Development of Children at Familial Risk for Dyslexia: Birth to Early School Age, The

Annals of Dyslexia, Dec 2004 by Lyytinen, Heikki, Aro, Mikko, Eklund, Kenneth, Erskine, Jane, Et al

Children at risk for familial dyslexia (n = 107) and their controls (n = 93) have been followed from birth to school entry in the Jyv�skyl� Longitudinal study of Dyslexia (JLD) on developmental factors linked to reading and dyslexia. At the point of school entry, the majority of the at-risk children displayed decoding ability that fell at least 1 SD below the mean of the control group. Measures of speech processing were the earliest indices to show both group differences in infancy and also significant predictive associations with reading acquisition. A number of measures of language, including phonological and morphological skill collected repeatedly from age three, revealed group differences and predictive correlations. Both the group differences and the predictive associations to later language and reading ability strengthened as a function of increasing age. The predictions, however, tend to be stronger and the spectrum of significant correlations wider in the at-risk group. These results are crucial to early identification and intervention of dyslexia in at-risk children.

It is widely held that dyslexia runs in the family (Hallgren, 1950). Rutter, Tizard, & Whitmore (1970) observed concordance rates of 34% in the families (parents and siblings) of reading disabled children but this was only 9% in the families of a control group, a result close to the distributions that we observe in the data summarized here. There is also evidence for a higher concordance rate of dyslexia in monozygotic (66%) when compared with dizygotic (43%) twins (DeFries & Gillis, 1993) and an increased risk for dyslexies relative to controls (Gilger, Pennington, & DeFries, 1991). Pennington (1991) summarizes:

Evidence supports the view that dyslexia is familial (about 35% to 40% of first degree relatives are affected), heritable (with a heritability of about 50%), heterogeneous in its mode of transmission (with evidence for both polygenic and major gene modes of the disorder), and linked in some families (less than 20% of the families studied, Pennington, 1990) to genetic markers on chromosome 15, and possibly in others to genetic markers on chromosome 6 (p. 48).

As Pennington (1990) argued some time ago, it is more likely that behavioral genetics will identify a few major loci for the transmission of both reading ability and disability, rather than a single locus for dyslexia. Nonetheless, more recently, molecular genetics has identified the first candidate gene associated with dyslexia, the results to which our families have made a contribution (Taipale, et al., 2003).

Scarborough (1990) published the first prospective study of familial dyslexia that compared the development (from 2.5 years of age) of three groups of children (at risk who were later diagnosed at school age, at risk who developed with age appropriate reading, and normal readers). The Jyv�skyl� Longitudinal study of Dyslexia (JLD) follows along similar lines to Scarborough with the exception that the children in this study have been followed from birth with the purpose of determining whether familial or genetic influences on development could be observed prior to any potential compensation from environmental factors. In addition, the JLD also incorporates brainrelated measures that have allowed very early access to the processing of sound in the brain.

Although developmental dyslexia is acknowledged to be language related (e.g., Adams, 1990; Catts, 1989), consensus has yet to be reached with regard to the more specific nature of the causal factors. However, within the last two decades, there has been a growing body of evidence suggestive of a "phonological core deficit" (Stanovich, 1986) as underlying developmental dyslexia (Bradley & Bryant, 1983; Catts, 1989; Snowling, et al., 1986; Vellutino, 1979). Snowling (1998) suggests that this core deficit is a result of

poorly specified phonological representations [which] underlie reading and spelling, [and] compromise literacy development by placing limitations on [the] ability to establish the mappings between letter strings and phonology that are critical for learning to read (p. 5).

This claim rests on the assumption that the acquisition of normal reading skill parallels the development of a metalinguistic awareness of segmentation of speech into its component sounds. The awareness of component sounds is arguably a more critical aspect of early literacy development in orthographies such as English where these segments are less explicitly and irregularly marked in the written language. This is in contrast with more regular languages such as Finnish, especially around the time of development when letters begin to assist segmentation-related awareness. Finnish has single-letter graphemes (with only one exception)-a context where the letters explicitly reveal the phonemic makeup of the words-as Cossu, et al. (1988) have suggested, facilitating the child's development of sensitivity to sublexical structures.

Phonological awareness is held to comprise a range of subskills that reflect awareness of different sound units, including small units (phonemes) (Muter, et al., 1998; Snowling, 1980) and larger units such as onsets or rhymes (Goswami & Bryant, 1990). Muter (1998) claims that application of rules of grapheme to phoneme conversion is followed by formation of direct connections between orthographic and phonological entities. Similarly, Ehri (1998) argues that phonological recoding skill is essential from the outset and is required to establish a vocabulary of known words. This sight vocabulary is then addressed when approaching unknown words, either through what she terms "graphophonic recoding" (p. 108) or by analogy with known words. The analogy-based support is not needed nor applied by children when acquiring reading skill in a highly regular orthography because decoding proceeds fully on a phoneme-letter basis (Holopainen, Ahonen, & Lyytinen, 2002).