ORÍGENES DE LA ARGUMENTACIÓN
1. ARGUMENTACIÓN PRE ARISTOTÉLICA
Predictors of lexical development in children with Down syndrome were studied at the group level (Chapter 4) and at the individual, case level (Chapter 5). At the group level, the lexical development of expressive vocabulary in children with Down syndrome was predicted by both internal and external factors: (a) the children’s adaptive level of functioning predicting both receptive and spoken expressive vocabulary, (b) the children’s receptive vocabulary predicting total expressive vocabulary, (c) the maternal educational level predicting early total expressive vocabulary, (d) the children’s early levels of communicative intent predicting later spoken expressive vocabulary, (e) the children’s phonological and phonemic awareness predicting later spoken expressive vocabulary, and (f) the children’s growth in attention skills predicting growth in spoken expressive vocabulary. The receptive vocabulary development could mainly be explained by early levels of receptive vocabulary and their developmental ages. These results closely relate to findings that receptive vocabulary does not show the same weakness as other aspects of language development in children with Down syndrome (Naess et al., 2011) and is more (developmentally) age appropriate. The study described in Chapter 4 is one of the first longitudinal attempts to monitor the lexical development of children with Down syndrome and is the first to include a wide range of internal and external predictors. The results show that many of the same predictors that play a role in lexical development in typically developing children are also found in our longitudinal study. This adds value to the more recent research base that shows that the language development of children with Down syndrome, when it is measured in both spoken and signed modalities, is delayed rather than deviant from typical development (Polisenka, & Kapalková, 2014). Combined with the results of recent research, this thesis (Chapters 3 and 4) provides evidence that the lexical development of children with Down syndrome thus seems to closely resemble that of typically developing children. Children with Down syndrome only differ in the use of multimodal communication, such as the use of manual signs next to speech. Their production of manual signs predicted the spoken vocabulary size of children one year later (Özçaliskan et al., 2016). In the present thesis, the number of manual signs alone did not predict subsequent spoken vocabulary size. This difference may be due to the age range of the children in the two studies. The children in Özçaliskan’s study were on average 2;6 years old, and were thus younger than the children in the present study. The children in the studies of the present thesis, who were between 2 and 8 years-old, may already be beyond the stage in lexical development to see the direct, compensatory effect of manual sign use on later vocabulary development, since most of the
children in the present thesis already started to speak a significant amount of words.
Next to modalities of communication, another important predictor of lexical development of children with Down syndrome lies in their direct environment. In Chapter 4, educational level of mothers was found to be a significant predictor of their children’s vocabulary size. This relationship has earlier been found in typically developing children (Taylor et al., 2013) and children with language impairments (Beitchman et al., 2008). Maternal education was also identified by Couzens, Haynes and Cuskelly (2012) to contribute to the cognitive development in individuals with Down syndrome. Direct parental input and parental use of lexical words, which may be influenced by educational levels, was not investigated in the present thesis and is regarded as a limitation of the study in Chapter 4. Kay-Raining Bird and Cleave (2015) found that mothers, irrespective of whether their child had Down syndrome, language impairments or typical development, fine-tune their input in ways that reflect their children’s vocabulary knowledge equally well. Mothers of children with Down syndrome were found to talk more than mothers in the other two groups, which may be due to a higher degree of repetition in speech. However, further analyses (MacDonald, New, Cleave, & Kay-Raining Bird, n.d., in Kay-Raining Bird & Cleave, 2015) demonstrated that mothers go beyond simply repeating the same information by providing additional contextual and semantic information. The role of the environment was further specified in Chapter 5. Next to the involvement in interpersonal interactions and relationships that influence receptive and expressive vocabulary development, other environmental factors should be accounted for, such as the use the availability of products and (assistive) technology in the child’s environment, acceptance of and attitudes about disabilities and AAC in the community and the availability of services, systems and policies for (health) care and education for children with Down syndrome.
Most predictors of lexical development have been determined within group studies. Group studies, as well as the described Down syndrome behavioural phenotype, can help professionals and families anticipate to areas of relative strength and weakness in children with Down syndrome and plan interventions accordingly without losing sight of individual differences. But, behavioural phenotypes are probabilistic and not deterministic; they can offer guidance about the probability of certain characteristics being present, but in planning interventions for children with Down syndrome, individual differences should be accounted for (McDaniel & Yoder, 2016). To account for these individual differences in internal and external factors contributing to lexical development in intervention planning, speech-language pathologists may benefit from looking
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beyond the children’s strengths and weaknesses in speech and language skills alone to those in other developmental domains that are interacting with their language skills (McDaniel & Yoder, 2016). Chapter 5 shows this ‘whole person’ perspective within a multiple case-study design, based on the ICF-CY and the body structures, body functions, activity and participation domains and the environmental factors playing a role in lexical development and communication. The Down syndrome behavioural phenotype, as described in the introduction, could not account for the individual differences in these ICF-CY domains, although children shared vocabulary sizes and/or mental ages. This shows the value of also conducting in-depth case-studies, next to group studies.