...reading problems were considered at risk for dyslexia; the other half were not at risk. A novel analysis, mixture modeling, revealed four subgroups with differential developmental trajectories to early reading. The subgroups who showed either a "dysfluent trajectory" (n = 12; 11 at risk vs. 1 control) or a "declining trajectory" (n = 35; 24 vs. 11) contained more children with familial risk for dyslexia. The subgroup showing an "unexpected trajectory" contained equal numbers of at-risk and non-risk children (n = 67; 33 vs. 34). The subgroup displaying a "typical trajectory" (n = 85, 38 vs. 47) contained more children born without dyslexia risk. This differential development of skills revealed that there are at least three troubled routes along which a child may ultimately encounter difficulties in reading acquisition. The most explicit routes are characterized by problems in either phonological awareness, naming speed, or letter knowledge-problems that increase in severity with age.

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One of the best ways to understand developmental disorders is through a prospective follow-up study, especially if it is initiated before the environment can exert its effect on the emerging developmental routes. The Jyvaskyla Longitudinal Study of Dyslexia (JLD) is one such prospective follow-up from birth to school age of 200 Finnish children, half of whom were at familial risk for dyslexia. Over a period of 9 years, the children underwent a plethora of assessments drawing on all psychological disciplines. These included measures assessing early auditory cognition (with event-related potentials, or ERPs), early language (expressive and receptive), cognitive abilities, and recognized correlates to literacy, including phonological awareness, naming speed, letter naming, and early decoding skills. Typically, Finnish children attend preschool or kindergarten at age 6, with formal schooling and reading instruction beginning at age 7 when they enter first grade. The youngest of the JLD children were about to enter third grade, so we had access to data covering birth to second grade. We drew on these data to identify factors in the development of the children's language skills that predict the children's ease in reading acquisition and their problems in foundation-level reading.

We now know that dyslexia is affected by genes, and recently it has been connected even to a single gene (Taipale et al., 2003). It is also clear from the earlier literature (Scarborough, 1990; Snowling, Gallagher, & Frith, 2003) that two domains are integral to dyslexia: the general language domain, including speech and language development, and the more literacy-specific language domain, including the phonological domain. We believe they should be considered separately to ascertain the extent to which they are distinct with regard to reading. During the first years of life, isolating different courses in terms of language development is complex. Nonetheless, even relatively undifferentiated indices of the development of early language, such as measures of early speech comprehension and production, create a basis for the prediction of language skills that emerge later.

During their exposure to spoken language, children's evolutionarily more universal sensitivity to sounds becomes redundant as they learn to home in on those aspects of language that discriminate meaning within their particular language environment (Kuhl, Williams, Lacerda, Stevens, & Lindblom, 1992). Such results show how interest in and implicit attention to aspects of sounds drills the brain's sensitivity to those aspects in a vein similar to how Gibson (1970) describes the development of perception in general.

Elsewhere (for a review of results, see H. Lyytinen, Leppanen, Richardson, & Guttorm, 2003) we have reviewed our data connecting speech sound processing in the brain and accuracy of speech perception to familial risk for dyslexia. The brains of the JLD at-risk infants differed from controls at birth in terms of the processing of speech sounds (Guttorm, Leppanen, Richardson, & Lyytinen, 2001; Guttorm, Leppanen, Tolvanen, & Lyytinen, 2003). In addition, preparedness to develop accuracy in the sensitivity to semantically distinctive features (i.e., categorical perception) also varied between the JLD infants with and without risk, as reflected in brain-event-related potential recordings (Leppanen, Eklund, & Lyytinen, 1997) and in behavioral measurements using a head-turn paradigm (Richardson, 1998). Importantly, and as one of the catalysts to the current pursuit, these speech-processing differences documented immediately after birth and the brain and behavioral indices collected at the age of 6 months were also found to correlate to later language development and to reading acquisition (Guttorm et al., 2005; for a review of the latest psychophysiological findings from the JLD see H. Lyytinen, Guttorm, et al., 2005).

The most prominent subdomain of language development relative to routes to reading acquisition is the phonological domain (Vellutino, Fletcher, Snowling, & Scanlon, 2004). Studies of at-risk children have shown deficits in phonological awareness (Scarborough, 1990), and regression techniques have revealed that the phonological processing abilities of kindergartners and preschoolers can predict later reading ability and, ultimately, reading disability (Lundberg, 1994; Wagner, Torgesen, & Laughon, 1993). Intervention studies with at-risk children have also shown that phonologically based reading intervention is critical to skilled reading (Shaywitz et al., 2004).

Our early predictive signs based on speech processing and differentiating children with and without familial risk for reading difficulties reveal an atypicality that compromises phonological accuracy and possibly affects language development in general (Guttorm et al., 2005; see also H. Lyytinen, Guttorm, et al., 2005, for review and the most recent evidence). Perceptual problems may delay the acquisition of explicit phonemic awareness required for reading (see, e.g., Goswami & Bryant, 1990, for a review). At the same time, insensitivity reflected in speech processing may also affect language development in a more general way. For instance, it is assumed that optimal vocabulary growth is facilitated by the combination of a rich linguistic environment (diversity of input) and intact abilities for continuous phonological restructuring of the representations forming one's vocabulary (e.g. Fowler, 1991; Hoff & Naigles, 2002; Metsala & Walley, 1998).

Research demonstrating the existence of multiple, autonomous routes to compromised reading acquisition is relatively sparse. Unitary claims (Snowling, 1998; Stanovich, 1990), for the most part, have focused on "core" features such as a phonological deficit as underlying reading problems, while previous attempts to differentiate the characteristic error patterns of dyslexia into distinct phonological (Beauvois & Derouesne, 1979) or surface (Coltheart, Masterson, Byng, Prior, & Riddoch, 1983; Marshall & Newcombe, 1973) subtypes have eluded replication. Moreover, in consistent languages such as Finnish, (1) surface-type errors are redundant in the language. In consistent orthographies with low levels of ambiguity in the relationship between letters and sounds, the focus leans toward dysfluent reading (Wimmer, 1993) as the primary characteristic, with a more secondary emphasis placed on errors. The degree of individual variation and overlap with regard to the deficits displayed by dyslexic individuals has made the categorical separation of distinct patterns difficult (Leinonen et al., 2001; see Erskine & Seymour, 2005, for their Cognitive Mosaic Model). Alongside this individual variation, the preparatory skills for a multifactorial ability such as reading are unlikely to vary dichotomously but continuously. It seems apparent, therefore, that different constellations of specific weaknesses form critical "drops" in the skill profiles that may compromise reading acquisition and/or the automatization of reading skill and that such characteristics may form differentiable developmental profiles that we are able to detect using the present methodology.

Rationale for Selection of Critical Developmental Skill Domains

A comprehensive developmental data set spanning 6 years was used for the modeling. Although we acknowledge that basic processes, such as speech perception, create the foundation for the later phonological development (e.g., Bradley & Bryant, 1983), in the present context we restricted our analysis to the use of our early behavioral measures. Seven skill domains were produced from these early measures, including (1) receptive and (2) expressive language skills, (3) inflectional morphology skills (general language domain), (4) memory, (5) retrieving words efficiently from memory (naming speed), (6) letter knowledge, and (7) phonological awareness skills. The development of knowledge relating to visual language (e.g., letter knowledge) is an integral part of the foundation skills necessary for reading acquisition and thus constitutes a necessary component of the analyses.

It is well documented that language begins to differ between children with and without risk for dyslexia prior to 3 years of age (Locke et al., 1997) and not only continues to do so but also correlates to later reading beyond this age (H. Lyytinen, Aro, Eklund, et al., 2004; Scarborough, 1990). The earliest significant predictive correlations from our data are from age 2 years onward (H. Lyytinen, Aro, Holopainen, et al., 2006). For the present modeling we used composite scores of both receptive and expressive language from ages 1 to 5.5 years, capitalizing on parental ratings of early language (MacArthur Communicative Development Inventory, CDI; Fenson et al., 1994) and well-known tests (e.g., Reynell Developmental Language Scales, Reynell & Huntley, 1987; Peabody Picture Vocabulary Test-R, Dunn & Dunn, 1981; Boston Naming Test, Kaplan, Goodglass, & Weintraub, 1983).

Our third developmental domain comprised inflectional morphology. The Finnish language contains fusional agglutinative morphology with very rich and complex sequential inflections and frequent stem variations. A single word can have hundreds or even thousands of variants (Karlsson, 1999). Many of these variations are differentiated by single phonemes (e.g., kodissa [at home]; kodista [from home]). Thus, a child who is able to inflect Finnish accurately must have a proficient implicit ability to manipulate small phonological units. This makes morphological skills highly interesting in our present modeling, as these skills must be mastered (and are easily available for assessment) before the higher-level phonological awareness skills can emerge (i.e., explicit manipulation of phoneme level units). In contrast to English, morphological awareness plays no explicit role in the pronunciation of the Finnish written language. The Finnish writing system is entirely phonological. Every word can be read accurately through knowledge of the letter-sounds and by assembling these in a row as dictated by letters in the written word. However, the morphology affects the average length of Finnish words, which are much longer than in most non-agglutinative languages such as English. Furthermore, as fluency of reading (necessary for reading continuous text due to working memory limits) is the aspect of reading that is most often compromised among Finnish individuals with dyslexia (see, e.g., Holopainen, Ahonen, & Lyytinen, 2001), morphology,...

NOTE: All illustrations and photos have been removed from this article.



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