A Correlation between Speech Intelligibility and Vowel Acoustic Measurements in Persian Speaking Spastic Cerebral Palsy Children with Dysarthria

Document Type : مقالات علمی پژوهشی

Authors
M. Mokhlesin 1
M. Nourbakhsh 2
B. Mansuri 3
R. Nahidi 4
N. Tahmasebi 5
1 Neuromuscular Rehabilitation Research Center, Semnan University of Medical Sciences, Semnan, Iran & Speech Therapy Department, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran
2 Linguistics Department Faculty of Literature, Alzahra University, Tehran, Iran
3 Neuromuscular Rehabilitation Research Center, Semnan University of Medical Sciences, Semnan, Iran
4 Student Research Committee, Semnan University of Medical Sciences, Semnan, Iran
5 Speech Therapy Department, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran
10.48311/LRR.15.4.275
Abstract
One of the dysarthria features is inaccurate articulation of consonants and vowels, which is the main cause of speech unintelligibility. However, it is not clear what acoustic measures are more related to speech intelligibility in dysarthria. On the other hand, the number of vowels, their articulation and acoustic characteristics are various in different languages. As far as we know, no study investigated vowels articulation in developmental dysarthria in Persian, so there is not enough information about movement details which make their inaccuracies articulation. Hence, the present study aimed to investigate some acoustic measures including vowel formants, vowel space area (VSA), formant centralization ratio (FCR) and vowel articulation index (VAI) and aim to study their correlation with speech intelligibility in Persian speaking children with dysarthria due to spastic cerebral palsy. In this cross ـ sectional study, 11 monolingual children with dysarthria aged 3 ـ 9 years and their typically developing matched peers were selected through convenience sampling. For each of the six Persian vowels, two monosyllable words were acoustically analyzed using PRAAT software. To determine intelligibility of words and vowels, the percentage of correct detected ones was calculated. Results showed a significant difference between the two groups in the third formant of the vowels /a/ and /u/ and the second formant of the vowel /o/. The vowel space area and intelligibility were significantly reduced in dysarthria. VAS was more correlated with intelligibility and is a good acoustic measurement to assess speech intelligibility in dysarthria. However all three studied acoustic indices had moderate significant correlation with intelligibility.



1. Introduction

Abnormalities in power, speed, range, muscle tone, stability, and accuracy of movements necessary for respiration, phonation, production, articulation, or prosodic aspects of speech lead to dysarthria (Duffy, 2019). Depending on the location of the damage in the nervous system, there are different types of dysarthria and each creates different speech characteristics (Duffy, 2019). Cerebral palsy is one of the main causes of dysarthria in children. One of the main and distinctive features of dysarthria is imprecise production, which is the main factor for low speech intelligibility (De Bodt et al., 2002). However, there is limited information on the motor features that cause this inaccuracy in children’s production, and awareness of these motor events is a prerequisite for providing appropriate therapeutic approaches (Allison et al., 2017). If we consider syllables as speech motor units (Yunusova et al., 2008) and vowels as the core components of syllables, vowel distortion can affect the structure of syllables and consequently speech. Describing the vowels of a language has more auditory aspects than articulatory ones. However, gradually linguistics realized that if the vowels of a language are displayed based on F1 and F2 features, the result will be a two-dimensional space that they named vowel space. Since speech intelligibility is the primary goal of clinical interventions, and since vowels are a good predictor of speech intelligibility in dysarthria, it is necessary to accurately determine the correctness and accuracy of vowels with the help of appropriate diagnostic or perceptual tools to assess the effects of direct and indirect therapies on the proper production of vowels and the improvement of speech intelligibility. This diagnosis guides therapists in selecting appropriate treatment methods or continuing therapy.

Research Question(s)

Is the formant of each of the 6 vowels in Persian-speaking children with spastic dysarthria different from that of typically developing children? Are speech intelligibility, acoustic parameters of vowel space, vowel production indices, and formant centralization ratio different in children with dysarthria based on age? Is there a correlation between some acoustic parameters of vowels and speech intelligibility?





2. Literature Review

In 2019, a study was conducted in China on the Mandarin language with the aim of determining acoustic parameters for speech evaluation in children with dysarthria due to cerebral palsy. Thirty children aged 5 to 15 with cerebral palsy and 13 typically developing children participated in the study. Formants F1 and F2 were calculated for vowels /a, i, u/. The VSA, VAI, and FCR indices were also calculated and analyzed. In children with cerebral palsy, there were significant differences in F1 and F2 for the vowel /a/ and F2 for the vowel /i/, while there were no significant differences in F1 and F2 for the vowel /u/ and F1 for the vowel /i/. There were significant differences between the two groups in the FCR, VAI, and VSA indices (Mou et al., 2019). In another study in 2017 aimed at improving the understanding of speech characteristics related to dysarthria in English-speaking children with cerebral palsy in the United States, formants were analyzed at the word and sentence levels. The speech of 10 children with dysarthria due to cerebral palsy was compared to 10 typically developing children. The results showed that children with dysarthria had a smaller vowel space compared to typically developing children at both the word and sentence levels (Allison et al., 2017). The same researchers conducted another study in 2018 with the aim of determining acoustic characteristics of connected speech in children with dysarthria due to cerebral palsy and typically developing children. Twenty children with dysarthria were matched with typically developing children in terms of age and gender. The results showed that children with dysarthria differ from typically developing children in terms of speech-segmental and suprasegmental features (Allison & Hustad, 2018). In 2016, a study titled “Vowel Intelligibility in Children with or without Dysarthria” was conducted. Researchers in this exploratory study examined the vowel intelligibility of American English vowels in American children. Generally, children with dysarthria had lower vowel intelligibility compared to typically developing children, but there was no statistically significant difference in intelligibility among different vowels. These studies highlight the significance of focusing on the vowel system during therapy to enhance intelligibility (Levy et al., 2016).



3. Methodology

In the present cross-sectional study, 11 children with spastic dysarthria due to cerebral palsy were selected from rehabilitation clinics in Semnan city, and 11 typically developing age- and gender-matched children from preschools and primary schools in the same city. The inclusion criteria for children with cerebral palsy in the study were mild to moderate dysarthria due to spastic cerebral palsy, aged 3 to 9 years, and the ability to produce single words. Exclusion criteria were the presence of sensory or motor problems, bilingualism, dysphagia, and difficulties in producing consonants present in test words. Initially, 12 monosyllabic words with a CVC structure including 2 words for each of the 6 vowels in the Persian language were selected by a linguist and a speech-language pathologist, which were simple to produce in terms of consonants and did not affect the central vowel. Each word was uttered twice, so 4 words were spoken for each vowel, with 3 of them selected for formant analysis using PRAAT software. Subsequently, the spoken words by each child were presented to adults unfamiliar with the child, who were free of hearing or language issues, to write down the words after hearing them. The researchers determined the number of correct words and the vowels identified by adults to establish the percentage of vowel and speech intelligibility for each child. Finally, the two groups of children were compared in terms of vowel space and speech intelligibility.



4. Results

In this study, each group of children with normal speech and dysarthria included 4 girls and 7 boys with a minimum age of 3.5 and a maximum age of 9 years. The mean age in children with dysarthria was 1.9 ± 5.6 years and in the group with normal speech was 1.8 ± 5.7 years, with no significant difference between them (p > 0.05). Since results in speech production change with age due to vocal tract lengthening and children’s motor skill development, the children in this study were categorized into 2 subgroups of 3.6 to 6 years and 6 to 9 years. Comparison of vowels in children with dysarthria and their age-matched typically developing peers using the Mann-Whitney test showed a significant difference between the two groups only in 6 to 9-year-old children in the first formant of /i/ and /æ/ vowels, the second formant of /æ/ vowel, and also the third formant of /e/ and /a/ vowels. Speech intelligibility at the single-word and vowel level, acoustic parameters of vowel space, vowel production indices, and formant centralization ratio were compared between the two groups of children with dysarthria at different ages using the Mann-Whitney test, which showed a significant difference in word and vowel intelligibility between the two groups. All acoustic indices also showed a significant difference between the two groups. Vowel space, calculated based on the /a/, /i/, and /u/ side vowels in this study, was smaller in the group of children with dysarthria compared to the normal group. In both age groups, the dispersion of vowel production in the dysarthric group was higher compared to the normal children. The results of the Spearman correlation coefficient test showed a significant moderate relationship between all indices of vowels and speech intelligibility. This relationship was direct for the Vowel Space Area (VSA) and Vowel Articulatory Index (VAI) and inverse for the Formant Centralization Ratio (FCR).

 

Keywords

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Language Related Research
Volume 15, Issue 4 - Serial Number 82
September and October 2024
Pages 275-302