A case study of the speech, language and vocal skills of a set of monozygous twin girls: one twin with a cochlear implant

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1 Cochlear Implants International, 2(1), 1 16, 2001 Whurr Publishers Ltd 1 A case study of the speech, language and vocal skills of a set of monozygous twin girls: one twin with a cochlear implant MARY BELL, LOUISE HICKSON, GAIL WOODYATT, Department of Speech Pathology and Audiology, The University of Queensland, Brisbane, Queensland 4072, Australia DIMITY DORNAN, The Hear and Say Centre for Deaf Children, Brisbane, Queensland 4066, Australia ABSTRACT This case study reports on the speech, language and vocal skills of a set of monozygous twin girls; one with a pre-lingual hearing impairment and subsequent cochlear implant, the other with functionally normal hearing. The twins skills were analysed and compared on two occasions over a seven-month period. Results from standardized assessments indicate that the speech-production and language skills of both girls were within normal limits, although analysis of their pragmatic, syntactic and discourse skills indicated that the implanted twin had some persisting difficulties. Analysis of the voice of the implanted twin indicated qualitative and quantitative abnormalities. The results suggest the need for an investigation into the voice quality of children with cochlear implants. The study also indicates the importance of early intervention and the potential benefit of early implantation and/or signing pre-implant. Keywords: cochlear implants, monozygous twins, speech, language, voice Introduction Much has been written about the application and potential benefits of the cochlear implant for children with profound hearing losses. The level of speech perception typically provided by a cochlear implant should allow an increased opportunity to develop speech and oral language skills through listening (Staller et al., 1991; Boothroyd and Eran, 1994; Geers and Brenner, 1994; Miyamoto et al., 1997a; Snik et al., 1997). Thus, in recent years, expectations of the post-operative speech-production and language outcomes for children with cochlear implants have risen. Some aspects of language, particularly vocabulary development, have been extensively investigated in children with

2 2 Bell et al. cochlear implants, while other areas, such as syntax, semantics, phonology and pragmatics, have received less attention, along with investigations of speechproduction and vocal skills. Typically, the only children found to have attained age-appropriate skills within a time frame of two to three years post-implant are those who were deafened post-lingually (Dawson et al., 1995). While research supports an enhanced rate of speech-production and language development in pre-lingually deaf children with cochlear implants after two to three years of device use, as a group they continue to present with a delay overall (Hasenstab and Tobey, 1991; Geers and Moog, 1992, 1994; Geers and Tobey, 1992; Tobey et al., 1994; Waltzman et al., 1994, 1997; Dawson et al., 1995; Tye-Murray et al., 1995; Miyamoto et al., 1997a,b; Serry et al., 1997). In a best-case scenario, is it possible for a pre-lingually deaf child with a cochlear implant to bridge the gap between her hearing age (time post-implant) and chronological age within two to three years? If so, what factors are most likely to contribute to such an accelerated rate of development? The present study allowed a unique opportunity to answer these questions and investigate the speech, language and voice outcomes for a child with a cochlear implant, with her identical twin sister as a control. Rather like the study by Kent et al. (1987), which investigated the effect of hearing loss on vocal development in a set of monozygous twin boys, one with a profound hearing loss and one with normal hearing, this study offers a rare chance to study the effects of a cochlear implant and other factors in promoting speech and language development, with some control over environmental and genetic factors. This child s results indicate what may be possible in a best-case scenario for prelingually deaf children who receive a cochlear implant in their early years. Methods Participants A set of female monozygous twins, Caroline and Helen (pseudonyms), participated in this study. Caroline was diagnosed with profound bilateral hearing loss of unknown aetiology at four months of age and was fitted with high-powered hearing aids one month later. However, her access to speech with hearing aids was minimal, so she received a Nucleus-22 channel cochlear implant at 2:2 years of age. Soon after her diagnosis, Caroline was enrolled in a total communication (TC) programme. She also began weekly auditory verbal (A V) therapy sessions (Dornan, 1999) prior to receiving a cochlear implant, and these continued following activation of her device. Caroline s parents gradually phased out their use of signing after she received a cochlear implant. No formal assessments of Caroline s speech or language skills were available pre-implant, but Caroline s speech pathologist reported that her signing vocabulary was age-appropriate, while her spoken vocabulary was minimal. Caroline s speech perception results indicate that she receives substantial benefit from the

3 Speech, language and vocal skills with a cochlear implant 3 cochlear implant auditory alone. When measured at 2:6 years post-implant she scored 92% on the word intelligibility by picture identification test, a sixchoice closed-set test (Ross and Lerman, 1971), and 98% on the Bamford Kowal Bench sentences in 45 db sound pressure level (SPL) background noise (Bench and Doyle, 1979). Helen was thought to have normal hearing before the study began. However, audiological assessment conducted as part of this study indicated that she had a moderate to profound mixed loss in her left ear. Nonetheless, Helen s hearing was considered to be functionally normal, as her hearing thresholds in the right ear were within normal limits. Materials and procedures The twins were assessed individually by an unfamiliar examiner on two separate occasions when they were aged 4:4 and 4:11 years. Assessments of cognition, speech, language and voice were conducted. Some of Caroline s early language results at 2:11 years (nine months post-implant) and 3:10 years (20 months postimplant) were also available, as well as an assessment of Helen s receptive vocabulary at 3:10 years. These assessments were administered by their A V therapist. Cognition The test of visual motor integration (VMI) (Beery and Buktenica, 1989) was used to estimate the twins non-verbal intelligence. In this assessment, the child is required to copy geometrical figures of increasing complexity. Speech The speech assessment consisted of the Goldman Fristoe test of articulation (Goldman and Fristoe, 1986), a test of a child s ability to produce different sounds at a single-word level. Results from this test were analysed both phonetically (analysis of sound errors) and phonologically (analysis of phonological processes used). Any speech errors were identified and the phonological processes used were categorized, as specified by Dodd and Iacono (1989), as being either developmental (i.e. typical of the development of children with normal hearing) and/or unusual (i.e. not typical of the development of children with normal hearing). Language A battery of standardized language assessments were administered, as follows: The Reynell developmental language scales (RDLS) (Reynell, 1987), an assessment of the understanding of spoken language in children aged

4 4 Bell et al. between two and six years. It examines a child s understanding of simple object names and actions and longer utterances incorporating concepts such as quantity, position and colour. The clinical evaluation of language fundamentals preschool (CELF-Pre) (Semel et al., 1992), a comprehensive assessment of receptive and expressive language in children aged between three and seven years. Peabody picture vocabulary test revised (PPVT-R) (Dunn and Dunn, 1981), a measure of receptive vocabulary that can be used with children from two years of age to adult. No spoken output is required. Receptive vocabulary provides an estimate of verbal intelligence and relates to cognitive ability, but only in the absence of a specific language disorder (Dunn and Dunn, 1981). The expressive one-word picture vocabulary test revised (EOWPVT-R) (Gardner, 1987), a measure of expressive vocabulary of children aged between two and 11 years. All these assessments have been standardized with a normal-hearing population and yield standard score data. In addition to the standardized language assessments, a 30-minute language sample was obtained from both of the twins, to examine their communicative ability in an informal communicative setting. A series of language analyses were performed on each language sample and included: The language assessment and remediation screening procedure (LARSP) (Crystal et al., 1976), a comprehensive syntactic analysis which identifies seven stages of syntactic development, corresponding to chronological ages of 0:9 to 4:6 years and describes the syntactic characteristics of each of the seven stages. Dore s conversational acts (Dore, 1978), a coding scheme used to categorize the utterances of children between three and five years of age, in terms of form, function, semantic content and conversational contingency. Voice Both acoustic and perceptual analyses of voice were performed. Acoustic analysis was conducted on prolonged vocalizations of three vowels /a/, /i/ and /u/, a word (ladder) and a sentence (It s a story about a zoo). The Computerised Speech Laboratory (CSL) (model 4300, Kaye Elemetrics Corporation) was used to capture and analyse the data. Each production was captured using a 10 khz sampling rate. Mean fundamental frequency, jitter (variation in fundamental frequency), shimmer (variation in intensity) and harmonic-to-noise ratio (a general evaluation of the noise present in a signal, such as amplitude and frequency variations, turbulence noise, subharmonic components and/or phonation breaks) were calculated from the stable mid-portion of the vowel.

5 Speech, language and vocal skills with a cochlear implant 5 For the words and sentences, mean fundamental frequency was obtained along with total duration (seconds). Additional analysis of the twins voices was made at 4:11 years using the voice-quality assessment program of the Visi-Pitch II (model 3300, Kay Elemetrics Corporation). Jitter, shimmer, harmonic-to-noise ratio and voice turbulence index (the relative energy level of high-frequency noise in the sample) were calculated from a sustained /a/ for 3 s. Descriptive analysis of the twins voices was undertaken by 39 final-year speech pathology students at The University of Queensland. The students were asked to listen to a tape-recording of each child telling a story to the researcher and to subjectively describe the features of both twins voices, according to the Oates profile (Oates, 1983), including: frequency (modal, variability, range); intensity (modal, variability, range); phonation type (breathy, harsh, whispery, glottal fry, falsetto); continuity (tremor, diplophonia, phonation breaks, pitch breaks, loudness breaks); and velopharyngeal function (hypernasal, hyponasal, undetermined/confounded). The students were also asked to identify which child was hearing-impaired. Inter-rater reliability for this task was not measured. Results Cognition The VMI test was unavailable for use when the twins were 4:4 years of age. When tested at 4:11 years of age, the cognitive skills of both girls were within normal limits (WNL) for their age (i.e. within one standard deviation from the mean). Speech The results for articulation and phonology are summarized in Table 1. Both children showed limited evidence of delayed speech and phonology at the final assessment (Kilminster and Laird, 1978; Hodsen and Paden, 1983; Grunwell, 1987). Caroline exhibited one unusual process at 4:4 years, but this had resolved by 4:11 years. Helen did not exhibit any unusual processes. Language The standard score results obtained on a number of the quantitative language assessments are included in Table 2. At the final assessment, Caroline s standard scores are either WNL or more than one standard deviation above the mean, whereas those for Helen tend to be significantly above average. The ageequivalent scores obtained on the PPVT-R show that Caroline s rate of receptive vocabulary development (i.e. change in age-equivalent test score in months over a 12-month period) prior to being implanted and up to nine

6 6 Bell et al. Table 1: Summary of speech-production results Caroline (CI) Helen 4:4 years 4:11 years 4:4 years 4:11 years Phonetic errors /r im / /w/ a none /r im / /w/ a /r im / /w/ a /th imf / /f/ a /th imf / /f/ a Phonological Articulatory shift (/TH/ w m ) a Articulatory shift (/TH/ w m ) a Articulatory shift (/TH/ w m ) a Articulatory shift (/TH/ w m ) a processes Velar fronting (/g/ only w im ) b Gliding of /r/ w i and blends Velar fronting (/k/+/g/ w i ) b Articulatory shift (/th/ w imf ) a FCD (/f/ only) b FCD (/f/+/th/ only) b Velar fronting (/k/+/g/ w i ) b CR (fricative-stop blends only) b Devoicing (alveolar stops only w m ) c CI = cochlear implant; FCD = final consonant deletion; CR = cluster reduction. a Age-appropriate; b delayed phonological process; c unusual phonological process. w i, word initially; w m, word medially; w f, word finally.

7 Speech, language and vocal skills with a cochlear implant 7 months post-implant was nine months gain in 12 months. From 3:10 to 4:11 years of age (18 months to 33 months after implantation), Caroline s rate of receptive vocabulary development was 17 months gain in 12 months, while Helen s rate of vocabulary development for the same period was 14 months. Over the seven-month period of the study, Caroline s expressive vocabulary (as measured by the EOWPVT-R) had increased by 10 months and Helen s had increased by 19 months. Both girls scored at least WNL on the RDLS and CELF-Pre on the two test occasions. Caroline s age-equivalent scores on the RDLS indicated month-for-month progress (13 months in 13 months), while Helen had an enhanced rate of development (10 months in seven months). On the CELF-Pre, Caroline s age-equivalent scores indicated an enhanced rate of development (11 months in seven months), while Helen s scores indicated less than month-for-month progress (three months in seven months), although this is likely to be due to a ceiling effect on the test, as Helen s skills were already significantly above average when assessed at 4:4 years of age. Caroline s results for the CELF-Pre indicated a trend for her receptive skills to be in advance of her expressive skills. Overall, analysis of the twins LARSP profiles indicated that Helen had superior syntactic development, compared with Caroline, with a wider variety and greater complexity of syntactic structures in use. Caroline had a larger number of errors in all categories. Helen also demonstrated development in advance of Caroline in discourse level structures, such as the use of co-ordinating and subordinating markers and pronoun referents. Analysis of the twins language with the LARSP allowed errors to be observed that were not identified by the other language tests. At 4:4 and 4:11 years, both Caroline and Helen used a wide variety of the pragmatic functions as measured in Dore s conversational acts and had a similar proportion and distribution of pragmatic categories. Caroline appeared to be less communicative generally and this was reflected in a larger proportion of responses to requests. She also provided inadequate or ambiguous information to the listener on several occasions, whereas Helen was never marked as inappropriate on this measure. Voice, acoustic analysis Analysis of the twins voices was undertaken using the voice quality assessment program of the Visipitch-II. The results are summarized in Table 3. The normative data in the table are taken from the voice quality assessment program, other than the mean fundamental frequency for children of five years of age, which is based on data presented by Eguchi and Hirsh (1969). Normative values were not available for the highest, lowest or standard deviation of fundamental frequency. Caroline s results were well outside the norms provided for relative average perturbation (jitter), shimmer, voice turbulence index and noise-to-harmonic ratio, and were much greater than those obtained by Helen.

8 8 Bell et al. Table 2: Summary of language assessment standard scores Caroline (CI) Helen Test 2:11 years 3:10 years 4:4 years 4:11 years 3:10 years 4:4 years 4:11 years PPVT-R 82 c 87 a 103 a 112 a 105 a 117 b 114 a EOWPVT-R 104 a 106 a 110 a 118 b RDLS 104 a 113 a 108 a 117 b 125 b CELF-Pre 103 a 120 b 137 b 139 b CI = cochlear implant. a WNL. b More than one standard deviation above the mean. c More than one standard deviation below the mean.

9 Speech, language and vocal skills with a cochlear implant 9 Helen s results were outside the norms for shimmer and just outside the norms for voice turbulence index. Caroline s voice was characterized by a considerably larger standard deviation of fundamental frequency (71 Hz for Caroline, 7 Hz for Helen). Voice, descriptive analysis All 39 listeners correctly identified Caroline as the child who was hearingimpaired. For Caroline, the most commonly reported voice features were: phonation breaks (49%), limited pitch variability in conversation (52%), narrow pitch range (62%) and breathy phonation type (67%). For Helen, every measure was reported by the majority of students (> 75%) to be WNL for a child of this age and gender. Discussion This case study compared the speech-production, language and vocal skills of a pair of monozygous twin girls, one who is profoundly hearing-impaired and has a cochlear implant, and one who has functionally normal hearing. Results showed that both girls had speech-production and language skills that were essentially WNL for their age, although there were differences in performance between the twins. Cognition Children with hearing impairment often present with a delay, even on tests measuring non-verbal intelligence (Marschark, 1993). This was not true for Caroline, with both of the twins scoring WNL for their age on the test of cognition. Table 3: Voice parameters from the voice quality assessment program Voice parameter Normative values Caroline (CI) Helen Average fundamental frequency (Hz) 289 a Highest fundamental frequency (Hz) Lowest fundamental frequency (Hz) s.d. of fundamental frequency (Hz) Relative average perturbation % 0.68 b Shimmer % 3.81 b Noise-to-harmonic ratio 0.19 b Voice turbulence index 0.06 b CI = cochlear implant. a Eguchi and Hirsh (1969). b Voice quality assesment program, Visi-Pitch II.

10 10 Bell et al. Speech At the second assessment, when aged 4:11 years and with Caroline having 2:9 years of implant experience, the speech-production skills of both children were essentially WNL. Both of the twins persisted in using one developmental process, but their other errors were appropriate for their age. Thus, the speechproduction skills of this child with a cochlear implant, who was deafened prelingually, were essentially appropriate for her age by two to three years post-implant. The studies that are available do not typically discuss whether the speech-production skills of children with cochlear implants are WNL according to chronological age. Rather, they discuss the acquisition of various groups of sounds by children with cochlear implants (Blamey et al., 1992; Grogan et al., 1994; Osberger et al., 1994; Serry et al., 1997) or they compare their skills to those of children who have hearing aids and/or vibrotactile aids (Geers and Tobey, 1992; Tobey et al., 1994). Children who are hearing-impaired typically use a greater number of unusual phonological processes in their speech than children with normal hearing (Murphy and Dodd, 1995). Caroline used one unusual phonological process in her speech at 4:4 years of age, but did not use this process in her speech at 4:11 years of age. She did use a developmental process (final consonant deletion of fricatives) inconsistently on both test occasions. This phonological process is commonly observed in the speech of children who are hearingimpaired (Geffner, 1980; Bench, 1992) and who have cochlear implants (Grogan et al., 1994). Helen did not use any unusual phonological processes in her speech on either test occasion, although she persisted in using a developmental process (i.e. velar fronting). Children from multiple birth sets often persist in the use of developmentally delayed phonological processes in their speech (McEvoy and Dodd, 1992). In Caroline s case, it was difficult to separate the potential influences of both her hearing impairment and multiple birth status on her phonological development. Helen s continued use of a developmentally delayed phonological process in her speech may be attributable to the fact that she is from a multiple birth set. Language At the completion of the study, both twins scored WNL on all standardized language assessments. However, Helen s scores were consistently higher than Caroline s. This suggests that even though Caroline was performing at an ageappropriate level, she still had a possible language delay, compared to her genetic potential. Caroline s receptive and expressive vocabulary results indicate that she is a member of a small group of pre-lingually deaf children with cochlear implants implanted between two and five years of age who achieve age-appropriate skills

11 Speech, language and vocal skills with a cochlear implant 11 in these areas within a time frame of two to three years post-implant. Additionally, Caroline s receptive and expressive language skills, as measured by the CELF-Pre and RDLS, indicated that she had an age-appropriate mastery of syntax, grammar and semantics. As a group, the receptive and expressive vocabulary skills of pre-lingually hearing-impaired children with cochlear implants are typically substantially delayed at the two- to three-year post-implant mark, although their rate of development is much enhanced (Waltzman et al., 1994, 1997; Dawson et al., 1995; Miyamoto et al., 1997b). Caroline s rate of receptive vocabulary development was enhanced when measured post-implant, and at that time, was greater than that of Helen. Her receptive vocabulary skills first fell WNL at 3:10 years of age, 1:8 years post-implant. Helen s rate of expressive vocabulary development was greater than Caroline s in this period, but Caroline s rate of development remained better than expected for children with normal hearing. When first measured at 3:10 years of age (20 months postimplant), Caroline s syntactic, semantic and grammatical skills were already within the normal range for her chronological age. She continued to make month-for-month progress when assessed at 4:4 and 4:11 years of age. Her rate of development of these language skills at this time was either equal to or faster than expected from children with normal hearing. There are a number of possible reasons for Caroline s excellent speech and language skills, at approximately two and a half years post-implant. One factor was that she received her cochlear implant early, when aged 2:2 years, and received intervention from a young age, while still in the critical period for language acquisition. Early implantation has been identified as a contributing factor to later speech perception and/or language success by a number of authors (Fryauf-Bertschy et al., 1992; Tye-Murray et al., 1995; Cowan et al., 1997; Gibson et al., 1997; Miyamoto et al., 1997a; Tyler et al., 1997). Caroline also had an age-appropriate signing vocabulary pre-implant, thus providing her with a solid language base for the cochlear implant and A V intervention to build on. As a consequence of both her early intervention and age-appropriate signing vocabulary pre-implant, Caroline s language delay at the time of receiving her cochlear implant was small. Another factor that was thought to have contributed to Caroline s success was that she developed excellent speechperception ability through her device. This was likely to have allowed her to access and learn spoken language through overhearing, rather than only accessing language specifically directed to her (Flexer 1994). Similarly, enhanced speech intelligibility has also been found to be positively correlated with better speech recognition skills (Tye-Murray et al., 1995; O Donoghue et al., 1999). It was also felt that the high quality of intervention provided by Caroline s parents contributed to her success, as well as having a hearing twin sister with excellent speech and language skills. Analysis of the twins language skills using the LARSP provided an opportunity to examine their connected language in more depth. The LARSP revealed some subtle differences in discourse and showed that Caroline made a greater

12 12 Bell et al. number of syntactic errors than Helen. These errors were not evident in any of the other language assessments used in the study. This indicates that a comprehensive evaluation of the language skills of children with cochlear implants must include an analysis of their discourse skills, as well as standardized tests of language ability. Consistent with her speech-production and language skills, Caroline s pragmatic skills were more characteristic of a child with normal hearing than a child with a profound hearing loss. She did, however, provide inadequate or ambiguous information to the listener on several occasions during the assessment. This pragmatic feature is a frequent error of school-aged children with mild-to-moderate hearing impairment (Elfenbein et al., 1994). Voice Caroline s vocal characteristics, in contrast to her speech-production and language skills, were more like a child with a hearing impairment than a child with normal hearing. Her vocal characteristics were both qualitatively and quantitatively abnormal. After listening to a tape-recording of both girls speaking, all the judges were able to identify Caroline as the child who was hearingimpaired. The features that were most frequently chosen to describe her voice included: breathiness, narrow pitch range and phonation breaks. These are all commonly reported features of the voices of speakers who are hearing impaired (Boone, 1966; Markides, 1970; Nickerson, 1975; Monsen, 1978; Osberger and McGarr, 1982). There is very little information available in the literature regarding the vocal characteristics of children who have cochlear implants and no source of comparison for Caroline s results. This is an area that warrants further detailed research. The acoustic analysis results suggest that Caroline had difficulty monitoring her voice, reflected by the variability in performance between different productions of the same stimulus and the variability within a single production (i.e. large standard deviations in fundamental frequency, shimmer, noise-toharmonic ratio and voice turbulence index). Children who are hearingimpaired typically have significantly greater average standard deviations in vocal tasks than children with normal hearing (Ryalls and Larouche, 1992). This variability in production is reported as being the hallmark of speakers who are hearing-impaired (Markides, 1970; Nickerson, 1975). Speakers who are hearing-impaired often over-aspirate voiced stops and fricatives in an attempt to increase tactile feedback (Bench, 1992). Such overaspiration results in increased spectral noise, which may explain the breathy perceptual quality of Caroline s voice and her high noise-to-harmonic ratio. Similarly, the voice turbulence index is thought to correlate with turbulence caused by incomplete or loose adduction of the vocal folds. This may also explain the breathy perceptual quality of Caroline s voice.

13 Speech, language and vocal skills with a cochlear implant 13 Clinical implications There are three major implications for the habilitation of children with cochlear implants which arise from this study. Caroline s abnormal vocal quality suggests the need for further intervention targeting voice. Her difficulties with the production of various suprasegmental features of speech are curious, given that the cochlear implant is designed to provide prosodic cues relating to duration, fundamental frequency and intensity (Clark, 1993). Caroline s abnormal voice results are especially curious when it is considered that she received substantial benefit from the implant and, when aided, had speech-perception ability equivalent to that of a person with a mild-to-moderate hearing loss unaided. This would allow her to hear speech at soft conversational levels. People with losses in this range do not typically have associated abnormal voice features (Markides, 1970). Second, Caroline s excellent language skills indicate the potential benefit of early implantation and/or signing pre-implant, so that language delays at the time of implantation are restricted. Finally, the impact of early intervention cannot be overlooked. Caroline began to acquire language, both signed and spoken, and received therapy during critical periods for speech and language acquisition. Her excellent speech-production and language results indicate the value of such therapy and demonstrate the importance of early intervention. Conclusions The research presently available on the speech-production and language skills of children with cochlear implants as a group indicates that these children generally do not achieve age-appropriate skills within two to three years of device use, although their rate of development is enhanced. There is very little detailed information about the skills of children with cochlear implants in more than one area of communicative ability. The present study, which looked at a number of communicative areas, found that a child (Caroline) who was pre-lingually hearing-impaired and received a cochlear implant at 2:2 years of age, was able to develop essentially age-appropriate speech-production and language skills after two to three years of device use. In-depth analysis of her pragmatic and discourse skills indicated some persisting difficulties. The excellent speech-production and language results of this child are thought to be due to a number of a factors, namely that she underwent cochlear implantation early; that she received intensive A V therapy pre- and post-implant; that her signing skills were at an age-appropriate level pre-implant; and that she received substantial speech-perception benefit from her implant. One notable result of the study was that the voice of the twin with the cochlear implant was found to be both qualitatively and quantitatively

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