The influence of cochlear implantation on vowel articulation.

PURPOSE: Speech of deaf persons differs considerably from that of speakers with normal hearing. The purpose of this study was to investigate the acoustic changes in articulation of corner vowels in deaf children and adults after cochlear implantation.
METHODS: Thirteen prelingually deaf children and 12 postlingually deaf adults were included in the study. Voice samples of the isolated corner vowels /a/, /i/ and /u/ were analyzed before and 6-12 months after the implantation. The frequencies of the first (F1) and second (F2) formants, the F1/F2 ratio of all three corner vowels, and the area of the vowel triangle were calculated and compared before and 6-12 months after the implantation. RESULT: In the adults, no significant differences were detected in the formant frequencies, the F1/F2 ratio or the area of the vowel triangle. However, significant change in formant frequencies was detected in the group of 13 prelingually deaf children. After the implantation the F1 of /u/ decreased significantly, and favorable decreases of the F1 of /i/ and the F1/F2 ratio in /i/ and /u/ were close to being statistically significant. All changes caused better phonological difference between the two vowels. The significant change in the F1 of /u/ and the change of F1 of /i/ resulted in the expansion of the vowel space, which was expressed as an increase in the area of the vowel triangle.
CONCLUSIONS: We suggest that in children the acquired hearing ability and further development of neuromuscular control of articulation are the reasons for the significant improvement after cochlear implantation. The results also suggest that the area of the vowel triangle is a useful and sensitive indicator of the more precise articulation after implantation. In order to achieve better and faster improvement of articulation, regular speech therapy should be included in the rehabilitation of deaf persons after cochlear implantation.