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Chapter 46 Dyslexia

Developmental dyslexia (or specific reading disability) is defined as an unexpected difficulty in accuracy or fluency of reading for an individual’s chronological age, intelligence, level of education, or professional status. Dyslexia is, at its core, a problem with phonological processing: that is, getting to the elemental sounds of spoken language, affecting both spoken and written language. As a consequence, individuals who are dyslexic require accommodations for their lack of reading and/or oral fluency. As dyslexic children progress in school, given good instruction, reading accuracy often improves; however, lack of fluency persists and remains a lifelong problem. Dyslexia is the most common and most comprehensively studied of the learning disabilities, affecting 80 percent of all individuals identified as learning-disabled. Historically, dyslexia in adults was first observed in the latter half of the 19th century, and developmental dyslexia in children was first reported in 1896 [Morgan, 1896]. Although the diagnosis and implications of dyslexia were often uncertain in the past, advances in our knowledge of the epidemiologic, neurobiologic, and cognitive influences on the disorder allow it to be approached within the framework of a traditional medical model. This chapter reviews these advances and their implications for the approach to children and adults with dyslexia.


Perhaps the most consistent and enduring core of the definition is the concept of dyslexia as an unexpected difficulty in reading. “Unexpected” refers to the presence of a reading difficulty in a child (or adult) who appears to have all of the factors (intelligence, motivation, exposure to reasonable reading instruction) present to be a good reader but who continues to struggle [Shaywitz, 1998]. Recent evidence provides empiric support for defining dyslexia as an unexpected difficulty in reading. Using data from the Connecticut Longitudinal Study, we [Ferrer et al., 2010] demonstrated that, in typical readers, reading and IQ development are dynamically linked over time. Not only do reading and IQ track together over time, they also influence one another. Such mutual interrelationships are not perceptible in dyslexic readers, suggesting that reading and cognition develop more independently in these individuals (Figure 46-1). These findings provide the first empirical demonstration of a coupling between cognition and reading in typical readers, confirming the general public perception that if you are a good reader, you are likely to be very intelligent and, conversely, if you struggle to read, you may be less intelligent. These new data demonstrating a developmental uncoupling between cognition and reading in dyslexic readers indicate that dyslexia is a special case that violates the assumption that reading and IQ are always linked, and confirm that, in dyslexia, one can be highly intelligent and still struggle to read.

Our findings of an uncoupling between IQ and reading in dyslexia, and the influence of this uncoupling on the developmental trajectory of reading, provide evidence to support the conceptual basis of dyslexia as an unexpected difficulty in reading in children who otherwise have the intelligence to learn to read but struggle to read fluently. Based on dynamic models, the uncoupling of reading and cognition observed demonstrates that, in the special case of dyslexia, a child or adult can be both bright and accomplished along with a much lower level of reading than expected for a person of that level of intelligence, education, or professional status. It also demonstrates that, in dyslexia, the reading difficulty is unexpected for an individual’s level of intelligence or education; that is, the difficulty is defined as a disparity existing within the individual. The implication is that, for individuals who are dyslexic, the appropriate comparison is between a person’s ability and his/her reading. Thus, in dyslexia, a highly intelligent person may read at a level above average but below that expected, based on his/her intelligence, education, or accomplishments. These new findings provide an explanation for the “unexpected” nature of developmental dyslexia and also supply the long-sought empirical evidence for the seeming paradox involving cognition and reading in individuals with developmental dyslexia.

More challenging has been the question of how to operationalize the unexpected nature of dyslexia. Thus, using differing methods and criteria, definitions have attempted to capture the “unexpected” nature of dyslexia by requiring a discrepancy of a certain degree between a child’s measured IQ and his reading achievement. For example, schools have typically relied on criteria based on an absolute discrepancy, most commonly one or one-and-one-half standard deviations between standard scores on IQ and reading tests. We want to emphasize that the difficulty has been not with the concept of a disparity, but rather with the real-life practical effect of implementing this model in a primary school setting. For example, children who were clearly struggling as early as kindergarten or first grade had to wait, often until third grade or later, until their failure in reading was of such a magnitude that they met discrepancy requirements. Attempts to clarify the criteria by meta-analyses comparing discrepant to simply low-achieving poor readers (defined on the basis of a reading score below a certain cut point, e.g., below a standard score of 90) find overlap between the two groups on reading-related constructs but differences on IQ-related measures [Stuebing et al., 2002]. In addition, studies examining growth curve models for low-achieving and discrepant readers indicate comparable reading plateaus (level of reading achievement) reached by the two groups but with the IQ-discrepant readers showing the lowest achievement level at any IQ level during the school years [Francis et al., 1996]. Not only do poor readers identified by either discrepancy or low-achievement criteria resemble one another on measures of reading and growth rates of reading, but each group also differs along multiple dimensions from groups of typically achieving boys and girls [Lyon et al., 2001].

These findings have strong educational implications. It is not valid to deny the education services available for disabled or at-risk readers either to low-achieving, nondiscrepant children, or to those children who are not low-achieving but who, at the same time, are reading below a level expected for their ability. The observed similarity of the discrepant and low-achieving groups in reading-related constructs argues for identification approaches that include both low-achieving children and those struggling readers who are discrepant but who do not satisfy an arbitrary cut point for designation as low-achieving. Seventy-five percent of children identified by discrepancy criteria also meet low-achievement criteria in reading; the remaining 25 percent who meet only discrepancy criteria may fail to be identified and yet still be struggling to read [Shaywitz et al., 1992].

Difficulties in identifying younger children based solely on a discrepancy score bring into focus the fact that dyslexia, as most other disorders in medicine, is a clinical diagnosis. Accordingly, while it may not yet be possible to demonstrate a quantitative disparity between ability and achievement in the lowest grades, it is still possible to demonstrate the fundamental concept of an unexpected difficulty in reading. Here, a history of core symptoms (weaknesses and strengths, see below), observation of oral reading demonstrating inaccurate and lack of fluent reading, and cognitive and psychological processing test scores indicating reading and, particularly in younger children, phonological processing difficulties (as well as strengths discussed below) should provide the necessary evidence to diagnose dyslexia. Dyslexia is neither diagnosed nor accurately represented by a single score on a test, but by consideration of a broader clinical picture conforming to the known characteristics of the disorder.


Epidemiologic data indicate that, like hypertension and obesity, dyslexia fits a dimensional model. Within the population, reading ability and reading disability occur along a continuum, with reading disability representing the lower tail of a normal distribution of reading ability [Shaywitz et al., 1992]. Dyslexia is perhaps the most common neurobehavioral disorder affecting children, with prevalence rates ranging from 5 to 17.5 percent [Interagency Committee on Learning Disabilities, 1987; Shaywitz, 2003]. Longitudinal studies, prospective [Francis et al., 1996; Shaywitz et al., 1999] and retrospective [Bruck, 1992; Felton et al., 1990], indicate that dyslexia is a persistent, chronic condition; it does not represent a transient developmental lag (Figure 46-2). Over time, poor readers and good readers tend to maintain their relative positions along the spectrum of reading ability [Francis et al., 1996; Shaywitz et al., 1995].


Fig. 46-2 Trajectory of reading skills over time in nonimpaired and dyslexic readers.

Numbers on the ordinate are Rasch scores (W scores) from the Woodcock–Johnson reading test [Woodcock and Johnson, 1989], and numbers on the abscissa are ages in years. Dyslexic and nonimpaired readers improve their reading scores as they get older, but the gap between the dyslexic and nonimpaired readers remains. Dyslexia is a deficit, not a developmental lag.

(Adapted from Shaywitz S. Overcoming dyslexia: A new and complete science-based program for reading problems at any level. New York: Alfred A. Knopf, 2003. Copyright 2003 by S. Shaywitz. Adapted with permission.)


Dyslexia is both familial and heritable [Pennington and Gilger, 1996]. Family history is one of the most important risk factors, with 23–65 percent of children who have a parent with dyslexia reported to have the disorder [Scarborough, 1990]. A rate among siblings of affected persons of approximately 40 percent and among parents of 27–49 percent [Pennington and Gilger, 1996] provides opportunities for early identification of affected siblings and often for delayed but helpful identification of affected adults. Given that dyslexia is familial and heritable, initial hopes that dyslexia would be explained by one or just a few genes have been disappointing. Thus, along with a great many common diseases, genome-wide association studies (GWAS) in dyslexia have so far identified genetic variants that account for only a very small percentage of the risk – less than 1 percent [Meaburn et al., 2008]. Current evidence suggests “that common diseases involve thousands of genes and proteins interacting on complex pathways” [Duncan, 2009], and that, similar to experience with other complex disorders (heart disease, diabetes), it is unlikely that a single gene or even a few genes will identify people with dyslexia. Rather, dyslexia is best explained by multiple genes, each contributing a small amount of the variance. Thus, current evidence suggests that the etiology of dyslexia is best conceptualized within a multifactorial model, with multiple genetic and environmental risk and protective factors leading to dyslexia.

Cognitive Influences

Among investigators in the field, there is a strong consensus supporting the phonologic theory. This theory recognizes that speech is natural and inherent, but that reading is acquired and must be taught. To read, the beginning reader must connect the letters and letter strings (i.e., the orthography) to something that already has inherent meaning – the sounds of spoken language. In the process, a child has to develop the insight that spoken words can be pulled apart into the elemental particles of speech (i.e., phonemes) and that the letters in a written word represent these sounds [Shaywitz, 2003]; such awareness is largely deficient in dyslexic children and adults [Liberman and Shankweiler, 1991; Shankweiler et al., 1979; Shaywitz, 2003]. Results from large and well-studied populations with reading disability confirm that, in young school-age children [Stanovich and Siegel, 1994] and in adolescents [Shaywitz et al., 1999], a deficit in phonology represents the most robust and specific correlate of reading disability [Morris et al., 1998; Ramus et al., 2003]. Such findings form the basis for the most successful and evidence-based interventions designed to improve reading [Report, 2000].

Implications of the Phonologic Model of Dyslexia

Reading comprises two main processes: decoding and comprehension [Gough and Tunmer, 1986]. In dyslexia, a deficit at the level of the phonologic module impairs the ability to segment the spoken word into its underlying phonologic elements. As a result, the reader experiences difficulty, initially in spoken language and then in written language. The phonologic deficit is domain-specific; that is, it is independent of other, nonphonologic abilities. In particular, the higher-order cognitive and linguistic functions involved in comprehension, such as general intelligence and reasoning, vocabulary [Share and Stanovich, 1995], and syntax [Shankweiler et al., 1995], are generally intact. This pattern – a deficit in phonologic analysis contrasted with intact higher-order cognitive abilities – offers an explanation for the paradox of otherwise intelligent, often gifted people who experience great difficulty in reading [Ferrer et al., 2010; Shaywitz, 1996, 2003].

Neurobiologic Studies

Neural Systems for Reading

Our understanding of the neural systems for reading emerged more than a century ago, with descriptions of adults who (usually due to a stroke) suddenly lost their ability to read, a condition termed acquired alexia. These postmortem studies, pioneered by Dejerine as early as 1891, suggested that a portion of the left posterior brain region (which includes the angular gyrus and supramarginal gyrus in the inferior parietal lobule and the posterior aspect of the superior temporal gyrus) is critical for reading [Dejerine, 1891]. Another left posterior brain region, one more ventral in the occipito-temporal area, was also described by Dejerine [Dejerine, 1892] as critical in reading. Within the last two decades, the development of functional brain imaging, particularly functional magnetic resonance imaging (fMRI), has provided the most consistent and replicable data on the location of the neural systems for reading and how they differ in dyslexic readers. fMRI is noninvasive and safe, and can be used repeatedly, properties which make it ideal for studying people, especially children. The signal used to construct MRI images derives from the determination of the blood oxygen level-dependent (BOLD) response; the increase in BOLD signal in regions that are activated by a stimulus or task results from the combined effects of increases in the tissue blood flow, volume and oxygenation, and in cognitive tasks the changes are typically in the order of 1–5 percent. Details of fMRI are reviewed in Anderson and Gore [1997], Frackowiak et al. [2004], and Jezzard et al. [2001]. To date, fMRI in dyslexic individuals can be carried out reliably only at a group level. The technology for determining brain activation at an individual subject level remains a work in progress.

Reflecting the language basis for reading and dyslexia, three neural systems critical in reading and dyslexia are localized in the left hemisphere (Figure 46-3): two left hemisphere posterior systems, one around the occipito-temporal region and another in the left occipito-temporal region, and an anterior system around the inferior frontal gyrus (Broca’s area) [Brambati et al., 2006; Helenius et al., 1999; Kronbichler et al., 2006; Nakamura et al., 2006; Paulesu et al., 2001; Shaywitz et al., 2002; Shaywitz et al., 2003; Shaywitz et al., 1998].

Many brain imaging studies in children and adults with developmental dyslexia (see below) have documented the importance of the left occipito-temporal system in reading, and its properties involving word analysis, operating on individual units of words (e.g., phonemes). In our figure we refer to the occipito-temporal system, which encompasses portions of the supramarginal gyrus in the inferior parietal lobule, portions of the posterior aspect of the superior temporal gyrus, and in some studies, may even encompass portions of the angular gyrus in the parietal lobe. The second posterior reading system is localized in the left occipito-temporal area, which Cohen and Dehaene have termed the visual word-form area (VWFA) [Cohen et al., 2000; Dehaene et al., 2005; Vinckier et al., 2007]. Just how the VWFA functions to integrate phonology (sounds) and orthography (print) is as yet unknown, though some have suggested that visual familiarity, phonological processing, and semantic processing all make significant but different contributions to activation of the word-form region [Cohen et al., 2004; Henry et al., 2005; Johnson and Rayner, 2007; Xue et al., 2006]. Still another reading-related neural circuit involves an anterior system in the left inferior frontal gyrus (Broca’s area), a system that has long been associated with articulation and also serves an important function in word analysis [Fiez and Peterson, 1998; Frackowiak et al., 2004].

The Reading Systems in Dyslexia in Children and Adults

Converging evidence from many laboratories around the world has demonstrated what has been termed “a neural signature for dyslexia”: that is, inefficient functioning of left posterior reading systems during reading real words and pseudowords, and often what has been considered as compensatory overactivation in other parts of the reading system. This evidence from functional brain imaging has, for the first time, made visible what previously was a hidden disability (Figure 46-4).


Fig. 46-4 Neural signature for dyslexia.

A neural signature for dyslexia is illustrated in this schematic view of left hemisphere brain systems in (left) nonimpaired and (right) dyslexic readers. In typical readers, the three systems provided in Figure 46-3 are shown. In dyslexic readers, the anterior system is slightly overactivated compared with systems of typical readers; in contrast, the two posterior systems are underactivated. This pattern of underactivation in left posterior reading systems is referred to as the neural signature for dyslexia.

(Adapted from Shaywitz S. Overcoming dyslexia: A new and complete science-based program for reading problems at any level. New York: Alfred A. Knopf, 2003. Copyright 2003 by S. Shaywitz. Adapted with permission.)

For example, in a study from our own research group, we [Shaywitz et al., 2002] used fMRI to study 144 dyslexic and nonimpaired boys and girls as they read pseudowords and real words. Our results indicated significantly greater activation in typical readers than in dyslexic readers during phonologic analysis in the posterior reading systems. Our data converge with reports from many investigators using functional brain imaging in dyslexia that show a failure of left hemisphere posterior brain systems to function properly during reading (reviewed in Richlan et al. [2009]; Shaywitz and Shaywitz [2005]). Recent studies report similar findings in German [Kronbichler et al., 2006] and Italian [Brambati et al., 2006] dyslexic readers.