Effects of Formant Bandwidth on the Identification of Synthetic Vowels by Cochlear Implant Recipients

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Abstract
The main objective was to investigate whether the broadening and narrowing of formant bandwidths had a significant effect on the identification of vowels often confused by Nucleus cochlear implant recipients using the Spectral Peak (SPEAK) speech coding strategy. Specifically, identification performance for synthetic vowels with the first two formants (F1 and F2) parametrically varied in bandwidth was explored. Eight implanted subjects identified synthetic versions of the isolated vowel sounds [Ι, ε, ʌ, ɝ] with F1 and F2 bandwidth manipulations, as well as foil tokens of [i, u, ɑ, æ, ʌ]. Identification performance was examined in terms of percent correct as well as error patterns. Further analyses compared patterns of electrode activation. In general, broader F1 bandwidths yielded poorer performance and narrower F1 bandwidths yielded better performance relative to identifications for the reference stimuli. However, similar manipulations of F2 bandwidths resulted in less predictable performance. Comparison of electrode activation patterns indicated a distinct sharpening or flattening in the F1 frequency region for subjects with the greatest performance extremes. Manipulation of F1 bandwidth can result in concomitant changes in electrode activation patterns and identification performance. This suggests that modifications in the SPEAK coding strategy for the F1 region may be a consideration. Similar manipulations of F2 bandwidth yielded less predictable results and require further investigation.

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