There is evidence for the involvement of a number of skills in the development of reading accuracy in children with Down syndrome. It appears that visual processing and
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memory, verbal memory, oral language and phoneme awareness may all correlate with the development of reading accuracy. However the research reviewed thus far has been concurrent and consequently it is difficult to establish the direction of the relationships, for example it could be that children with Down syndrome develop better oral language skills as a result of good reading accuracy skills, or that pre-existing good oral language skills help these children to become better readers, or indeed both. There have been a small number of longitudinal studies focussing on the development of reading accuracy, and these can help clarify these relationships.
Cupples and Iacono (2000) investigated the longitudinal relationship between
phonological awareness and reading accuracy with 19 children with Down syndrome aged 6-10 years, at two time-points which were approximately nine months apart. Regression analyses were conducted to examine the predictors of reading accuracy and nonword reading.
Chronological age, verbal short-term memory and receptive vocabulary were always entered at the first step; however the correlations of these variables with reading accuracy or their independent contributions to outcome variables in the regression are unfortunately not reported. Initial phoneme segmentation ability explained significant additional variance in nonword but not word reading after the autoregressors were controlled for. However, the mean phoneme segmentation scores were close to floor at both time points and several children were reported to not understand the task.
Roch and Jarrold (in press) conducted a follow-up study of 12 of the adolescents and adults with Down syndrome who had taken part in Roch and Jarrold (2008) four years previously. The aim was to examine whether there were any changes in the use of the visual versus phonological route to reading, and to investigate the longitudinal associations between nonword reading, irregular word reading and phonological awareness. To briefly revisit the Time 1 results, phonological awareness was strongly correlated with nonword reading whereas nonword reading and irregular word reading were not significantly correlated with each other. In contrast to these results, at the follow-up phonological awareness was no longer strongly correlated with nonword reading whereas the correlation between irregular and nonword reading was now significant. In regression analyses controlling for the autoregressor, nonword reading did not predict irregular word reading. In contrast irregular word reading did predict
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later nonword reading, although phonological awareness did not. The authors suggest that with developing word recognition skill, as indexed by irregular word reading, individuals with Down syndrome adopt a different strategy to reading nonwords, namely by the increasing use of visual analogies.
The above studies have concentrated on phonological awareness and different reading outcomes. As illustrated by concurrent studies, skills such as memory and oral language may also influence reading accuracy. One study to include a broader battery of tests is that of Kay- Raining Bird, Cleave and McConnell (2000). They conducted a longitudinal study over four and a half years with 12 children with Down syndrome, who were aged 6-11 years at the outset of the study. There were three time points, with a lag of three years between the first two time- points and 18 months between the second and third time-point. The children were assessed on a range of oral language, reading, memory and phonological awareness tasks at all time-points. Over the course of the study, there were significant improvements in reading accuracy,
nonword reading and rhyme generation, but not in phoneme segmentation and deletion, for which the scores were generally low. At every time-point, reading accuracy was better than nonword reading, which the authors suggested shows continued reliance on a visual reading strategy. In terms of the relationship between tasks, after controlling for age and nonverbal ability, the highest correlations for endpoint nonword reading were with initial phoneme segmentation and digit span; however the correlation with phoneme segmentation disappeared after controlling for digit span but not vice-versa. The highest correlations for Time 3 reading accuracy were with initial digit span, phoneme segmentation and phoneme deletion. Kay-Raining Bird et al. also included measures of receptive vocabulary and MLU, and the correlations between these and endpoint reading skills were low to moderate. However, the descriptive scores for the two language measures are not reported, so the correlations cannot be interpreted in the light of the distribution of scores. Given the small sample size and low scores on key measures, including nonword reading and phoneme segmentation, these results must be considered as preliminary.
Laws and Gunn (2002) examined the development of reading, oral language and cognition in a group of 30 individuals with Down syndrome over a period of five years. The individuals who took part were aged 10-24 years at the end of study. They were assessed on
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receptive language, nonverbal ability, nonword repetition, letter knowledge and reading accuracy at both time points. Hearing thresholds, MLU, alliteration matching and rime matching were also assessed at the second time point. The sample was divided into readers, defined as being able to read at least one word at Time 2, and non-readers. The readers were significantly older than the non-readers and had better nonverbal ability, receptive grammar and MLU. Furthermore, significantly more individuals in the reader group performed above chance on the rime and alliteration matching tasks. There were also group differences in the same direction for nonword repetition and receptive vocabulary, but these were not significant once hearing ability was controlled for. Over the course of the study, five individuals began to read; these individuals had significantly better nonverbal ability and better language skills than those who remained non-readers at the second time-point. The criterion for being a reader was rather conservative and unsurprisingly the range of scores within this group was large, but the between-group analyses do not capture this. Correlations were also conducted and initial receptive vocabulary, receptive grammar and letter-name knowledge were significantly correlated with later reading accuracy.
The longitudinal studies above did not include control groups, however it is important to use a control group to identify similarities and differences with typical development. Over a period of two years, Byrne, MacDonald and Buckley (2002) monitored the progress of 24 children with Down syndrome aged 4-12 years, 21 reading accuracy matched children aged 4-9 years and 32 average readers aged 4-9 years. The average readers were recruited from the same classroom as the children with Down syndrome, but were significantly younger and the reading accuracy matched control group showed delayed reading and language skills relative to their chronological age. The children were assessed on various skills including reading accuracy, verbal memory, visual memory, receptive vocabulary and receptive grammar. Over the two years, children with Down syndrome made significant progress in reading accuracy, although this was significantly slower than the average readers. Overall, children with Down syndrome showed relatively advanced reading accuracy compared to their language ability. The children with Down syndrome and reading accuracy matched children showed generally stronger correlations between reading accuracy and language than the average reader group, and when controlling for age, the correlation between reading accuracy and grammar was greater than
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that between reading accuracy and vocabulary, which was not significant. Furthermore, when age was controlled, the strongest correlations with reading accuracy for the children with Down syndrome were with both visual and verbal memory. Although typically developing control groups were used in this study, the average reader group were not matched on any variable and although the other group were matched on reading accuracy, they were considered to have delayed reading and therefore may not represent reading in typical development.
Hulme et al. (in press) compared a large group of individuals with Down syndrome with a typically developing group of children matched at the same level of reading accuracy at the beginning of the study. Forty-nine children and adolescents with Down syndrome with a mean age of 10;04 and 61 typically developing children with a mean age of 6;05 took part at three time points, each separated by approximately 12 months. Tasks of nonverbal ability, reading accuracy, vocabulary, letter-sound knowledge and phonological awareness were administered. As found in Byrne et al. (2002), the children with Down syndrome made significantly less progress on reading accuracy than the typically developing children. Path models were used to investigate whether earlier phoneme awareness, vocabulary and letter-sound knowledge predicted reading accuracy after controlling for the autoregressor. In the individuals with Down syndrome, no other variable predicted reading accuracy after the autoregressor, however in the typically developing group earlier phoneme awareness was found to predict reading accuracy development. Due to the strong autoregressive effect of reading accuracy for the individuals with Down syndrome, there was little variance remaining which the other measures could account for. Therefore it may be of interest to examine the predictors of reading
accuracy at Time 1. Phoneme awareness was a predictor of reading accuracy in the typically developing children but not vocabulary whereas the reverse was true for the children with Down syndrome. Letter-sound knowledge did not predict concurrent reading accuracy in either group.