Effects of vowel context on the recognition of initial and medial consonants by cochlear implant users

Scores on consonant-recognition tests are widely used as an index of speech-perception ability in cochlear implant (CI) users. The consonant stimuli in these tests are typically presented in the /alpha/ vowel context, even though consonants in conversational speech occur in many other contexts. For this reason, it would be useful to know whether vowel context has any systematic effect on consonant recognition in this population. The purpose of the present study was to compare consonant recognition for the /alpha, i/, and /u/ vowel contexts for consonants presented in both initial (Cv) and medial (vCv) positions.

Twenty adult CI users with one of three different implanted devices underwent consonant-confusion testing. Twelve stimulus conditions that differed according to vowel context (/alpha, i, u/), consonant position (Cv, vCv), and talker gender (male, female) were assessed in each subject.

Mean percent-correct consonant-recognition scores were slightly (5 to 8%) higher for the /alpha/ and /u/ vowel contexts than for the /i/ vowel context for both initial and medial consonants. This general pattern was observed for both male and female talkers, for subjects with better and poorer average consonant-recognition performance, and for subjects using low, medium, and high stimulation rates in their speech processors. In contrast to the mean data, many individual subjects demonstrated large effects of vowel context. For 10 of 20 subjects, consonant-recognition scores varied by 15% or more across vowel contexts in one or more stimulus conditions. Similar to the mean data, these differences generally reflected better performance for the /alpha/ and /u/ vowel contexts than for the /i/ vowel context. An analysis of consonant features showed that overall performance was best for the voicing feature, followed by the manner and place features, and that the place feature showed the strongest effect of vowel context. Vowel-context effects were strongest for the six consonants /d, j, n, k, m/, and /l/. For three of these consonants (/j, n, k/), the back vowels /alpha/ and /u/ produced substantially (30 to 35%) higher mean scores than the front vowel /i/. For each of the remaining three consonants, a unique pattern was observed in which a different single vowel produced substantially higher scores than the others. Several additional consonants (/s, g, w, b/, and /d/) showed strong context effects in either the initial consonant or medial consonant position. Overall, voiceless stop, nasal, and glide-liquid consonants showed the strongest effects of vowel context, whereas the voiceless fricative and voiceless affricate consonants were least affected. Consistent with the feature analysis, a qualitative assessment of phoneme errors for the six key consonants indicated that vowel-context effects stem primarily from changes in the number of place-of-articulation errors made in each context.

Vowel context has small but significant effects on consonant-recognition scores for the "average" CI listener, with the back vowels /alpha/ and /u/ producing better performance than the front vowel /i/. In contrast to the average results, however, the effects of vowel context are sizable in some individual subjects. This suggests that it may be beneficial to assess consonant recognition using two vowels, such as /alpha/ and /i/, which produce better and poorer performance, respectively. The present results underscore previous findings that poor transmission of spectral speech cues limits consonant-recognition performance in CI users. Spectral cue transmission may be hindered not only by poor spectral resolution in these listeners but also by the brief duration and dynamic nature of consonant place-of-articulation cues.