فراگویی صحیح واکهها از عوامل مهم ارتباط کلامی موفق است و مشخصات صوتشناختی، به بهترین وجه نحوه فراگویی را نشان میدهد. این مطالعه به تأثیر زمان جراحی کاشت حلزون بر اولین و دومین سازه واکههای فارسی و فضای واکهای در تولید کودکان ناشنوای کاشت حلزون (کودکان کاشت) میپردازد.
سازه اول و دوم واکههای فارسی در تولید کودکان کاشت قبل و بعد از سه سالگی و گروه کنترل شنوای همسن استخراج و مقایسه شد. برای مقایسه گروهها از آزمون کراسکال-والیس با سطح معنیداری 05/0 استفاده شد.
تفاوت معنیداری بین گروهها در سازه اول و دوم واکهها به استثنای هر دو سازه /a/ و سازه دوم /i/ و /e/ دیده شد. طبق آزمونهای تعقیبی منویتنی، کودکان شنوا و کاشت قبل از سه سال تفاوت معناداری در سازه اول /i/، /e/ و /u/ و سازه اول و دوم /o/ و /ɑ/ داشتند. کودکان شنوا و کاشت بعد از سه سال تفاوت معناداری در هشت مورد اولیه نشان دادند. کودکان کاشت قبل و بعد از سه سال نیز در سازه اول /i/ و /e/ و دوم /u/ و /o/ تفاوت آماری معنیداری داشتند.
بسامد سازه اول کودکان کاشت بیشتر از شنوایان بود یعنی واکهها را افتادهتر تولید کردند که احتمالا به دلیل آموزش با دهان باز مربیان برای مشاهدهپذیری محل تولید بود. اما بسامد سازه دوم آنها مؤید تأثیر مطلوب جراحی زودهنگام بر شناسایی محل تولید واکه است زیرا اگرچه همه کودکان کاشت واکههای پسین را جلوتر از محل طبیعی تولید کردند، اما این ویژگی در کودکان کاشت بعد از سه سالگی بارزتر بود.
عنوان مقاله [English]
The effect of cochlear implantation on the acoustic features of Farsi vowels by hearing-impaired children
Cochlear implantation has caused significant improvement in the language ability of cochlear implantees in comparison with the hearing aided and has been preferred to other treatments for sensory-neural sever to profound hearing loss. Although CI users have gained access to auditory feedback, there are still some gap in their speech articulation compared to the normal hearings. One area in which the difference lies is their shortcomings in the accurate production of speech segments. They evidence difficulties in the production of vowels which are determinant factors in speech intelligibility and successful communication. As studying vowels from an articulatory perspective does not provide objective results, researchers have shown an inclination for acoustic studies to investigate the differentiating features, namely the first and second formants (F1 and F2), of the vowels articulated by CI speakers. Frequency of the first formant of the vowels (F1) correlates with articulatory height of the vowels and frequency of the second formant (F2) with the tongue position (the front/back dimension). Therefore, F1/F2 ratio – vowel space – provides both an objective representation of the vowel articulation accuracy and the main acoustic cue in auditory perception for listeners. The studies on the acoustic features of the vowels have demonstrated indefinite and contrasting results in comparative investigations of CI users and normal hearing listeners. Some have reported a significantly reduced vowel space for CI users, however, an increase in the vowel space has been mentioned in others which evidence the positive impact of CI, and other studies have even claimed an improvement to the point of similarity of vowel spaces after implantation. Inconsistent findings seem to be due to methodological differences such as considering different variables such as post or pre-lingual deafness, CI adults or children, different ages at implantation and time span after the surgery. Among these factors, age at implantation has wide-ranging consequences and requires more in-depth studies. This study aims at investigating the effect of early cochlear implantation on the first and second formants (F1 and F2) of the Persian vowels produced by cochlear implanted (CI) children.
To this end, the vowel production of two groups of children, implanted before and after three years (CI<3, CI>3) and with at least three years of device use, were studied. All CIs had been diagnosed with severe congenital hearing loss at both ears without any other disability and physical condition. There was a control group of five normal hearing age-matched children (NH group). Each participant repeated after a speech therapist 36 CV syllables made up of eight Persian plosives as onset and six Persian vowels as nucleus. Using Praat software, frequency of the first and the second formants of the vowels produced by children in three groups were extracted and then compared with each other. To find between-group variances, Kruskal-Wallis test was used with the significance level set at 0.05.
Findings of this study demonstrated that there were significant differences among the groups in the frequency of the first and second formants of all Persian vowels except for F1 and F2 of /æ/ and F2 of /i/ and /e/ (eight cases). Mann–Whitney U test showed that CI3 and normal hearing groups demonstrated significant differences in the first mentioned eight cases (F1 of /i/ and /e/ and F1 and F2 of /u/, /o/ and /a/). CI3 groups were statistically different from each other in F1 of vowel /i/ and vowel /e/ and F2 of vowel /u/ and vowel /o/.
Considering the frequency of the first formant, we did not find any association between early implantation and F1 of the vowels by CIs. All CI users produced vowels with a more open mouth than the normal hearing, which seems to be due to the exaggerated training method of therapists producing the vowels with more open mouth to give a visual cue to CIS about the vowels’ articulation. In terms of F2, on the contrary, there is a clear relationship between age at implant and vowel production. Although all CIs showed lower F2 than NHs and articulated back vowels more front than their normal place, the deviation was more observable in CI children who had undergone surgery after the age 3 (CI>3). This indicates that early implantation positively affects the cochlear implanted ones in articulating vowels with normal like F2 as an important distinguishing acoustic feature of vowels.