Production and perception of stop consonants by a
hearing-impaired Brazilian subject
PaPI - 2005
Luisa Barzaghi, Sandra Madureira
Pontifícia Universidade Católica de São Paulo – Brasil
INTRODUCTION
The relationship between hearing impairment and speech perception and production has not been yet fully understood. Results of studies on the hearing-impaired subjects´ speech production have pointed out the following aspects: difficulties in implementing the voicing contrast (Monsen, 1976; McGarr, Löfqvist, 1982, 1988); lack of transitional cues, indicating a lower degree of coarticulation (Tye-Murray, 1987); differences regarding F2 trajectories; longer duration of segments (Okalidou, Harris, 1999); stereotyped displacements of the tongue body and excessive opening of the jaw (Tye-Murray, 1991). On the other hand studies on the auditory perception of the stop consonants and other speech segments by hearing-impaired subjects, show that perception is altered by many factors and that the degree of hearing loss alone does not account for the variability in identifying and discriminating sounds, although, in general, it gets worse as the hearing loss increases (Tsui, Ciocca, 2000; Turner, Brus, 2001). Revoile et al (1982, 1999) show that hearing-impaired subjects tend to rely more in temporal cues and less in spectral cues to identify the voicing contrast in English. In identifying the stop place of articulation contrast hearing-impaired subjects are judged to rely more in transition cues than in the burst information (Sammeth et al, 1999).
OBJECTIVE AND THEORETICAL BACKGROUND
The aim of this experimental study, which follows Madureira, Barzaghi and Mendes (2002), is to investigate the production and perception of stop consonants of Brazilian Portuguese (PB) by a hearing-impaired subject (HI) in an interrelated way. A normal hearing-subject’s (NH) speech productions are taken as reference for the sake of spectrographic analysis and also used as stimuli for the discrimination and identification tasks.
The focus is on place and voicing contrasts. Spectrographic analysis and identification and discrimination tests are carried out. The Acoustic Theory of Speech Production (Fant, 1960) and Articulatory Phonology (Browman & Goldstein, 1986; 1990; 1992; Goldstein & Fowler, 2003) provides the theoretical background for the interpretation of the findings in this work.
METHOD
The corpus and subjects were the same used in a previous study (Madureira, Barzaghi & Mendes, 2002). All the subjects were native speakers of PB, from São Paulo, Brazil.
Production Task:
Two subjects participated in the production task: (1) a normal-hearing (NH) 35-year-old woman with clear speech; (2) a 16-year-old, female, with bilateral sensorineural hearing loss, severe in the right ear and profound in the left, acquired at 18 months old after meningitis. At the time of recording, she was attending a regular school and had been engaged in an oral approach rehabilitation program at Derdic-PUC/SP for 14 years (HI). The corpus consisted of ten repetitions of two-syllable words initiated by one of the six stops of PB (/p/, /b/, /t/, /d/, /k/, /g/) inserted in a carrier phrase Diga ____baixinho . The words are paroxytons, which happens to be the most frequent stress pattern in PB (Albano, 1995) and presents a CVCV pattern (V=/a/; /t/ as second consonant). The recording was made in studio, ten repetitions for the six sentences, randomly organized in lists and digitalizated at 22 kHz sampling rates with the help of CLS, Kay Elemetrics. The acoustic analysis (Multispeech, Kay Elemetrics) included: f0 and F1 frequency at the onset of the vowels following the stop consonants; relative duration of the stop consonants and their adjacent segments; F1, F2 and F3 of the vowels preceding and following the stop consonants; F1, F2 and F3 transitions from the stop consonants into the following vowels. Statistics analysis – in order to compare acoustic parameters related to voicing and place contrasts, in the production of each subject, a one-way ANOVA and post hoc comparison Sheffe’s test (alpha set at 0,05) were used.
Identification Task
One hundred and twenty normal-hearing undergraduated students acted as judges and evaluated the oral productions of both participants. The 60 students that judged the HI subject´s productions were not familiarized with hearing-impaired subjects’ speech. The six sentences of the fifth list recorded were randomly presented to the judges through head phones connected to a computer in a silent room. The intensity was adjusted to the most comfortable level for each subject. The subjects were told to write down what they had listen even though it had not sounded like a familiar word.
Discrimination Task
In order to analyze the perception of the stop consonants by the HI subject a closed-set, forced-choice discrimination test was developed (Boothroyd, 1985, 1996). The hearing-impaired subject evaluated both the normal-hearing subject's production and her own.
RESULTS
The results of the acoustic analysis and of identification and discrimination tests indicated the HI subject neither produce nor perceived the voicing contrasts: for the two acoustic voicing correlates investigated in this study (f0 e F1 frequency in the following vowel onset – Fig.1) no significant differences between minimal pairs were found, in accordance with the results of the analysis of VOT and consonant duration presented in our previous study based on the same data. For the NH subject significant differences were found regarding f0 (F(5,54)=30,511,p<0,001; Sheffe: /p x b/; /t x d/ e /k x g/ p<0,001) and F1 onset measures (F(5,54)=119,970, p<0,001; Sheffe: /p x b/ p<0.05; /t x d/ e /k x g/, p<0,001); In the identification task all her productions were correctly identified by the 60 judges.
Figure 1 – f0 and F1 frequency values in the stressed following vowel onset in words “pata, bata, tata, data, cata, gata” for the hearing-impaired (HI) and normal-hearing (NH )subjects
In the identification task the voiced stop consonants produced by the HI subject were only 6% (11 in 180 presentations) correctly identified by the judges (Table 1). In the voicing discrimination task the HI subject showed no correct answers above chance level, either when evaluating the NH subject's productions or her own (Table 2).
Table 1: Judges (n=60) responses to the stop consonants as produced by the hearing- impaired subject (HI)
Table 2: Discrimination task results: hearing- impaired subject’s responses - HI (% of right answers above chance level) to the NH subject’s and her own stop consonants productions
On the other hand, the place of articulation contrast was better produced and perceived than the voicing contrast: judges identified 87% correctly the bilabial stop consonants (105 times in 120 presentations), 64% the alveolar (77 in 120) and 88% the velar (106 in 120) (Table 1). The place of articulation contrast was also better discriminated by the HI subject who presented a higher percentage of expected correct answers in the discrimination test, when the contrast involved bilabial and velar places of articulation - bilabial x velar: 95%; bilabial x alveolar: 35%; alveolar x velar: 60%. She discriminated the NH productions of plosives better than her own - bilabial x alveolar and alveolar x velar: no correct answer above chance level; bilabial x velar: 20% (Table 2) . These results were interpreted in relation to the characteristics of the oral production of stops by the HI subject: the second formant transitions were flat and first and third formants undifferentiated (Fig. 2); the formants onset frequency were also undifferentiated across place as it is showed in Fig. 3 (we found significant differences for the normal-hearing subject’s productions: for F2, F(5,54)=260,548, p<0,001); segments in stressed position were found to be very long (Fig. 4); vowels in stressed position, following all plosives, exhibited very high F1 frequency values which was interpreted as suggesting a greater magnitude of jaw opening (Fig. 5). Figures 4 and 5 presents data related to bilabial plosives but equivalent results were found to the alveolar and velar place of articulation.
Figure 2 - Frequency values in F1, F2 e F3 transition between speech segments in stressed position in the target words “pata, bata, tata, data, cata, gata” for the normal-hearing (NH) and hearing-impaired (HI) subjects
Figure 3 - Frequency values in F2 e F3 stressed vowel onsets (/pa/, /ba/, /ta/, /da/, /ka/, /ga/) for the normal-hearing (NH) and hearing-impaired (HI) subjects
Figure 4 - Relative duration of segments /a Cata/ (where C is /p/ or /b/) and other phonetic unities in the carrier phrase “diga Cata baixinho” as produced by the normal-hearing (NH) and hearing-impaired (HI) subjects
Figure 5 - F1 frequency values of vowels in three positions: post-stressed vowel in the word preceding the target words /pata, bata/ and stressed and post-stressed vowel in these target words, as produced by the normal-hearing (NH) and the hearing-impaired (HI) subjects
FINAL CONSIDERATIONS
Perception and production results were found to be coherent. The analysis of spectrographic data and of the results of the perceptual tasks indicates voicing contrasts were not produced nor perceived by the HI subject. The velar and bilabial place of articulations were better perceived by the HI subject, and also better produced, considering the judges’ evaluation. The analysis of formant transitions indicated lack of cues related to place of articulation and suggests that its identification by the judges has been mostly cued by the burst, and that the burst cue was less effective regarding the identification of the alveolar place of articulation (Smits et al, 1996). The greater difficulty experienced by the HI subject in perceiving the place contrasts in her own productions suggests that the HI subject relied more in the transition cues provided by the coarticulated speech of the NH subject.
Spectrographic analysis of the HI stop speech productions revealed undifferentiated transitions and extremely lengthened stressed vowels with high F1 values, suggesting a greater magnitude of jaw opening in stressed syllable positions. The analysis of the data in the light of Articulatory Phonology enabled us to consider the coordination of the opening gesture of the glottis associated to a greater magnitude of the tongue body gesture represented in the gestural score by a wider box relative to the constriction grade of the tongue body. The fact that the greater magnitude of the tongue body gesture can be represented in the gestural score enhances the explanatory power in relation to the dynamics of speech production implemented by the HI subject to produce stops followed by a low central vowel in stressed syllable position, since a vowel gesture specified with a greater magnitude when preceded by gestures involving complete closure such is the case of plosives imply in greater laryngeal tension making the vibration of the vocal cords more difficult. Accordingly to this is the fact that the HI subject systematically produced voiceless plosives in the stressed syllables of the target words “pata, bata, tata, data, cata, gata” but often produced the voiced alveolar plosive instead of its voiceless counterpart in the unstressed syllable onset. Specifying scaling differences related to the magnitude of the articulatory gestures are helpful to capture the interaction between prosody and segment as described here.
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