(NB version 4 or above)

Designed and built by
Martyn Holland
May 2001.
Click to comment.

© 2002. Copyright of these documents remains with the authors. Documents may be reproduced in part or as a whole for educational and review purposes only, and with suitable acknowledgement to the authors.

Spectral and temporal cues to pitch in noise-excited vocoder simulations of continuous-interleaved-sampling (CIS) cochlear implants
Tim GREEN, Andrew FAULKNER and Stuart ROSEN

Abstract
Four-band and single-band noise-excited vocoders were used in acoustic simulations to investigate spectral and temporal cues to melodic pitch in the output of a cochlear implant speech processor. Noise carriers were modulated by amplitude envelopes extracted by half-wave rectification and low-pass filtering at 32 or 400 Hz. The four-band, but not the single-band processors, may preserve spectral correlates of fundamental frequency (F0). 400 Hz envelope smoothing preserves temporal correlates of F0, which are eliminated with 32 Hz smoothing. Inputs to the processors were sawtooth frequency glides, in which spectral variation is completely determined by F0, or synthetic diphthongal vowel glides, whose spectral shape is dominated by varying formant resonances. Normal listeners labelled the direction of pitch movement of the processed stimuli. For processed sawtooth waves, purely temporal cues led to decreasing performance with increasing F0. With purely spectral cues, performance was above chance despite the limited spectral resolution of the processors. For processed diphthongs, performance with purely spectral cues was at chance, showing that spectral envelope changes due to formant movement obscured spectral cues to F0. Performance with temporal cues was poorer for diphthongs than for sawtooths, with very limited discrimination at higher F0. In conclusion, for speech signals through a typical cochlear implant processor, spectral cues to pitch will have little utility, while temporal envelope cues are useful only at low F0.

Auditory-visual L2 speech perception: effects of visual cues and acoustic-phonetic context for Spanish learners of English
Marta ORTEGA-LLEBARIA, Andrew FAULKNER and Valerie HAZAN

Abstract
This study was designed to identify English speech contrasts that might be appropriate for the computer-based auditory-visual training of Spanish learners of English. It examines auditory-visual and auditory consonant and vowel confusions by Spanish speaking students of English and a native English control group. 36 Spanish listeners were tested on their identification of 16 consonants and 9 vowels of British English. For consonants, both L2 learners and controls showed significant improvements in the audiovisual condition, with larger effects for syllable final consonants. The patterns of errors by L2 learners were strongly predictable from our knowledge of the relation between the phoneme inventories of Spanish and English. Consonant confusions which were language-dependent - mostly errors in voicing and manner - were not reduced by the addition of visual cues whereas confusions that were common to both listener groups and related to acoustic-phonetic sound characteristics did show improvements. Spanish listeners did not use visual cues that disambiguated contrasts that are phonemic in English but have allophonic status in Spanish. Visual features therefore have different weights when cueing phonemic and allophonic distinctions.

The right information matters more than frequency-place alignment: simulations of cochlear implant processors with an electrode array insertion depth of 17 mm
Andrew FAULKNER, Stuart ROSEN and Clare NORMAN

Abstract
It has been claimed that speech recognition with a cochlear implant is dependent on the correct frequency alignment of analysis bands in the speech processor with characteristic frequencies (CFs) at electrode locations. However the cochlear position of the most apical electrode often has a CF of 1 kHz or more, and the use of filters aligned in frequency to relatively basal electrode arrays leads to significant loss of lower frequency speech information. This study simulates a cochlear implant array with 8 electrodes spaced 2-mm apart, inserted to a relatively shallow depth within the typical range, such that the most apical element is at a CF of 1851 Hz. Two noise-excited vocoder speech processors for this simulated electrode location have been compared, one with CF-matched filters, and one with filters matched to CFs at basilar membrane locations 6mm more basal than electrode locations.

An extended crossover training design examined pre- and post-training performance in the identification of vowels and words in sentences for both processors. The shifted processor led to higher post-training scores than the frequency-aligned processor for a male talker with both vowels and sentences. For a female talker, post-training vowel scores did not differ significantly between processors, while sentence scores were higher with the frequency-aligned processor. Training effects were significant only for the shifted processor. The effects of upward spectral shifting were significantly reduced with a few hours of experience. In the case of a shallow electrode insertion, it seems likely that speech processors should cover the most informative frequency range irrespective of electrode position and frequency misalignment.

Lenition degrades information: consonant allophony in Ibibio
John HARRIS and Eno-Abasi URUA

Abstract
Consonantal lenition degrades information in the speech signal and should be understood as having an analogous impact on phonological representations. The point is illustrated by a phonological and instrumental study of Ibibio (Lower Cross, Nigeria). The more general claim is that informational asymmetries in the speech signal are matched by informational asymmetries in phonology.

A perceptual interference account of acquisition difficulties for non-native phonemes
Paul IVERSON, Patricia K. KUHL, Reiko AKAHANE-YAMADA, Eugen DIESCH, Yoh'ich TOHKURA, Andreas KETTERMANN, and Claudia SIEBERT

Abstract
This article presents a new account of how early language experience can impede the acquisition of non-native phonemes during adulthood. The hypothesis is that early language experience alters relatively low-level perceptual processing, and that these low-level changes interfere with the formation and adaptability of higher-level linguistic representations. Supporting data is presented from an experiment that tested the perception of English /r/ and /l/ by Japanese, German, and American adults. The underlying perceptual spaces for these phonemes were mapped using multidimensional scaling, and were compared to native-language categorization judgments. The results demonstrate that Japanese adults are most sensitive to an acoustic cue, F2, that is irrelevant to the English /r/-/l/ categorization. German adults, in contrast, have relatively high sensitivity to more critical acoustic cues. The results show how language-specific perceptual processing can alter the relative salience of within- and between-category acoustic variation, and thereby interfere with second language acquisition.