Which best describes the relationship between phonological and lexical development?

65.1 Introduction

Developmental dyslexia is a neurobiological disorder with a strong genetic basis. Various theories have been proposed to account for the cognitive origin of developmental dyslexia. Although the phonological theory remains as the most plausible account of the disorder, recent findings suggest that other impairments – a rapid automatized naming or visual attention span disorder – might independently contribute to developmental dyslexia. Dyslexia in the child is characterized by atypical functioning of the left-sided neural network of reading which includes regions of the frontal, parietal, and temporal lobes. The dyslexia candidate susceptibility genes which have been identified are involved in brain development.

Abstract

Developmental Dyslexia (DD) significantly interferes with academic, personal, social and emotional functioning. Nevertheless, established therapeutic options are still scarce. Research has begun to emerge studying the potential action of transcranial direct current stimulation (tDCS) for ameliorating reading. However, there are still open questions regarding the most suitable tDCS protocol in young with DD.

The current study tested the effectiveness of a short, intensive and multi-session tDCS protocol and presented preliminary data from a randomized sham-controlled crossover trial. Twenty-seven children and adolescents with DD were randomly assigned to active tDCS or sham tDCS. Active tDCS group received five daily-consecutive sessions of left anodal/right cathodal set at 1 mA for 20 min over parieto-occipital regions. Reading measures, including text, high frequency word, low frequency word and non-word lists, were recorded before, immediately after the treatment and 1-week later.

We found that only the active tDCS group ameliorated non-word reading speed immediately after and 1-week later the end of the treatment compared to the baseline.

Some suggestions for the development of future tDCS protocols in children and adolescents with DD are given.

Children with developmental dyslexia have specific problems with reading and spelling. These problems have no apparent environmental or neurological cause, and are most likely due to difficulties in the accurate specification and neural representation of the sequential sounds of speech. The normal development of lexical representation is outlined, followed by a survey of the abnormal lexical development characteristic of developmental dyslexia. Possible basic auditory processing deficits are considered. Relevant information from brain imaging and genetic linkage studies is discussed. The manifestation of developmental dyslexia in languages other than English and possible subtypes of developmental dyslexia are considered in arriving at a phenotype for this disorder.

Developmental dyslexia and developmental dysgraphia are learning disabilities of reading and writing arising during the normal course of development despite adequate learning, instructional opportunities, and intelligence. Research has yielded much information on the different ways in which differences in abilities are seen in children's reading and writing behaviors. There are different theories of the causes and consequences of dyslexia and dysgraphia. Recent research in genetics, brain imaging and brain processing, and behavioral sciences supports a link between reading and writing skills and specific linguistic skills. This research is informing intervention and instructional treatments for children with and without disabilities.

Abstract

Developmental dyslexia is a neurodevelopmental disorder defined by poor word-reading ability. Dyslexia exists in every culture studied and affects approximately 10% of the population. Its etiology is multifactorial and includes several genes and environmental risk factors. Dyslexia is a language-based disorder whose primary underlying deficit involves poor phonological processing, although phonological deficits interact with other neurocognitive risk and protective factors. At the brain level, dyslexia is associated with abnormal structure and function of left hemisphere reading and language networks. Effective treatments include direct, explicit instruction in phonological processing, phonics, and reading fluency.

Developmental dyslexia, which affects in the range of 5–12% of the population, is a condition in which children who receive regular reading instruction and who possess typical intellectual skills show significant difficulties in the decoding of written text and hence impeded ability to extract meaning. Difficulties in phonological coding have been shown to be a core deficit in children with dyslexia. In addition, there exists a large corpus of research indicating a variety of deficits in sensorimotor performance. Genetic and brain imaging studies support biological aberrations within these disparate domains. The most promising avenues for treatment reside in behavioral interventions involving language, the precise benefits of which continue to be investigated.

Introduction

Developmental dyslexia is defined as deficient literacy acquisition despite adequate intellectual ability and sufficient educational exposure (American Psychiatric Association, 2013; World Federation of Neurology, 1968). It has been established that dyslexia is a heritable neurobiological disorder (Scerri & Schulte-Korne, 2010) with neural differences throughout a broad network of brain regions, including the cerebellum (for reviews, see Eckert, 2004; Richlan, 2012; Sun, Lee, & Kirby, 2010; Turkeltaub, Eden, Jones, & Zeffiro, 2002). Although it has been proposed that cerebellar dysfunction could be a cause of dyslexia (Nicolson & Fawcett, 2011; Nicolson, Fawcett, & Dean, 2001), the literature suggests that the cerebellum is one of many regions affected in dyslexia, and its specific role in the etiology of dyslexia is not yet clear. Supporting evidence for cerebellar dysfunction in developmental dyslexia comes from behavioral, clinical, and neuroimaging sources (for reviews, see Stoodley & Stein, 2011, 2013), which detail the presence of structural and functional differences in the cerebellum, and deficits on classically “cerebellar” measures of posture, balance, eye movements, and motor learning. In addition, our increasing understanding of the role of the human cerebellum in cognition—including language and reading—expands the potential impact of cerebellar dysfunction beyond the motor domain, yielding a more persuasive theoretical argument for its role in a developmental reading disorder. One of the most compelling potential contributions of the cerebellum to developmental dyslexia may be in the realm of learning: the cerebellum is part of the procedural learning system (Nicolson & Fawcett, 2011), and dyslexia is, at its core, a deficit in the acquisition of a complex skill. Indeed, in the Diagnostic and Statistical Manual of the American Psychiatric Association, 5th edition, dyslexia is classified as a specific learning disorder (American Psychiatric Association, 2013).

The most robust behavioral manifestation of dyslexia involves a deficit in phonological processing—difficulty in translating letters into the sounds they represent, breaking down words into their constituent phonemes, and phonemic manipulation—and hence it has been proposed that phonological deficits are the main cause of this reading disorder (Bradley & Bryant, 1983; Lieberman, Shankweiler, Fischer, & Carter, 1974; Ramus, 2004; Shankweiler & Liberman, 1972; Snowling, 1981, 2000). Multiple lines of research have sought to determine the physiological underpinnings of the phonological deficit in dyslexia (for reviews, see Peterson & Pennington, 2012; Stein, 2001), and it has been suggested that further evidence is necessary to prove the causality of the phonological deficit to dyslexia (Castles & Coltheart, 2004). Beyond phonological processing, dyslexia is also associated with slow, laborious reading, which is often the most robust indicator of dyslexia in regular orthographies (Wimmer, 1993). Based on this, Wolf and colleagues hypothesized that there are two core deficits in dyslexia: the phonological deficit and a fluency deficit, and the most impaired readers have a “double deficit” characterized by both inaccurate decoding and slow, dysfluent reading (Wolf & Bowers, 1999).

Cerebellar dysfunction could potentially impact both phonological processing and fluent, speeded reading through different mechanisms. In pursuit of the potential physiological underpinnings of reading difficulties in dyslexia, several deficits on sensorimotor tasks have been described, from difficulties in basic visual and auditory processing (for review, see e.g., Stein, 2001) to a variety of motor deficits, including differences in eye movements, balance and postural stability, motor processing speed, and implicit motor learning (for reviews, see Stoodley & Stein, 2011, 2013; see Ramus, 2003 for the argument that these may be associated with, but not causal to, developmental dyslexia). Such motor deficits, which often characterize patients with cerebellar disorders, together with the lack of fluency in dyslexia, led to the cerebellar deficit hypothesis of dyslexia (Nicolson et al., 2001). Nicolson and Fawcett hypothesized that impaired procedural learning, resulting from cerebellar dysfunction, can explain both the literacy and sensorimotor symptoms of dyslexia (Nicolson & Fawcett, 2011; Nicolson et al., 2001). They propose that the phonological deficit arises from the effect of cerebellar dysfunction on the articulatory system; incoordination and clumsiness result from a lack of cerebellar modulation of motor function; visual sequencing problems in reading could be due to poor cerebellar control of eye movements and visual attention; and, finally, a deficit in procedural learning would have deleterious effects on literacy skill acquisition.

Here we review the evidence for cerebellar dysfunction in developmental dyslexia, from movement to cognition. We discuss the role of the cerebellum in typical reading development, the evidence for reading deficits in cerebellar patients, cerebellar structural and functional differences reported in dyslexia, performance of dyslexic populations on “cerebellar” tasks, and the potential role of the procedural learning system in this developmental learning disorder.

Prevalence

DD occurs in all parts of the world, in all social classes, and irrespective of intelligence. Prevalence figures cited in the literature, however, can be highly variable (i.e., 5–17% of school-aged children; Shaywitz et al., 2008). This variability is likely the result of (1) assessment methods (i.e., whether based on individual diagnoses as in clinical practice, in which choice of tests and type of investigation is dependent on each case or based on a more general, one-for-all testing battery as in epidemiological studies); (2) differences in diagnostic criteria; and (3) linguistic and socio-cultural/-economic differences between studies. One study found dyslexia to affect 3.6–8.5% of children in Italy and 4.5–12% of children in the United States, where the range for each country was explained by differences in definition and diagnosis of dyslexia. A study in the United Kingdom found an incidence of 3.6% for children on the Isle of Wight compared to 9.3% for children in London of less favorable socioeconomic status. A vast study undertaken in Connecticut, USA (Shaywitz et al., 1990) found a prevalence of 17.5% when all backgrounds were taken together, irrespective of diagnostic criteria.

Prevalence may differ according to gender, with clinicians typically citing a ratio of 2–3 boys for every girl. Some authors (Shaywitz et al., 1990) have argued that gender differences are an artifact of sampling bias (i.e., boys tend to be more disruptive in class so are referred more often for assessment); however, a more recent meta-analysis (Rutter et al., 2004) has shown that four independent scientific investigations have in fact observed a significantly higher male-to-female prevalence ratio (2–4:1). Possible mechanisms cited in the literature may relate to differences in hormonal effects during neurodevelopment (Galaburda and Habib, 1987).

DD does run in families, with sometimes different clinical symptoms and manifestations within the same family. Heritability is thought to be between 50% and 65%.

Developmental Dyslexia

Developmental dyslexia is “characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities. These difficulties typically result from a deficit in the phonological component of language that is often unexpected in relation to other cognitive abilities and the provision of effective classroom instruction” (Lyon et al., 2003, p. 2). Comprehension of text can be impacted as a secondary consequence of poor decoding (for specific language impairment leading to reading problems, see Nation, Cocksey, Taylor, & Bishop, 2010).

Dyslexia affects 5–12% of the English-speaking population (rates vary for other languages; Brunswick, McDougall, & de Mornay Davies, 2010), and the incidence is two to three times higher in males than in females (Rutter et al., 2004). Dyslexia is highly heritable, with an estimated 30–50% chance of being passed from parent to child (Fisher & DeFries, 2002). Linkage and association studies have identified candidate genes (Scerri & Schulte-Körne, 2010).

Several lines of evidence converge in support of a causal role of poor PA in dyslexia (McCardle, Scarborough, & Catts, 2001): (i) Young children's PA skills predict later reading outcome; (ii) children and adults with dyslexia have poor PA skills; (iii) children with dyslexia have weaker PA than younger children matched on reading level; and (iv) interventions addressing weaknesses in PA are largely successful in bringing about gains in decoding. Together, these have led to the theory that a phonological core deficit best describes the condition of dyslexia (Stanovich & Siegel, 1994). However, orthographic complexity of a language exacerbates PA problems in dyslexia (Landerl et al., 2013); when grapheme–phoneme mapping is more direct, phonological deficits have a lesser role (Wimmer & Schurz, 2010). Moreover, the phonological core deficit theory does not explain all of the manifestations of this reading disability (Peterson & Pennington, 2012; Scarborough, 2005), and there is ongoing work in determining its exact etiology and other deficits, for example, involving visual or motor system dysfunction (see Ramus, 2004; Vellutino et al., 2004).