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Journal of Intellectual and Developmental Disability Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/cjid20
Vocabulary development in children with Down syndrome: Longitudinal and cross-sectional data a
a
Laura Zampini & Laura D'Odorico a
Department of Psychology, University of Milano-Bicocca, Milan, Italy Published online: 26 Sep 2013.
To cite this article: Laura Zampini & Laura D'Odorico (2013) Vocabulary development in children with Down syndrome: Longitudinal and cross-sectional data, Journal of Intellectual and Developmental Disability, 38:4, 310-317 To link to this article: http://dx.doi.org/10.3109/13668250.2013.828833
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Journal of Intellectual & Developmental Disability, 2013 Vol. 38, No. 4, 310–317, http://dx.doi.org/10.3109/13668250.2013.828833
ORIGINAL ARTICLE
Vocabulary development in children with Down syndrome: Longitudinal and cross-sectional data
LAURA ZAMPINI & LAURA D’ODORICO
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Department of Psychology, University of Milano-Bicocca, Milan, Italy
Abstract Background Research findings on vocabulary development1 in children with Down syndrome are inconsistent. This study aimed to analyse the developmental trend of vocabulary growth in children with Down syndrome and the relationships between vocabulary and chronological and developmental age. Method Children’s vocabulary size was assessed by a parental report (the Italian version of the MacArthur–Bates Communicative Development Inventories [CDI]). A longitudinal study, involving 18 children with Down syndrome, and a cross-sectional study, involving 27 children with Down syndrome, were conducted. Results The variability in children’s vocabulary size increased from a chronological age of 36 months. Both vocabulary size (from 36 months) and developmental level (from 42 months) were significant predictors of lexical outcomes at 48 months. A comparison with normative data showed that children with Down syndrome had a significantly lower vocabulary size than typically developing children at the same developmental age. Conclusion Although there are similarities with vocabulary growth in typically developing children, lexical development in children with Down syndrome appears to lag behind their cognitive development.
Keywords: Down syndrome, vocabulary size, longitudinal design, cross-sectional design, developmental age
Introduction Overall, language development in children with Down syndrome seems to proceed at a slower pace than does cognitive development because these children show substantial problems in linguistic skills relative to their developmental age (Abbeduto, Warren, & Conners, 2007). However, the lexical abilities of children with Down syndrome appear to be less severely compromised than syntactic competencies (Chapman, Schwartz, & Kay-Raining Bird, 1991; Fowler, 1990; Laws & Bishop, 2003; Vicari, Caselli, & Tonucci, 2000). In fact, data from previous research show the existence of a wider gap between vocabulary and morphosyntactic skills in children with Down syndrome in comparison not only to typically developing children but also to children with developmental disabilities (e.g., children with specific language impairments or children with mixed aetiology intellectual disability; Caselli, Monaco, Trasciani, & Vicari, 2008; Chapman, 2006).
Research findings concerning whether children with Down syndrome show linguistic deficits relative to cognitive development are inconsistent (Roberts, Price, & Malkin, 2007). Some authors (e.g., Miller, 1992, 1999; Roberts, Price, Barnes, et al., 2007) argue that both syntactic and lexical development are significantly delayed in children with Down syndrome in comparison to typically developing children of the same mental age. Other authors (e.g., Galeote, Sebastián, Checa, Rey, & Soto, 2011), however, have found that lexical development in children with Down syndrome does not lag behind cognitive development. It has to be noted that the inconsistency in the results could be partially due to the methods used; for instance, some studies assessed children’s lexical production by standardised tests (e.g., Roberts, Price, Barnes, et al., 2007), whereas other studies used parental inventories (e.g., Galeote et al., 2011) or parental observation (e.g., Oliver & Buckley, 1994).
Correspondence: Laura Zampini, Department of Psychology, University of Milano-Bicocca, Piazza dell’Ateneo Nuovo, 1, 20126 Milan, Italy. E-mail: laura. [email protected] © 2013 Australasian Society for Intellectual Disability, Inc.
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Vocabulary development in Down syndrome Regarding the acquisition of first words, Berglund, Eriksson, and Johansson (2001) found that first words usually emerge between 12 and 24 months of age in children with Down syndrome, not much later than for typically developing children. However, it was observed that the acquisition process for new words proceeds at a slower pace than in typically developing children matched for linguistic or mental age. While the phenomenon of a vocabulary spurt (i.e., a rapid increase in the rate at which young children learn new words) generally occurs at approximately 18 months in typically developing children, data regarding the onset of a vocabulary spurt in children with Down syndrome are inconsistent. Oliver and Buckley (1994), for instance, found that the vocabulary spurt can appear as early as 30 months in children with Down syndrome, whereas Caselli, Marchetti, and Vicari (1994) showed that this phenomenon can occur later, for example, when children are 5 or 6 years old. However, evidence in the literature suggests that not all children with Down syndrome exhibit a vocabulary spurt; Oliver and Buckley (1994), analysing the records of language development of 12 children with Down syndrome, found that a vocabulary spurt was identifiable in only five of them. Regarding vocabulary composition, there are significant differences between children with Down syndrome and typically developing children. Data on the vocabulary composition of children with Down syndrome show that the percentage of free morphemes (i.e., pronouns, articles, and conjunctions) is lower than expected based on children’s lexical sizes because these children show a specific impairment in producing function words even with large vocabulary sizes. This result has been found in both Englishspeaking children (Chapman, Seung, Schwartz, & Kay-Raining Bird, 1998) and Italian-speaking children (Vicari et al., 2000; Zampini, & D’Odorico, 2011). One of the most frequently used methodologies for assessing young children’s vocabulary size is parental report. A number of studies on lexical development in children with Down syndrome have been conducted using parental reports, particularly the MacArthur–Bates Communicative Development Inventories (CDI; Fenson et al., 1993), which is a vocabulary checklist designed to assess children’s word production and comprehension. The validity of the productive section of the CDI has been demonstrated by Miller, Sedey, and Miolo (1995) for English-speaking children and by Zampini and D’Odorico (2009) for Italian-speaking children. Some of the parental report studies have been conducted on a wide range of children, using different
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versions of the CDI, adapted to different languages or specifically developed to test children with developmental delays. For instance, Berglund et al. (2001) employed the Swedish Early Communicative Development Inventory to assess the vocabulary production of 330 children with Down syndrome; Caselli et al. (1998) used the Italian version of the CDI to assess the vocabulary size of 40 children with Down syndrome. In addition, Galeote et al. (2011) employed an adaptation of the Spanish version of the CDI, specifically developed for children with Down syndrome, to assess the vocabulary size of 186 children. Although a large amount of data has been collected on the lexicons of children with Down syndrome, only a few studies have used a longitudinal design to assess the vocabulary development of children with Down syndrome (e.g., Cardoso-Martins, Mervis, & Mervis, 1985). For instance, Oliver and Buckley (1994), using monthly diary records based on parental observation, investigated the lexical development of 17 children with Down syndrome ranging in chronological age from 1–4 years. In particular, they followed nine children with Down syndrome until the children reached the 10-word stage, reporting that the chronological age range in which this goal was reached was 19–38 months. Moreover, in the same study, Oliver and Buckley investigated the emergence of two-word combinations in 14 children with Down syndrome, reporting that the age range in which this goal was reached was 25–52 months. In addition, Miller (1999) followed 20 children with Down syndrome (with a developmental age ranging from 12 to 17 months) for 2 years, finding that the differences between children with Down syndrome and typically developing children, matched for developmental age, widened as their mental age increased. In particular, at 17 months typically developing children had a vocabulary size 4 times larger than children with Down syndrome. Due to the lack of data on trends in vocabulary development, the first aim of the present study was to assess, with a longitudinal design, the lexical development of a group of Italian children with Down syndrome. The relationships of vocabulary production to both children’s chronological and developmental ages were considered. Moreover, a second part of the study aimed to analyse the relationship between children’s vocabulary size and developmental age using a cross-sectional design. The aim of this second study was to verify the existence of a deficit in children’s vocabulary production with respect to their cognitive level because the research findings on this topic are still inconsistent.
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Method
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Study 1. Longitudinal trends in vocabulary development
Participants. The Italian children with Down syndrome participating in the present study were selected from a sample of toddlers (n = 60) attending a monitoring program on language development at the Department of Psychology of the University of Milano-Bicocca, Italy. All participants were treated in accordance with the American Psychological Association’s ethical guidelines and their parents signed informed consent for participation in the study. All selected participants were monolingual Italian speaking, had free trisomy 21, as shown by karyotype analysis, and had normal or corrected-tonormal vision and hearing. The children’s developmental age was evaluated using the Brunet–Lézine Scale of Psychomotor Development (Brunet & Lézine, 1967). For the first study, the linguistic and psychomotor development of 18 children with Down syndrome was studied longitudinally. Ten 2-year-old children were followed for a 2-year period until they reached the age of 4 years. Moreover, eight other 3-year-old children were followed for a 1-year period until they reached the age of 4 years. Procedure. Children’s vocabulary development was assessed at 6-month intervals by administering the Italian version of the MacArthur–Bates Communicative Development Inventories: “Il Primo Vocabolario del Bambino” (PVB; Caselli & Casadio, 1995). The PVB consists of two different forms: the first form, called “Words and Gestures,” is the infant form and is designed for use with 8- to 16-month-old typically developing children, whereas the second form, called “Words and Sentences,” is the toddler form and is designed for use with 16- to 30-month-old children. The Words and Gestures form includes a vocabulary list of 408 words for assessing both comprehension and production and a section that evaluates gesture use. The Words and Sentences form includes a vocabulary production checklist of 670 words and a section in which morphosyntactic skills are investigated. For the present study, only word production was considered. Parents were asked to mark all of the words that their children were able to spontaneously produce. At the beginning, the first form of the inventory was administered to all parents. If the number of marked words was over 250, the second form, which contains late-acquired words, was administered as well, and the number of
words marked on this form was considered the child’s lexical size. Data analyses. First, the relationships between children’s vocabulary size and both chronological and developmental age were calculated using Spearman’s rho coefficient (we opted for nonparametric tests, due to the small sample size). Second, the predictive role of early vocabulary size on subsequent lexical development was analysed calculating Spearman’s rho coefficients between the PVB scores at Time 1 and the PVB scores at the subsequent sessions. Third, children were divided into three groups based on their vocabulary size at the last session (“low outcome group,” “medium outcome group,” and “high outcome group”); a Kruskal–Wallis test (a nonparametric test for independent samples) was used to verify the existence of significant differences among the groups in lexical and psychomotor development.
Study 2. Cross-sectional data for developmental age Participants. For the second study, 27 children were selected from the sample of 60 toddlers attending our monitoring program on language development. The inclusion criteria for these children were a developmental age of approximately 18 (M = 18.22, range: 16–20), 24 (M = 24.11, range: 23–25), and 30 (M = 29.22, range: 28–30) months. Nine children were included in each group. Procedure. The procedure was the same as the first study: children’s lexical size was assessed by administering the PVB. Data analyses. Descriptive data on word production at different developmental ages (i.e., 18, 24, and 30 months) in children with Down syndrome were compared with PVB’s normative data on typically developing children (Caselli, Pasqualetti, & Stefanini, 2007). The Italian normative sample is made up of 752 typically developing children ranging in age from 18 to 36 months of chronological age. As these children had been selected excluding those with developmental disorders or perinatal problems and those with a preterm birth, their developmental age was supposed to be equivalent to their chronological age. To analyse the existence of significant differences in vocabulary size between children with Down syndrome and typically developing children at the same developmental age, a Wilcoxon test (a nonparametric test for two independent samples)
Vocabulary development in Down syndrome was used for each developmental age considered (i.e., 18, 24, and 30 months). Results
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Study 1. Longitudinal trends in vocabulary development Descriptive statistics for children’s chronological ages, developmental ages, and vocabulary sizes for each session are shown in Table 1. Data analyses showed that children’s vocabulary size appeared to be related to their developmental age. The correlation coefficients (Spearman’s rho) were significant at 24 months (rho = .654, p = .040), at 36 months (rho = .668, p = .003), at 42 months (rho = .667, p = .005), and at 48 months (rho = .686, p = .002), but they were not significant at 30 months (rho = .550, p = .099). On the contrary, no significant relationships were found between vocabulary size and children’s chronological age (all rhos −.116 to .221, all ps > .377). To analyse the predictive role of early vocabulary size on subsequent lexical development, the correlations between the PVB score at Time 1 and the PVB scores at the subsequent sessions were
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calculated. Because children’s developmental age was concurrently related to their vocabulary size, the correlations among the PVB scores were calculated while controlling for the effects of developmental age. These partial correlations, controlling for developmental age, are reported in Table 2. Although vocabulary size at 24 months was not correlated with subsequent lexical size, children’s PVB sizes at 30, 36, and 42 months appeared to be significantly correlated with the subsequent vocabulary assessment. Because children’s vocabulary size is characterised by wide individual differences at 48 months, data analyses aimed to determine the age at which predictive indexes of lexical development could be identified. For this purpose, children were divided into three groups based on their vocabulary size at 48 months: children with less than 50 words (M = 19, SD = 17.01, range: 1–43) were included in the low outcome group (n = 5), children with a vocabulary size ranging from 100 to 250 words (M = 163, SD = 26.61, range: 135–210) were included in the medium outcome group (n = 7), and children with more than 300 words (M = 411, SD = 94.96, range: 316–581) were included in the high outcome group (n = 6). The existence of these three distinct groups was confirmed by statistical analyses. A
Table 1. Chronological age (CA), developmental age (DA), and vocabulary size (PVB) at the five sessions M 24 months
(n = 10)
30 months
(n = 10)
36 months
(n = 18)
42 months
(n = 18)
48 months
(n = 18)
CA DA PVB CA DA PVB CA DA PVB CA DA PVB CA DA PVB
25.00 15.90 9.30 31.10 18.90 17.90 36.94 20.29 47.56 43.33 22.94 116.61 49.00 25.28 205.50
SD
Range
0.82 1.52 4.83 0.99 1.37 11.18 0.87 3.41 61.83 0.84 3.24 127.55 0.84 4.24 169.78
24–26 13–18 4–21 30–32 17–21 4–40 36–39 12–25 0–243 42–45 16–28 0–499 48–50 17–30 1–581
Table 2. Partial correlations, controlling for developmental age (DA), among PVB vocabulary sizes
a
PVB 24 months PVB 30 monthsb PVB 36 monthsc PVB 42 monthsd
PVB 30 months
PVB 36 months
PVB 42 months
PVB 48 months
Rho
p
Rho
p
Rho
p
Rho
p
.507
.163
.532 .843
.140 .004
.603 .691 .760
.086 .039 .001
.522 .858 .941 .941
.149 .003 .000 .000
Controlling for DA at a24 months,b30 months,c36 months, and d42 months.
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Kruskal–Wallis test showed that these groups’ vocabulary sizes at 48 months differed significantly (K = 15.07, p = .001); more specifically, a Mann– Whitney test run as a post hoc test showed that the three groups differed significantly from one another: low < medium (p = .003); low < high (p = .004); medium < high (p = .001). Longitudinal trends of vocabulary development by outcome group are shown for each child in Figure 1. Data analyses aimed to determine whether individual differences in lexical size at 24, 30, 36, or 42 months could explain the variability in children’s lexical outcomes. To test whether children with
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different lexical outcomes at 48 months differed in their first stages of vocabulary acquisition, a comparison among the three groups was performed using a Kruskal–Wallis test. The results, reported in Table 3, showed that children’s vocabulary size was significantly different among the three groups at 36 and 42 months of chronological age. In particular, the low outcome group had a mean number of words that was significantly lower than that of the other two groups, and the medium outcome group had a mean vocabulary size lower than that of the high outcome group. On the contrary, there were no significant differences between groups when children were 24 and 30 months old. Therefore, only at 36 and 42 months could vocabulary size explain individual differences at 48 months. In the same manner, data analyses aimed to determine whether individual differences in children’s developmental ages at 24, 30, 36, 42, or 48 months could explain the variability in children’s lexical outcomes. As shown in Table 4, there were significant differences in children’s developmental ages only at 42 and 48 months; in these stages, the children in the low outcome group showed a mean developmental age, as assessed by the Brunet–Lézine Scale of Psychomotor Development, that was significantly lower than that for the other two groups. Therefore, only at 42 and 48 months could developmental age explain the variability in children’s lexical outcomes. Study 2. Cross-sectional data for developmental age Compared to the typically developing children, the children with Down syndrome showed impairments in cognitive development. However, it is interesting to determine whether their lexical development proceeds at the same pace as that for typically developing children of the same developmental age. Therefore, data on word production of a group of children with Down syndrome were compared with data
Figure 1. Vocabulary trends for each child in the three groups.
Table 3. Group differences in PVB vocabulary sizes Medium outcome
Low outcome M
SD
M
SD
High outcome M
SD
K
p
PVB 24 months PVB 30 months PVB 36 months
6.50 8.50 7.20
3.54 6.36 7.43
7.00 14.00 23.00
1.83 5.83 10.66
13.00 26.50 109.83
5.66 12.23 75.42
4.84 4.27 12.97
.089 .118 .002
PVB 42 months
10.20
9.39
72.43
27.92
256.83
129.05
15.05
.001
Mann–Whitney
Low < Medium, p = .018 Low < High, p = .004 Medium < High, p = .005 Low < Medium, p = .003 Low < High, p = .004 Medium < High, p = .001
Vocabulary development in Down syndrome
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Table 4. Group differences in developmental age (DA) Medium outcome
Low outcome
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M
SD
M
High outcome
SD
M
SD
K
p
DA 24 months DA 30 months DA 36 months DA 42 months
15.50 17.00 16.25 18.75
0.71 0.00 3.86 2.22
15.00 18.75 21.14 24.00
1.83 0.96 2.04 2.19
17.00 20.00 22.00 24.67
0.82 0.82 2.37 2.16
3.87 6.28 5.93 7.67
.144 .043 .052 .022
DA 48 months
20.00
3.16
27.00
2.77
27.67
2.25
9.08
.011
from typically developing children extracted from the PVB’s normative data (Caselli et al., 2007). The productive vocabulary size of children with Down syndrome at the developmental ages of 18, 24, and 30 months was compared with that of typically developing children at a chronological age equal to the developmental age of children with Down syndrome. Twenty-seven children with Down syndrome were divided into three groups (each consisting of nine children) according to their developmental ages, that is, 18, 24, and 30 months. The mean number of words produced by children with Down syndrome with a developmental age of 18 months was 22 (SD = 19.6, range: 0–50); children with Down syndrome at the developmental ages of 24 and 30 months had a
Mann–Whitney
Low < Medium, p = .019 Low < High, p = .010 Low < Medium, p = .005 Low < High, p = .009
mean number of words of 161.78 (SD = 121.51, range: 23–400) and 345 (SD = 144.73, range: 135– 581), respectively. Figure 2 jointly shows the estimated percentiles of PVB vocabulary production extracted from Caselli et al. (2007) and the medians computed on the children with Down syndrome included in the current study. It appears that the median productive vocabulary size of children with Down syndrome (i.e., the 50th percentile for children with Down syndrome in our sample) falls between the 10th and 25th percentiles of the population of typically developing children. This ranking was found at each developmental age examined. A Wilcoxon test showed that there were significant differences between children with Down syndrome
Figure 2. Estimated percentiles of word production scores in typically developing children (extracted from Caselli et al., 2007) and medians of word production scores computed for the participants of Study 2.
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and typically developing children in word production at each age considered: 18 months (W = −2.67, p < .008), 24 months (W = −2.19, p < .028), and 30 months (W = −2.19, p < .028).
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Discussion The principal aims of the present study were, on the one hand, to analyse the developmental trends of vocabulary development in children with Down syndrome and, on the other hand, to analyse the relationships between lexical size and both chronological and developmental age. Data obtained from the completion of the PVB by parents of children with Down syndrome showed that children’s chronological age was not correlated with word production, therefore no linear relationships could be found between expressive lexical skills and biological and experiential growth. On the contrary, children’s word production increased with their developmental age, as commonly occurs in typical development, and its individual variability (expressed by standard deviation) tended to rise with developmental age. Nevertheless, children with Down syndrome at 18, 24, and 30 months of developmental age produced significantly fewer words than typically developing children matched on a cognitive level. Therefore, expressive lexical development in these children appeared to lag behind their cognitive development. This finding supports the claim that children with Down syndrome show an impairment in linguistic skills higher than would be predicted using their cognitive level, in comparison to both typically developing children and children with intellectual disability due to other aetiologies (e.g., children with Williams syndrome; Abbeduto et al., 2007; Fowler, 1990; Miller, 1988). Miller (1995) found that 65% of children with Down syndrome had a vocabulary size below the 10th percentile of typically developing children. Similarly, in the present study, it was found that the median word production of children with Down syndrome fell between the 10th and 25th percentiles of scores for typically developing children. Our results are in contrast with those of Galeote et al. (2011) who found that lexicon does not lag behind cognitive development in children with Down syndrome. Although the methods used in our study were similar to those used in Galeote et al.’s study (i.e., parental inventory and administration of Brunet–Lézine Scale of Psychomotor Development), it has to be noted that the inventories used were different. Our instrument was the standard Italian version of the MacArthur–Bates CDI, whereas the instrument used by Galeote et al. was
an inventory derived from the Spanish version of the MacArthur–Bates CDI, but specifically adapted to be used with children with Down syndrome; this inventory consists of a single form to assess both word comprehension and production across the developmental age range covered by the original CDI (i.e., 8–30 months). It is possible that the differences in the inventories used contributed to the differences in the results. The observation of longitudinal trends allowed us to demonstrate that the main changes in vocabulary size occurred at 36 months of chronological age; at this stage, wide individual variability was observed, as expressed by standard deviation and range of word production. Until 30 months of chronological age, all of the assessed children showed a vocabulary size below 50 words. Moreover, the increase in vocabulary size from 24 to 30 months was very limited, as shown by the growth curves. For this reason, the predictive value of vocabulary size at one stage on the subsequent lexical outcome began at 36 months of chronological age. In addition, from 42 months of chronological age, children’s developmental age also had a significant influence on subsequent lexical outcome. Although the small number of participants was a limitation of the study, especially because of the wide individual variability that characterises the population of children with Down syndrome, the present study still has applicative value, as it makes available prescriptive data on vocabulary development that clinicians could use to compare children’s lexical production. In particular, it could be useful to assess the position of each child with Down syndrome relative to the average population of children with Down syndrome. These data also have a predictive value, insofar as they allow children’s lexical development to be estimated on the basis of their current vocabulary size. Future research will be oriented to analyse longterm outcomes in lexical development, to assess if children with a higher vocabulary size at 48 months could also have a better outcome in the school age period. Acknowledgements The authors declare that there are no real or potential conflicts of interest related to the manuscript. Note 1. This study focused on the production of words in children with Down syndrome. Only the expressive vocabulary produced using speech, and not
Vocabulary development in Down syndrome vocabulary produced using gestures or augmentative and alternative communication, has been considered.
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References Abbeduto, L., Warren, S. F., & Conners, F. A. (2007). Language development in Down syndrome: From the prelinguistic period to the acquisition of literacy. Mental Retardation and Developmental Disabilities Research Reviews, 13, 247–261. doi:10.1002/mrdd.20158 Berglund, E., Eriksson, M., & Johansson, I. (2001). Parental reports of spoken language skills in children with Down syndrome. Journal of Speech, Language, and Hearing Research, 44, 179–191. doi:10.1044/1092-4388(2001/016) Brunet, O., & Lézine, I. (1967). Scala di sviluppo psicomotorio della prima infanzia [Infant Scale of Psychomotor Development]. Florence, Italy: Organizzazioni Speciali. Cardoso-Martins, C., Mervis, C. B., & Mervis, C. A. (1985). Early vocabulary acquisition by children with Down syndrome. American Journal of Mental Deficiency, 90, 177–184. Caselli, M. C., & Casadio, P. (1995). Il Primo Vocabolario del Bambino [Children’s first words]. Milan, Italy: Franco Angeli. Caselli, M. C., Marchetti, C., & Vicari, S. (1994). Conoscenze lessicali e primo sviluppo morfosintattico [Lexical knowledge and first morphosyntactic development]. In A. Contardi & S. Vicari (Eds.), Le persone Down. Aspetti neuropsicologici, educativi e sociali [People with Down syndrome: Neuropsychological, educational and social aspects] (pp. 28–48). Milan, Italy: Franco Angeli. Caselli, M. C., Monaco, L., Trasciani, M., & Vicari, S. (2008). Language in Italian children with Down syndrome and with specific language impairment. Neuropsychology, 22, 27–35. doi:10.1037/0894-4105.22.1.27 Caselli, M. C., Pasqualetti, P., & Stefanini, S. (2007). Parole e frasi nel “primo vocabolario del bambino.” Nuovi dati normativi fra 18 e 36 mesi e Forma breve del questionario [Words and sentences in “children’s first words.” New normative data between 18 and 36 months and short form of the inventory]. Milan, Italy: Franco Angeli. Caselli, M. C., Vicari, S., Longobardi, E., Lami, L., Pizzoli, C., & Stella, G. (1998). Gestures and words in early development of children with Down syndrome. Journal of Speech Language, and Hearing Research, 41, 1125–1135. Chapman, R. S. (2006). Language learning in Down syndrome: The speech and language profile compared to adolescents with cognitive impairment of unknown origin. Down Syndrome Research and Practice, 10, 61–66. doi:10.3104/ reports.306 Chapman, R. S., Schwartz, S. E., & Kay-Raining Bird, E. (1991). Language skills of children and adolescents with Down syndrome: I. Comprehension. Journal of Speech and Hearing Research, 34, 1106–1120. Chapman, R. S., Seung, H-K., Schwartz, S. E., & Kay-Raining Bird, E. (1998). Language skills of children and adolescents with Down syndrome: II. Production deficits. Journal of Speech, Language, and Hearing Research, 41, 861–873. Fenson, L., Dale, P. S., Reznick, J. S., Thal, D., Bates, E., Hartung, J. P., … Reilly, J. S. (1993). The MacArthur Communicative Development Inventories: User’s guide and technical manual. San Diego, CA: Singular.
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Fowler, A. E. (1990). Language abilities in children with Down syndrome: Evidence for a specific syntactic delay. In D. Cicchetti, & M. Beeghly (Eds.), Children with Down syndrome: A developmental perspective (pp. 302–328). New York, NY: Cambridge University Press. doi:10.1017/ CBO9780511581786.010 Galeote, M., Sebastián, E., Checa, E., Rey, R., & Soto, P. (2011). The development of vocabulary in Spanish children with Down syndrome: Comprehension, production, and gestures. Journal of Intellectual & Developmental Disability, 36, 184–196. doi:10.3109/13668250.2011.599317 Laws, G., & Bishop, D. V. M. (2003). A comparison of language abilities in adolescents with Down syndrome and children with specific language impairment. Journal of Speech, Language, and Hearing Research, 46, 1324–1339. doi:10.1044/ 1092-4388(2003/103) Miller, J. F. (1988). The developmental asynchrony of language development in children with Down syndrome. In L. Nadel (Ed.), The psychobiology of Down syndrome (pp. 167–198). Cambridge, MA: MIT Press. Miller, J. F. (1992). Development of speech and language in children with Down syndrome. In I. T. Lott & E. E. McCoy (Eds.), Clinical care for persons with Down syndrome (pp. 39–50). New York, NY: Academic Press. Miller, J. F. (1995). Individual differences in vocabulary acquisition in children with Down syndrome. In C. J. Epstein, T. Hassold, I. T. Lott, L. Nadel, & D. Patterson (Eds.), Etiology and pathogenesis of Down syndrome (pp. 93–103). New York, NY: Wiley-Liss. Miller, J. F. (1999). Profiles on language development in children with Down syndrome. In J. F. Miller, M. Leddy, & L. A. Leavitt (Eds.), Improving the communication of people with Down syndrome (pp. 11–39). Baltimore, MD: Brookes. Miller, J. F., Sedey, A. L., & Miolo, G. (1995). Validity of parent report measures of vocabulary development for children with Down syndrome. Journal of Speech, Language, and Hearing Research, 38, 1037–1044. Oliver, B., & Buckley, S. (1994). The language development of children with Down’s syndrome: First words to two-word phrases. Down Syndrome Research and Practice, 2, 71–75. doi:10.3104/reports.33 Roberts, J., Price, J., Barnes, E., Nelson, L., Burchinal, M., Hennon, E. A., … Hooper, S. R. (2007). Receptive vocabulary, expressive vocabulary, and speech production of boys with fragile X syndrome in comparison to boys with Down syndrome. American Journal on Mental Retardation, 112, 177–193. Roberts, J. E., Price, J., & Malkin, C. (2007). Language and communication development in Down syndrome. Mental Retardation and Developmental Disabilities Research Reviews, 13, 26–35. doi:10.1002/mrdd.20136 Vicari, S., Caselli, M. C., & Tonucci, F. (2000). Asynchrony of lexical and morphosyntactic development in children with Down syndrome. Neuropsychologia, 38, 634–644. doi:10.1016/S0028-3932(99)00110-4 Zampini, L., & D’Odorico, L. (2009). Communicative gestures and vocabulary development in 36-month-old children with Down’s syndrome. International Journal of Language & Communication Disorders, 44, 1063–1073. doi:10.1080/ 13682820802398288 Zampini, L., & D’Odorico, L. (2011). Lexical and syntactic development in Italian children with Down’s syndrome. International Journal of Language & Communication Disorders, 46, 386–396. doi:10.3109/13682822.2010.508764
Journal of Intellectual and Developmental Disability Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/cjid20
Vocabulary development in children with Down syndrome: Longitudinal and cross-sectional data a
a
Laura Zampini & Laura D'Odorico a
Department of Psychology, University of Milano-Bicocca, Milan, Italy Published online: 26 Sep 2013.
To cite this article: Laura Zampini & Laura D'Odorico (2013) Vocabulary development in children with Down syndrome: Longitudinal and cross-sectional data, Journal of Intellectual and Developmental Disability, 38:4, 310-317 To link to this article: http://dx.doi.org/10.3109/13668250.2013.828833
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Journal of Intellectual & Developmental Disability, 2013 Vol. 38, No. 4, 310–317, http://dx.doi.org/10.3109/13668250.2013.828833
ORIGINAL ARTICLE
Vocabulary development in children with Down syndrome: Longitudinal and cross-sectional data
LAURA ZAMPINI & LAURA D’ODORICO
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Department of Psychology, University of Milano-Bicocca, Milan, Italy
Abstract Background Research findings on vocabulary development1 in children with Down syndrome are inconsistent. This study aimed to analyse the developmental trend of vocabulary growth in children with Down syndrome and the relationships between vocabulary and chronological and developmental age. Method Children’s vocabulary size was assessed by a parental report (the Italian version of the MacArthur–Bates Communicative Development Inventories [CDI]). A longitudinal study, involving 18 children with Down syndrome, and a cross-sectional study, involving 27 children with Down syndrome, were conducted. Results The variability in children’s vocabulary size increased from a chronological age of 36 months. Both vocabulary size (from 36 months) and developmental level (from 42 months) were significant predictors of lexical outcomes at 48 months. A comparison with normative data showed that children with Down syndrome had a significantly lower vocabulary size than typically developing children at the same developmental age. Conclusion Although there are similarities with vocabulary growth in typically developing children, lexical development in children with Down syndrome appears to lag behind their cognitive development.
Keywords: Down syndrome, vocabulary size, longitudinal design, cross-sectional design, developmental age
Introduction Overall, language development in children with Down syndrome seems to proceed at a slower pace than does cognitive development because these children show substantial problems in linguistic skills relative to their developmental age (Abbeduto, Warren, & Conners, 2007). However, the lexical abilities of children with Down syndrome appear to be less severely compromised than syntactic competencies (Chapman, Schwartz, & Kay-Raining Bird, 1991; Fowler, 1990; Laws & Bishop, 2003; Vicari, Caselli, & Tonucci, 2000). In fact, data from previous research show the existence of a wider gap between vocabulary and morphosyntactic skills in children with Down syndrome in comparison not only to typically developing children but also to children with developmental disabilities (e.g., children with specific language impairments or children with mixed aetiology intellectual disability; Caselli, Monaco, Trasciani, & Vicari, 2008; Chapman, 2006).
Research findings concerning whether children with Down syndrome show linguistic deficits relative to cognitive development are inconsistent (Roberts, Price, & Malkin, 2007). Some authors (e.g., Miller, 1992, 1999; Roberts, Price, Barnes, et al., 2007) argue that both syntactic and lexical development are significantly delayed in children with Down syndrome in comparison to typically developing children of the same mental age. Other authors (e.g., Galeote, Sebastián, Checa, Rey, & Soto, 2011), however, have found that lexical development in children with Down syndrome does not lag behind cognitive development. It has to be noted that the inconsistency in the results could be partially due to the methods used; for instance, some studies assessed children’s lexical production by standardised tests (e.g., Roberts, Price, Barnes, et al., 2007), whereas other studies used parental inventories (e.g., Galeote et al., 2011) or parental observation (e.g., Oliver & Buckley, 1994).
Correspondence: Laura Zampini, Department of Psychology, University of Milano-Bicocca, Piazza dell’Ateneo Nuovo, 1, 20126 Milan, Italy. E-mail: laura. [email protected] © 2013 Australasian Society for Intellectual Disability, Inc.
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Vocabulary development in Down syndrome Regarding the acquisition of first words, Berglund, Eriksson, and Johansson (2001) found that first words usually emerge between 12 and 24 months of age in children with Down syndrome, not much later than for typically developing children. However, it was observed that the acquisition process for new words proceeds at a slower pace than in typically developing children matched for linguistic or mental age. While the phenomenon of a vocabulary spurt (i.e., a rapid increase in the rate at which young children learn new words) generally occurs at approximately 18 months in typically developing children, data regarding the onset of a vocabulary spurt in children with Down syndrome are inconsistent. Oliver and Buckley (1994), for instance, found that the vocabulary spurt can appear as early as 30 months in children with Down syndrome, whereas Caselli, Marchetti, and Vicari (1994) showed that this phenomenon can occur later, for example, when children are 5 or 6 years old. However, evidence in the literature suggests that not all children with Down syndrome exhibit a vocabulary spurt; Oliver and Buckley (1994), analysing the records of language development of 12 children with Down syndrome, found that a vocabulary spurt was identifiable in only five of them. Regarding vocabulary composition, there are significant differences between children with Down syndrome and typically developing children. Data on the vocabulary composition of children with Down syndrome show that the percentage of free morphemes (i.e., pronouns, articles, and conjunctions) is lower than expected based on children’s lexical sizes because these children show a specific impairment in producing function words even with large vocabulary sizes. This result has been found in both Englishspeaking children (Chapman, Seung, Schwartz, & Kay-Raining Bird, 1998) and Italian-speaking children (Vicari et al., 2000; Zampini, & D’Odorico, 2011). One of the most frequently used methodologies for assessing young children’s vocabulary size is parental report. A number of studies on lexical development in children with Down syndrome have been conducted using parental reports, particularly the MacArthur–Bates Communicative Development Inventories (CDI; Fenson et al., 1993), which is a vocabulary checklist designed to assess children’s word production and comprehension. The validity of the productive section of the CDI has been demonstrated by Miller, Sedey, and Miolo (1995) for English-speaking children and by Zampini and D’Odorico (2009) for Italian-speaking children. Some of the parental report studies have been conducted on a wide range of children, using different
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versions of the CDI, adapted to different languages or specifically developed to test children with developmental delays. For instance, Berglund et al. (2001) employed the Swedish Early Communicative Development Inventory to assess the vocabulary production of 330 children with Down syndrome; Caselli et al. (1998) used the Italian version of the CDI to assess the vocabulary size of 40 children with Down syndrome. In addition, Galeote et al. (2011) employed an adaptation of the Spanish version of the CDI, specifically developed for children with Down syndrome, to assess the vocabulary size of 186 children. Although a large amount of data has been collected on the lexicons of children with Down syndrome, only a few studies have used a longitudinal design to assess the vocabulary development of children with Down syndrome (e.g., Cardoso-Martins, Mervis, & Mervis, 1985). For instance, Oliver and Buckley (1994), using monthly diary records based on parental observation, investigated the lexical development of 17 children with Down syndrome ranging in chronological age from 1–4 years. In particular, they followed nine children with Down syndrome until the children reached the 10-word stage, reporting that the chronological age range in which this goal was reached was 19–38 months. Moreover, in the same study, Oliver and Buckley investigated the emergence of two-word combinations in 14 children with Down syndrome, reporting that the age range in which this goal was reached was 25–52 months. In addition, Miller (1999) followed 20 children with Down syndrome (with a developmental age ranging from 12 to 17 months) for 2 years, finding that the differences between children with Down syndrome and typically developing children, matched for developmental age, widened as their mental age increased. In particular, at 17 months typically developing children had a vocabulary size 4 times larger than children with Down syndrome. Due to the lack of data on trends in vocabulary development, the first aim of the present study was to assess, with a longitudinal design, the lexical development of a group of Italian children with Down syndrome. The relationships of vocabulary production to both children’s chronological and developmental ages were considered. Moreover, a second part of the study aimed to analyse the relationship between children’s vocabulary size and developmental age using a cross-sectional design. The aim of this second study was to verify the existence of a deficit in children’s vocabulary production with respect to their cognitive level because the research findings on this topic are still inconsistent.
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Method
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Study 1. Longitudinal trends in vocabulary development
Participants. The Italian children with Down syndrome participating in the present study were selected from a sample of toddlers (n = 60) attending a monitoring program on language development at the Department of Psychology of the University of Milano-Bicocca, Italy. All participants were treated in accordance with the American Psychological Association’s ethical guidelines and their parents signed informed consent for participation in the study. All selected participants were monolingual Italian speaking, had free trisomy 21, as shown by karyotype analysis, and had normal or corrected-tonormal vision and hearing. The children’s developmental age was evaluated using the Brunet–Lézine Scale of Psychomotor Development (Brunet & Lézine, 1967). For the first study, the linguistic and psychomotor development of 18 children with Down syndrome was studied longitudinally. Ten 2-year-old children were followed for a 2-year period until they reached the age of 4 years. Moreover, eight other 3-year-old children were followed for a 1-year period until they reached the age of 4 years. Procedure. Children’s vocabulary development was assessed at 6-month intervals by administering the Italian version of the MacArthur–Bates Communicative Development Inventories: “Il Primo Vocabolario del Bambino” (PVB; Caselli & Casadio, 1995). The PVB consists of two different forms: the first form, called “Words and Gestures,” is the infant form and is designed for use with 8- to 16-month-old typically developing children, whereas the second form, called “Words and Sentences,” is the toddler form and is designed for use with 16- to 30-month-old children. The Words and Gestures form includes a vocabulary list of 408 words for assessing both comprehension and production and a section that evaluates gesture use. The Words and Sentences form includes a vocabulary production checklist of 670 words and a section in which morphosyntactic skills are investigated. For the present study, only word production was considered. Parents were asked to mark all of the words that their children were able to spontaneously produce. At the beginning, the first form of the inventory was administered to all parents. If the number of marked words was over 250, the second form, which contains late-acquired words, was administered as well, and the number of
words marked on this form was considered the child’s lexical size. Data analyses. First, the relationships between children’s vocabulary size and both chronological and developmental age were calculated using Spearman’s rho coefficient (we opted for nonparametric tests, due to the small sample size). Second, the predictive role of early vocabulary size on subsequent lexical development was analysed calculating Spearman’s rho coefficients between the PVB scores at Time 1 and the PVB scores at the subsequent sessions. Third, children were divided into three groups based on their vocabulary size at the last session (“low outcome group,” “medium outcome group,” and “high outcome group”); a Kruskal–Wallis test (a nonparametric test for independent samples) was used to verify the existence of significant differences among the groups in lexical and psychomotor development.
Study 2. Cross-sectional data for developmental age Participants. For the second study, 27 children were selected from the sample of 60 toddlers attending our monitoring program on language development. The inclusion criteria for these children were a developmental age of approximately 18 (M = 18.22, range: 16–20), 24 (M = 24.11, range: 23–25), and 30 (M = 29.22, range: 28–30) months. Nine children were included in each group. Procedure. The procedure was the same as the first study: children’s lexical size was assessed by administering the PVB. Data analyses. Descriptive data on word production at different developmental ages (i.e., 18, 24, and 30 months) in children with Down syndrome were compared with PVB’s normative data on typically developing children (Caselli, Pasqualetti, & Stefanini, 2007). The Italian normative sample is made up of 752 typically developing children ranging in age from 18 to 36 months of chronological age. As these children had been selected excluding those with developmental disorders or perinatal problems and those with a preterm birth, their developmental age was supposed to be equivalent to their chronological age. To analyse the existence of significant differences in vocabulary size between children with Down syndrome and typically developing children at the same developmental age, a Wilcoxon test (a nonparametric test for two independent samples)
Vocabulary development in Down syndrome was used for each developmental age considered (i.e., 18, 24, and 30 months). Results
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Study 1. Longitudinal trends in vocabulary development Descriptive statistics for children’s chronological ages, developmental ages, and vocabulary sizes for each session are shown in Table 1. Data analyses showed that children’s vocabulary size appeared to be related to their developmental age. The correlation coefficients (Spearman’s rho) were significant at 24 months (rho = .654, p = .040), at 36 months (rho = .668, p = .003), at 42 months (rho = .667, p = .005), and at 48 months (rho = .686, p = .002), but they were not significant at 30 months (rho = .550, p = .099). On the contrary, no significant relationships were found between vocabulary size and children’s chronological age (all rhos −.116 to .221, all ps > .377). To analyse the predictive role of early vocabulary size on subsequent lexical development, the correlations between the PVB score at Time 1 and the PVB scores at the subsequent sessions were
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calculated. Because children’s developmental age was concurrently related to their vocabulary size, the correlations among the PVB scores were calculated while controlling for the effects of developmental age. These partial correlations, controlling for developmental age, are reported in Table 2. Although vocabulary size at 24 months was not correlated with subsequent lexical size, children’s PVB sizes at 30, 36, and 42 months appeared to be significantly correlated with the subsequent vocabulary assessment. Because children’s vocabulary size is characterised by wide individual differences at 48 months, data analyses aimed to determine the age at which predictive indexes of lexical development could be identified. For this purpose, children were divided into three groups based on their vocabulary size at 48 months: children with less than 50 words (M = 19, SD = 17.01, range: 1–43) were included in the low outcome group (n = 5), children with a vocabulary size ranging from 100 to 250 words (M = 163, SD = 26.61, range: 135–210) were included in the medium outcome group (n = 7), and children with more than 300 words (M = 411, SD = 94.96, range: 316–581) were included in the high outcome group (n = 6). The existence of these three distinct groups was confirmed by statistical analyses. A
Table 1. Chronological age (CA), developmental age (DA), and vocabulary size (PVB) at the five sessions M 24 months
(n = 10)
30 months
(n = 10)
36 months
(n = 18)
42 months
(n = 18)
48 months
(n = 18)
CA DA PVB CA DA PVB CA DA PVB CA DA PVB CA DA PVB
25.00 15.90 9.30 31.10 18.90 17.90 36.94 20.29 47.56 43.33 22.94 116.61 49.00 25.28 205.50
SD
Range
0.82 1.52 4.83 0.99 1.37 11.18 0.87 3.41 61.83 0.84 3.24 127.55 0.84 4.24 169.78
24–26 13–18 4–21 30–32 17–21 4–40 36–39 12–25 0–243 42–45 16–28 0–499 48–50 17–30 1–581
Table 2. Partial correlations, controlling for developmental age (DA), among PVB vocabulary sizes
a
PVB 24 months PVB 30 monthsb PVB 36 monthsc PVB 42 monthsd
PVB 30 months
PVB 36 months
PVB 42 months
PVB 48 months
Rho
p
Rho
p
Rho
p
Rho
p
.507
.163
.532 .843
.140 .004
.603 .691 .760
.086 .039 .001
.522 .858 .941 .941
.149 .003 .000 .000
Controlling for DA at a24 months,b30 months,c36 months, and d42 months.
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Kruskal–Wallis test showed that these groups’ vocabulary sizes at 48 months differed significantly (K = 15.07, p = .001); more specifically, a Mann– Whitney test run as a post hoc test showed that the three groups differed significantly from one another: low < medium (p = .003); low < high (p = .004); medium < high (p = .001). Longitudinal trends of vocabulary development by outcome group are shown for each child in Figure 1. Data analyses aimed to determine whether individual differences in lexical size at 24, 30, 36, or 42 months could explain the variability in children’s lexical outcomes. To test whether children with
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different lexical outcomes at 48 months differed in their first stages of vocabulary acquisition, a comparison among the three groups was performed using a Kruskal–Wallis test. The results, reported in Table 3, showed that children’s vocabulary size was significantly different among the three groups at 36 and 42 months of chronological age. In particular, the low outcome group had a mean number of words that was significantly lower than that of the other two groups, and the medium outcome group had a mean vocabulary size lower than that of the high outcome group. On the contrary, there were no significant differences between groups when children were 24 and 30 months old. Therefore, only at 36 and 42 months could vocabulary size explain individual differences at 48 months. In the same manner, data analyses aimed to determine whether individual differences in children’s developmental ages at 24, 30, 36, 42, or 48 months could explain the variability in children’s lexical outcomes. As shown in Table 4, there were significant differences in children’s developmental ages only at 42 and 48 months; in these stages, the children in the low outcome group showed a mean developmental age, as assessed by the Brunet–Lézine Scale of Psychomotor Development, that was significantly lower than that for the other two groups. Therefore, only at 42 and 48 months could developmental age explain the variability in children’s lexical outcomes. Study 2. Cross-sectional data for developmental age Compared to the typically developing children, the children with Down syndrome showed impairments in cognitive development. However, it is interesting to determine whether their lexical development proceeds at the same pace as that for typically developing children of the same developmental age. Therefore, data on word production of a group of children with Down syndrome were compared with data
Figure 1. Vocabulary trends for each child in the three groups.
Table 3. Group differences in PVB vocabulary sizes Medium outcome
Low outcome M
SD
M
SD
High outcome M
SD
K
p
PVB 24 months PVB 30 months PVB 36 months
6.50 8.50 7.20
3.54 6.36 7.43
7.00 14.00 23.00
1.83 5.83 10.66
13.00 26.50 109.83
5.66 12.23 75.42
4.84 4.27 12.97
.089 .118 .002
PVB 42 months
10.20
9.39
72.43
27.92
256.83
129.05
15.05
.001
Mann–Whitney
Low < Medium, p = .018 Low < High, p = .004 Medium < High, p = .005 Low < Medium, p = .003 Low < High, p = .004 Medium < High, p = .001
Vocabulary development in Down syndrome
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Table 4. Group differences in developmental age (DA) Medium outcome
Low outcome
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M
SD
M
High outcome
SD
M
SD
K
p
DA 24 months DA 30 months DA 36 months DA 42 months
15.50 17.00 16.25 18.75
0.71 0.00 3.86 2.22
15.00 18.75 21.14 24.00
1.83 0.96 2.04 2.19
17.00 20.00 22.00 24.67
0.82 0.82 2.37 2.16
3.87 6.28 5.93 7.67
.144 .043 .052 .022
DA 48 months
20.00
3.16
27.00
2.77
27.67
2.25
9.08
.011
from typically developing children extracted from the PVB’s normative data (Caselli et al., 2007). The productive vocabulary size of children with Down syndrome at the developmental ages of 18, 24, and 30 months was compared with that of typically developing children at a chronological age equal to the developmental age of children with Down syndrome. Twenty-seven children with Down syndrome were divided into three groups (each consisting of nine children) according to their developmental ages, that is, 18, 24, and 30 months. The mean number of words produced by children with Down syndrome with a developmental age of 18 months was 22 (SD = 19.6, range: 0–50); children with Down syndrome at the developmental ages of 24 and 30 months had a
Mann–Whitney
Low < Medium, p = .019 Low < High, p = .010 Low < Medium, p = .005 Low < High, p = .009
mean number of words of 161.78 (SD = 121.51, range: 23–400) and 345 (SD = 144.73, range: 135– 581), respectively. Figure 2 jointly shows the estimated percentiles of PVB vocabulary production extracted from Caselli et al. (2007) and the medians computed on the children with Down syndrome included in the current study. It appears that the median productive vocabulary size of children with Down syndrome (i.e., the 50th percentile for children with Down syndrome in our sample) falls between the 10th and 25th percentiles of the population of typically developing children. This ranking was found at each developmental age examined. A Wilcoxon test showed that there were significant differences between children with Down syndrome
Figure 2. Estimated percentiles of word production scores in typically developing children (extracted from Caselli et al., 2007) and medians of word production scores computed for the participants of Study 2.
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and typically developing children in word production at each age considered: 18 months (W = −2.67, p < .008), 24 months (W = −2.19, p < .028), and 30 months (W = −2.19, p < .028).
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Discussion The principal aims of the present study were, on the one hand, to analyse the developmental trends of vocabulary development in children with Down syndrome and, on the other hand, to analyse the relationships between lexical size and both chronological and developmental age. Data obtained from the completion of the PVB by parents of children with Down syndrome showed that children’s chronological age was not correlated with word production, therefore no linear relationships could be found between expressive lexical skills and biological and experiential growth. On the contrary, children’s word production increased with their developmental age, as commonly occurs in typical development, and its individual variability (expressed by standard deviation) tended to rise with developmental age. Nevertheless, children with Down syndrome at 18, 24, and 30 months of developmental age produced significantly fewer words than typically developing children matched on a cognitive level. Therefore, expressive lexical development in these children appeared to lag behind their cognitive development. This finding supports the claim that children with Down syndrome show an impairment in linguistic skills higher than would be predicted using their cognitive level, in comparison to both typically developing children and children with intellectual disability due to other aetiologies (e.g., children with Williams syndrome; Abbeduto et al., 2007; Fowler, 1990; Miller, 1988). Miller (1995) found that 65% of children with Down syndrome had a vocabulary size below the 10th percentile of typically developing children. Similarly, in the present study, it was found that the median word production of children with Down syndrome fell between the 10th and 25th percentiles of scores for typically developing children. Our results are in contrast with those of Galeote et al. (2011) who found that lexicon does not lag behind cognitive development in children with Down syndrome. Although the methods used in our study were similar to those used in Galeote et al.’s study (i.e., parental inventory and administration of Brunet–Lézine Scale of Psychomotor Development), it has to be noted that the inventories used were different. Our instrument was the standard Italian version of the MacArthur–Bates CDI, whereas the instrument used by Galeote et al. was
an inventory derived from the Spanish version of the MacArthur–Bates CDI, but specifically adapted to be used with children with Down syndrome; this inventory consists of a single form to assess both word comprehension and production across the developmental age range covered by the original CDI (i.e., 8–30 months). It is possible that the differences in the inventories used contributed to the differences in the results. The observation of longitudinal trends allowed us to demonstrate that the main changes in vocabulary size occurred at 36 months of chronological age; at this stage, wide individual variability was observed, as expressed by standard deviation and range of word production. Until 30 months of chronological age, all of the assessed children showed a vocabulary size below 50 words. Moreover, the increase in vocabulary size from 24 to 30 months was very limited, as shown by the growth curves. For this reason, the predictive value of vocabulary size at one stage on the subsequent lexical outcome began at 36 months of chronological age. In addition, from 42 months of chronological age, children’s developmental age also had a significant influence on subsequent lexical outcome. Although the small number of participants was a limitation of the study, especially because of the wide individual variability that characterises the population of children with Down syndrome, the present study still has applicative value, as it makes available prescriptive data on vocabulary development that clinicians could use to compare children’s lexical production. In particular, it could be useful to assess the position of each child with Down syndrome relative to the average population of children with Down syndrome. These data also have a predictive value, insofar as they allow children’s lexical development to be estimated on the basis of their current vocabulary size. Future research will be oriented to analyse longterm outcomes in lexical development, to assess if children with a higher vocabulary size at 48 months could also have a better outcome in the school age period. Acknowledgements The authors declare that there are no real or potential conflicts of interest related to the manuscript. Note 1. This study focused on the production of words in children with Down syndrome. Only the expressive vocabulary produced using speech, and not
Vocabulary development in Down syndrome vocabulary produced using gestures or augmentative and alternative communication, has been considered.
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