Faculty Bibliography

Cochlear implant (CI) users differ in their ability to perceive and recognize speech sounds. Two possible reasons for such individual differences may lie in their ability to discriminate formant frequencies or to adapt to the spectrally shifted information presented by cochlear implants, a basalward shift related to the implant's depth of insertion in the cochlea. In the present study, we examined these two alternatives using a method-of-adjustment (MOA) procedure with 330 synthetic vowel stimuli varying in F1 and F2 that were arranged in a two-dimensional grid. Subjects were asked to label the synthetic stimuli that matched ten monophthongal vowels in visually presented words. Subjects then provided goodness ratings for the stimuli they had chosen. The subjects' responses to all ten vowels were used to construct individual perceptual 'vowel spaces.' If CI users fail to adapt completely to the basalward spectral shift, then the formant frequencies of their vowel categories should be shifted lower in both F1 and F2. However, with one exception, no systematic shifts were observed in the vowel spaces of CI users. Instead, the vowel spaces differed from one another in the relative size of their vowel categories. The results suggest that differences in formant frequency discrimination may account for the individual differences in vowel perception observed in cochlear implant users

This study examined two possible reasons underlying longitudinal increases in vowel identification by cochlear implant users: improved labeling of vowel sounds and improved electrode discrimination. The Multidimensional Phoneme Identification (MPI) model was used to obtain ceiling estimates of vowel identification for each subject, given his/her electrode discrimination skills. Vowel identification scores were initially lower than the ceiling estimates, but they gradually approached them over the first few months post-implant. Taken together, the present results suggest that improved labeling is the main mechanism explaining post-implant increases in vowel identification

Although cochlear implants improve the ability of profoundly deaf children to understand speech, critics claim that the published literature does not document even a single case of a child who has developed a linguistic system based on input from an implant. Thus, it is of clinical and scientific importance to determine whether cochlear implants facilitate the development of English language skills. The English language skills of prelingually deaf children with cochlear implants were measured before and after implantation. We found that the rate of language development after implantation exceeded that expected from unimplanted deaf children (p < .001) and was similar to that of children with normal hearing. Despite a large amount of individual variability, the best performers in the implanted group seem to be developing an oral linguistic system based largely on auditory input obtained from a cochlear implant

A 'multidimensional phoneme identification' (MPI) model is proposed to account for vowel perception by cochlear implant users. A multidimensional extension of the Durlach-Braida model of intensity perception, this model incorporates an internal noise model and a decision model to account separately for errors due to poor sensitivity and response bias. The MPI model provides a complete quantitative description of how listeners encode and combine acoustic cues, and how they use this information to determine which sound they heard. Thus, it allows for testing specific hypotheses about phoneme identification in a very stringent fashion. As an example of the model's application, vowel identification matrices obtained with synthetic speech stimuli (including 'conflicting cue' conditions [Dorman et al., J. Acoust. Soc. Am. 92, 3428-3432 (1992)] were examined. The listeners were users of the 'compressed-analog' stimulation strategy, which filters the speech spectrum into four partly overlapping frequency bands and delivers each signal to one of four electrodes in the cochlea. It was found that a simple model incorporating one temporal cue (i.e., an acoustic cue based only on the time waveforms delivered to the most basal channel) and spectral cues (based on the distribution of amplitudes among channels) can be quite successful in explaining listener responses. The new approach represented by the MPI model may be used to obtain useful insights about speech perception by cochlear implant users in particular, and by all kinds of listeners in general