Chapter 1 Developmental Dyslexia and Specific Language Impairment. The same or different? Developmental dyslexia is defined as a failure to learn to read properly, despite normal intelligence, normal hearing, adequate classroom exposure and the absence of physical, emotional or socioeconomic problems (Vellutino, 1979). According to the International Dyslexia Association, dyslexia is a specific language-based reading disorder. Specific Language Impairment (SLI) is defined as a developmental language disorder in which affected children fail to acquire language properly, despite having normal non-verbal intelligence, normal hearing and no known neurological dysfunctions or behavioural, emotional or social problems (Leonard, 1998) Both disorders are defined by way of exclusion: a child is said to be specifically reading impaired or specifically language impaired when the relevant impairment is not caused by a known handicap. However, for both disorders this entails that it is not clear what is specific about them. To make matters worse, both dyslexia and SLI are heterogeneous disorders. Common profiles of dyslexia and SLI do exist, but they don’t fit all individuals suffering from either of these disorders. This makes it even less likely that all children diagnosed as dyslexic or language impaired will exhibit the same symptoms. Furthermore, there seems to be a considerable overlap between these two clinical populations. Children with dyslexia and SLI often suffer from the same symptoms, including poor phonological processing, poor short-term memory skills and difficulties in speech perception. Problems with morphosyntax are mainly associated with SLI, but dyslexic children have also been found to experience problems in this domain. Thus, some children fit the criteria of both disorders and are labelled as dyslexic or language impaired by chance, depending on the type of clinician that the child first encounters. McArthur et al. (2000), for example, found that 50% of the children in their study that were diagnosed as dyslexic also fit the defining criteria of SLI, and vice versa. This seems an unhealthy situation, to say the least. Despite a great amount of research on this topic, the literature remains unclear as to whether dyslexia and SLI are essentially the same or different. This chapter provides a condensed literature study of dyslexia and of SLI, and of the possible relation between them. 8 Dyslexia and SLI: same or different? 1.1. General characteristics of developmental dyslexia Individuals with dyslexia find it difficult to read. Apart from this core deficit, dyslexics often exhibit difficulties in related areas such as spelling and writing. Problems with spoken language are also not uncommon: dyslexics may find it difficult to express concepts clearly or to fully comprehend what others mean when they speak. Such language difficulties are usually subtle (and therefore difficult to recognize), but they can lead to significant problems. In addition to problems in the language domain, dyslexics are often said to have a visual impairment as well (Lovegrove, 1994; Stein et al., 2001).1 Some researchers believe that it is this visual impairment that causes difficulties with the processing of letters and words on a page of text. These difficulties include unstable binocular fixations, poor vergence and increased visual crowding. It has been known for some time that dyslexia runs in families; dyslexic parents are more likely to have children who are dyslexic than non- dyslexic parents. Whereas dyslexia affects around 3-10% of the population at large, the risk of first-degree relatives of dyslexics to develop the disorder is estimated to be around 40% (Gilger et al., 1991). The impact of dyslexia depends on the severity of the condition and on the amount of remediation that an individual has received. Some dyslexics, for example, do not have much difficulty with early reading and spelling tasks but do experience great problems when more complex language skills are required (such as learning a foreign language, understanding textbook material and writing essays). Other areas that are typically affected in dyslexia are short-term memory, mathematics, attention and sequencing. 1.2. Theories of developmental dyslexia 1.2.1. The phonological deficit theory Perhaps the best-known theory of developmental dyslexia is the phonological deficit theory. This theory assumes the representation, storage and/or retrieval of speech sounds to be impaired in dyslexic individuals (Ramus, 2003). A pre-requisite of learning how to read is the acquisition of the grapheme-phoneme correspondences of an alphabetic system. In other words, a child has to discover that letters and constituent sounds of speech are connected. Theorists explain dyslexia by claiming that if speech sounds are poorly represented, stored or retrieved this would result in a poor understanding of the grapheme-phoneme correspondences of a particular 1 The visual theory of dyslexia will not be discussed in any detail in this dissertation. The interested reader is referred to the work of John Stein, e.g. Stein et al., 2001 and to the anatomical studies of Livingstone et al., 1991 and Cornelissen et al., 1995). Dyslexia and SLI: same or different? 9 language (Snowling, 1981, 2001); Brady and Shankweiler, 1991). Supporters of the phonological deficit theory believe that phonology has a central and causal role in dyslexia, suggesting a straightforward link between a cognitive deficit and the behavioural problem. The phonological deficit theory is supported by findings that dyslexics perform poorly on tasks involving phonological awareness. Phonological awareness refers to the conscious segmentation and manipulation of speech sounds. The ability to analyze words into consonants and vowel segments is a typical example of a phonological awareness skill. According to Snowling (2001) poor verbal short term memory and slow automatic naming also points to a basic phonological deficit. At the neurological level, functional brain imaging studies (Pugh et al., 2000, Shaywitz et al., 2002) and anatomical work (Galaburda et al., 1985) suggest that a congenital dysfunction of the left perisylvian brain areas forms the basis of the phonological deficit. In its strongest version, the theory claims that the cognitive deficit in dyslexia is specific to phonology. This claim has caused an ongoing debate. Challengers of the phonological deficit theory believe that dyslexia is much more far-reaching than the theory suggests, with its origin in general sensory, motor and learning processes. The existence of phonological problems in dyslexia is not disputed, but such problems are argued to represent only one facet of a more general disorder. Phonological deficits, for example, could be secondary to a more basic auditory deficit. This view forms the basis of the next theory. 1.2.2. The temporal processing deficit The temporal processing deficit hypothesis (also known as the rapid auditory processing theory) challenges the specificity of the phonological deficit in dyslexia by claiming that phonological problems arise as a result of a more basic auditory deficit (Tallal, 1980, Tallal et al., 1993). Like the phonological theory, the auditory processing theory assumes that a cognitive deficit lies at the heart of dyslexia. Originally, Tallal introduced the rapid auditory processing theory in the 1970’s as an explanation of SLI, but went on to suggest that it could also explain the problems of dyslexics. The main idea of this theory is that dyslexia is the result of a perturbation of auditory language processing in the temporal domain. The consequence of a ‘temporal processing deficit’ is that children are not fully capable of perceiving and processing short or rapidly varying acoustic events, including those crucial to the recognition of speech sounds (Tallal et al. 1996). According to Tallal et al. (1993, 1996) a failure to correctly represent short sounds and fast transitions would cause further difficulties; in particular when such acoustic events are cues to phonemic contrasts (like in 10 Dyslexia and SLI: same or different? ‘ba’ versus ‘da’). This claim is compatible with various indications that dyslexic children have deficient speech sound representations (Liberman, 1973; Lyytinen et al. 1992). The auditory processing deficit is further supported by findings of Tallal and her colleagues that dyslexics perform poorly on auditory tasks like tone discrimination and temporal order judgement as well as on backward masking.2 The nature of the temporal processing deficit is controversial. It is not clear whether the processing difficulties concern temporal features of speech. As Bishop (1997) puts it, the correct label might be rapid processing. Rate could be more important than the sequential ordering of stimuli. Researchers are in disagreement about the nature of the underlying factor that causes brief, rapidly presented sequences to be difficult to process. 1.2.3. The cerebellar theory The problems faced by dyslexic individuals are not confined to reading and spelling. Dyslexics appear to have a general impairment in their ability to perform skills automatically. This ability is thought to be dependent upon the cerebellum. The cerebellar theory (Fawcett & Nicolson, 2004) has a biological basis in assuming that the cerebellum of dyslexics is mildly dysfunctional. As a result, a number of cognitive difficulties arise. The cerebellum plays an important role in motor control and thus in speech articulation.