2. Considerations for expressions of dyslexia

It is clear that depending on what is emphasized in any given language (e.g. fluency in German; visual–spatial memory in Chinese; phonological skills in English), there will be somewhat different manifestations of dyslexia, as well as different predictors of reading failure. Cross-language studies highlight the importance, not only of regular language features, but also the influence of the writing system (orthography) on reading performance [14]. The type of orthography that the child is acquiring is a primary cultural factor that influences reading acquisition in both typical and atypical development [15]. It has been considered that the cognitive processes underpinning reading ability may be differently involved in

It seems inevitable that some kind of labels would need to be created to identify these special readers. The dual route model of reading constitutes the background of dyslexia subtyping [1]. Its central axiom is that no single processing procedure produces the correct pronunciations of both nonwords or pseudo-words (e.g. slint) and exception or irregular words (e.g. pint) [2]. It is theorized that nonwords can only be correctly pronounced using the grapheme-phoneme correspondence rules, the "non-lexical" route; exception words require an additional procedure, the "lexical" route, because they cannot be pronounced by the rules and readers must use context to figure them out. In many studies, subjects are classified in terms of accuracy either as "phonological dyslexics" when pseudo-word reading ability is impaired but irregular word reading is spared, or as "surface dyslexics" when the reverse occurs- irregular word reading is compromised while pseudo-word reading is intact [3]. For example, in accuracy-based studies in Spanish, surface dyslexics were more frequent than phonological dyslexics [4]. However unlike English, most orthographies have highly regular grapheme-phoneme correspondences with relatively few "exception" words [5], so the applicability of the dual-route framework beyond English has been questioned [6]. More recently, researchers have focused on those children who display Single Deficits (phonological processing weakness) and those who have the dreaded Double Deficit or Double Dissociation (phonological processing and processing speed deficits) [7]. They have worked hard comparing disabled reading and cognitive skill performances with normal readers who are carefully matched by chronological age or reading level (reading age), who are younger, or who represent a different ethnicity. The subtyping, use of labels, and multi-control-group comparisons all serve to refine and focus the discussion of how

To a lesser extent, investigators have used neurobiological technology to explore various brain activations: post-mortem studies of brains of individuals with dyslexia [8], Magnetoencephalography (MEG) and Magnetic Source Imaging (MSI) to provide information both on the spatial localization and on the timing of neurophysiological processes [9], positron emission topography (PET) to examine differences in resting state blood flow in regions of interest in the brain [10], and computerized tomography (CT) and structural magnetic resonance imaging (MRI) to examine noninvasively structural brain differences [11]. Decreased activation in the left temporal–parietal cortex of adults with dyslexia was first found using functional magnetic resonance imaging (fMRI) by Constable et al. [12]. These technologies were developed and implemented in an effort to understand better the growth of the phonological and visual processing systems and verbal retrieval systems in beginning learners that for many readers seem automatic. The crucial issue is the reliability of the different sub-types, which differ according to the type of response taken into account (accuracy, speed, or both) and the type of orthography (opaque/

these students learn to read fluently or not.

Neurodevelopment and Neurodevelopmental Disorder

transparent) being evaluated [13].

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2. Considerations for expressions of dyslexia

It is clear that depending on what is emphasized in any given language (e.g. fluency in German; visual–spatial memory in Chinese; phonological skills in English), there will be somewhat different manifestations of dyslexia, as well as different predictors of reading failure. Cross-language studies highlight the importance, not only of regular language features, but also the influence of the writing system (orthography) on reading performance [14]. The type of orthography that the child is acquiring is a primary cultural factor that influences reading acquisition in both typical and atypical development [15]. It has been considered that the cognitive processes underpinning reading ability may be differently involved in

producing the symptoms of Developmental Dyslexia, depending on orthographic transparency [16]. Generally, languages that are considered more transparent with regular orthographies are Spanish, German, Finnish, Dutch, Greek, Italian and Hebrew, while English and French are considered less regular and therefore, more opaque.

A particularly challenging example is found in the standard Arabic language. Most Arabic words are morphologically derived from roots and written Arabic uses three basic diacritical marks corresponding to short vowels. Arabic script is also made with different degrees of internal connectivity or ligation between the letters within a word. In Algeria, standard Arabic is the first written language taught in the first 3 years of schooling, and there is a transition from vowelized to un-vowelized forms of reading starting from the third grade. Although the vowelized form of Arabic is highly transparent, the non-vowelized form is rather opaque [13]. Clearly, even though the language itself is fairly regular, the features of orthography present unique difficulties to students.

A central hypothesis in the area of reading accuracy and speed across orthographies is the Psycholinguistic Grain Size Theory of reading. This idea suggests that differences in reading accuracy and speed across orthographies reflect basic differences in the nature of the phonological recoding and reading strategies [17]. Learning to read in orthographically inconsistent languages cannot rely on letter to sound correspondences (small grain size), forcing the reader to develop flexible unit size recoding strategies, such as morphological units, analogy and whole-word recognition. It would follow that these processing differences would also reflect variable activations in key processing areas in the brain. This theory would seem to impact the less regular features of a language and, particularly, languages such as English and French that are highly irregular and opaque.

Another important idea in this field is the growing body of work demonstrating the predictive ability of "rapid automatized naming" or RAN tasks [18] in reading performance. Several researchers have used these tasks where children are presented with separate arrays of different primary colors, common objects, numerals from 0 to 9, and/or single letters, and are timed while they name the stimuli as quickly as they can. It has been claimed that RAN, and in particular, the RAN alphanumeric component (digit naming and letter naming), is associated with reading success [19]. A predominate and somewhat controversial view is that reading and RAN could be linked together through the general phonological processing system because they both tap the speed of accessing phonological representations in long-term memory [20]. However, some studies suggest that RAN is independent of phonological processing and can, itself, account for variance in reading. This implies that a naming deficit is directly related to orthographic processing- if letters are recognized at a slower rate, letter representations of words are not activated with sufficient speed to create a strong trace of common orthographic features [21]. Further support for this view is found in research that confirms that later in reading skill development, the role of non-alphanumeric RAN diminishes, while that of alphanumeric RAN (letters and digits) increases and becomes the sole predictor of reading at this stage [22]. From a global perspective, this naming speed deficit seems to be more prominent than the phonological deficit, and this appears to be true in both transparent orthographies like Spanish, Finnish, and German, as well as in entirely different and diverse orthographies, such as Hebrew, Chinese, and Japanese [23].
