, even when they are playing the same note. The answers to these questions might, at first, seem obvious; one could say that brass instruments all make the same kind of sound because they are all made of brass, and the different members of the family sound different because they are different sizes. But answers at this level just prompt more questions, such as: What do we mean when we say the members of a family produce the same sound? What is it that is actually the same, and what is it that is different, when different instruments within a family play the same melody on the same notes? To answer these and similar questions, we examine the relationship between the physical variables of musical instruments, such as the length, mass, and tension of a string, and the variables of auditory perception, such as pitch, timbre, and loudness. The discussion reveals that there are three acoustic properties of musical sounds, as they occur in the air, between the instrument and the listener, that are particularly useful in summarizing the effects of the physical properties on the musical tones they produce, and in explaining how these musical tones produce the perceptions that we hear.

T'ain't the way you say it, it's what you say--perceptual continuity of voice and top-down restoration of speech.

Phonemic restoration, or top-down repair of speech, is the ability of the brain to perceptually reconstruct missing speech sounds, using remaining speech features, linguistic knowledge and context. This usually occurs in conditions where the interrupted speech is perceived as continuous. The main goal of this study was to investigate whether voice continuity was necessary for phonemic restoration. Restoration benefit was measured by the improvement in intelligibility of meaningful sentences interrupted with periodic silent gaps, after the gaps were filled with noise bursts. A discontinuity was induced on the voice characteristics. The fundamental frequency, the vocal tract length, or both of the original vocal characteristics were changed using STRAIGHT to make a talker sound like a different talker from one speech segment to another. Voice discontinuity reduced the global intelligibility of interrupted sentences, confirming the importance of vocal cues for perceptually constructing a speech stream. However, phonemic restoration benefit persisted through all conditions despite the weaker voice continuity. This finding suggests that participants may have relied more on other cues, such as pitch contours or perhaps even linguistic context, when the vocal continuity was disrupted.

Factors limiting vocal-tract length discrimination in cochlear implant simulations.

Perception of voice characteristics allows normal hearing listeners to identify the gender of a speaker, and to better segregate speakers from each other in cocktail party situations. This benefit is largely driven by the perception of two vocal characteristics of the speaker: The fundamental frequency (F0) and the vocal-tract length (VTL). Previous studies have suggested that cochlear implant (CI) users have difficulties in perceiving these cues. The aim of the present study was to investigate possible causes for limited sensitivity to VTL differences in CI users. Different acoustic simulations of CI stimulation were implemented to characterize the role of spectral resolution on VTL, both in terms of number of channels and amount of channel interaction. The results indicate that with 12 channels, channel interaction caused by current spread is likely to prevent CI users from perceiving VTL differences typically found between male and female speakers.

Using Zebra-speech to study sequential and simultaneous speech segregation in a cochlear-implant simulation

Previous studies have suggested that cochlear implant users may have particular difficulties exploiting opportunities to glimpse clear segments of a target speech signal in the presence of a fluctuating masker. Although it has been proposed that this difficulty is associated with a deficit in linking the glimpsed segments across time, the details of this mechanism are yet to be explained. The present study introduces a method called Zebra-speech developed to investigate the relative contribution of simultaneous and sequential segregation mechanisms in concurrent speech perception, using a noise-band vocoder to simulate cochlear implants. One experiment showed that the saliency of the difference between the target and the masker is a key factor for Zebra-speech perception, as it is for sequential segregation. Furthermore, forward masking played little or no role, confirming that intelligibility was not limited by energetic masking but by across-time linkage abilities. In another experiment, a binaural cue was used to distinguish the target and the masker. It showed that the relative contribution of simultaneous and sequential segregation depended on the spectral resolution, with listeners relying more on sequential segregation when the spectral resolution was reduced. The potential of Zebra-speech as a segregation enhancement strategy for cochlear implants is discussed.