Finished thinking
Peterson RL, Pennington BF.
Lancet (London, England). 2012;379(9830):1997-2007. doi:10.1016/S0140-6736(12)60198-6.
Dyslexia is a neurodevelopmental disorder that is characterised by slow and inaccurate word recognition. Dyslexia has been reported in every culture studied, and mounting evidence draws attention to cross-linguistic similarity in its neurobiological and neurocognitive bases. Much progress has been made across research specialties spanning the behavioural, neuropsychological, neurobiological, and causal levels of analysis in the past 5 years. From a neuropsychological perspective, the phonological theory remains the most compelling, although phonological problems also interact with other cognitive risk factors. Work confirms that, neurobiologically, dyslexia is characterised by dysfunction of the normal left hemisphere language network and also implicates abnormal white matter development. Studies accounting for reading experience demonstrate that many recorded neural differences show causes rather than effects of dyslexia. Six predisposing candidate genes have been identified, and evidence shows gene by environment interaction.
Shaywitz SE, Shaywitz JE, Shaywitz BA.
Current Opinion in Psychiatry. 2021;34(2):80-86. doi:10.1097/YCO.0000000000000670.
Purpose Of Review: Within the past decade tremendous advances have occurred in our understanding of dyslexia.
Recent Findings: Reliable data now validate the definition of dyslexia as an unexpected difficulty in reading in an individual who has the ability to be a much better reader. That dyslexia is unexpected is now codified in US federal law (PL 115-391). Replicated studies using functional brain imaging have documented a neural signature for dyslexia. Epidemiologic, longitudinal data now demonstrate that dyslexia is highly prevalent, affecting 20% of the population, affecting boys and girls equally. These data further demonstrate that the achievement gap between dyslexic and typical readers is now evident as early as first grade and persists. Evidence-based, efficient, inexpensive screening tools now offer the possibility of universal screening to identify children at risk for dyslexia as early as first grade. Specialized schools which focus on dyslexic students provide welcoming communities, ensuring that dyslexic children will not only survive but thrive.
Summary: Taken together, these findings indicate that we must act and act now to ensure that this 21st century knowledge of dyslexia is disseminated to educators, policy makers, and most of all to parents of dyslexic children.
Snowling MJ, Hulme C, Nation K.
Oxford Review of Education. 2020;46(4):501-513. doi:10.1080/03054985.2020.1765756.
Dyslexia is a difficulty in learning to decode (read aloud) and to spell. DSM5 classifies dyslexia as one form of neurodevelopmental disorder. Neurodevelopmental disorders are heritable, life-long conditions with early onset. For many years, research on dyslexia proceeded on the basis that it was a specific learning difficulty - specific meaning that the difficulty could not be explained in terms of obvious causes such as sensory problems or general learning difficulties (low IQ). However, the failure to find qualitative differences in reading, and phonological skills, between children with dyslexia and children with more general learning problems led this kind of 'discrepancy' definition to fall from favour. The Rose Review stated that dyslexia can occur across the IQ range and that poor decoding skills require the same kinds of intervention irrespective of IQ. In this paper, we argue that loosening the criteria for dyslexia has influenced common understanding of the condition and led to diagnostic confusion. In the longer term, the use of the term may need to change. Implications for research and practice are discussed.
Sheryl Handler MD FAAO, Walter M. Fierson MD FAAO FAAP, Gregg T. Lueder MD FAAP, et al
American Academy of Ophthalmology
2-4 It is characterized by difficulties with accurate and/or fluent sight word recognition and by poor spelling and decoding abilities. These difficulties are unexpected in relation to the child's other cognitive skills. Dyslexia has been identified as having a strong genetic basis. 2,8,9 Recent genetic-linkage studies have identified many loci at which dyslexia-related genes are encoded. Approximately 40% of siblings, children, or parents of an affected individual will have dyslexia. Although dyslexia is often inherited, it may exist in the absence of a family history. Dyslexia can be mild or severe, occurs throughout the world, seems to affects boys more than girls, 10 involves children with all levels of intelligence, and can persist for a lifetime. 2,4,5,8,11,12 Dyslexia is identified in some people early in their lives but in others is not diagnosed until much later, when more complex reading and writing skills are required. People with dyslexia can be very bright and may be gifted in math, science, the arts, or even in unexpected areas such as writing. 12 Dyslexia should be separated from other secondary forms of reading difficulties caused by visual or hearing disorders, mental retardation, and experiential or instructional deficits. 2,8 Early reading difficulties may be caused by experiential and instructional deficits. 8 It is important to identify and address such causes of secondary reading difficulties. 5,8
Oral language development has been found to play a critical role in learning to read. 1 Unlike speaking, reading and writing do not develop naturally and require active learning. Reading is more difficult than speaking, because children must be aware of the sound structure in spoken language and then break the alphabetic code to acquire the sound/symbol connection.
English is a phonemically complex language in which the 26 letters of the alphabet create 44 sounds, or phonemes, in approximately 70 letter combinations. 6,7,13 The phonemic complexity of an alphabet-based language corresponds to the prevalence of dyslexia, pointing to the linguistic origin of dyslexia.
6,7,13 The phonemic complexity of an alphabet-based language corresponds to the prevalence of dyslexia, pointing to the linguistic origin of dyslexia. 8,14 Reading involves the integration of multiple factors related to a person's experience, ability, and neurologic functioning. Most people with dyslexia have a neurobiological deficit in the processing of the sound structure of language, called a phonemic deficit, 1,2,4-8,11,13,15 which exists despite relatively intact overall language abilities. 2,4-7 Children with more severe forms of dyslexia may have a second deficit in naming letters, numbers, and pictures, creating a double deficit, 8,16 or they may have problems with their attention or working memory. 8 Other children may have trouble orienting, recognizing, and remembering letter combinations. 8,17 This difficulty may be a neuromaturational delay that improves with development. Importantly, the definition of dyslexia does not include reversal of letters or words or mirror reading or writing, which are commonly held misconceptions. 8,12,14
Research has shown that most children and adults with reading disabilities experience a variety of problems with language 1,2,4-8,11,13 that stem from altered brain function 2,4,8,18-29 There is solid scientific evidence that supports the neurologic basis for the phonological coding deficit theory of reading disabilities. 2,4,8,18-29 Scientific research using functional MRI studies and positron emission tomography scans has shown that reading takes place predominantly in left-hemisphere sites including the inferior frontal, superior temporal, parietotemporal, and middle temporal-middle occipital gyri in typical readers. Children with dyslexia, on the other hand, use different areas of the brain when reading. 2,4,18-29 People with dyslexia demonstrate a dysfunction in the left-hemisphere posterior reading systems and show compensatory use of the inferior frontal gyri of both hemispheres and the right occipitotemporal area.
Provazza S, Adams AM, Giofrè D, Roberts DJ.
Frontiers in Psychology. 2019;10:2725. doi:10.3389/fpsyg.2019.02725.
Developmental dyslexia is a reading disorder characterized by problems in accurate or fluent reading. A deficiency in phonological processing is thought to underpin the reading difficulties of individuals with developmental dyslexia and a variety of explanations have been proposed including deficits in phonological awareness and verbal memory. Recent investigations have begun to suggest that developmental deficits in the acquisition of reading may also co-occur with visual processing deficits, which are particularly salient for visually complex stimuli, yet these deficits have received relatively little attention from researchers. To further explore the nature of phonological and visual processing in developmental dyslexia, we administered a series of non-reading tasks tapping both domains. Unsurprisingly, individuals with developmental dyslexia performed worse than typically developing readers in phonological tasks. More intriguingly, they also struggled with visual tasks, specifically when discriminating between novel visual patterns, and in visuo-spatial working memory, which requires greater attentional control. These findings highlight that individuals with developmental dyslexia present not only with phonological impairments but also difficulties in processing visual materials. This aspect has received limited attention in previous literature and represents an aspect of novelty of this study. The dual phonological and visual impairments suggest that developmental dyslexia is a complex disorder characterized by deficits in different cognitive mechanisms that underpin reading.
Farah R, Ionta S, Horowitz-Kraus T.
Frontiers in Psychology. 2021;12:708863. doi:10.3389/fpsyg.2021.708863.
Dyslexia is a neurobiological learning disability in the reading domain that has symptoms in early childhood and persists throughout life. Individuals with dyslexia experience difficulties in academia and cognitive and emotional challenges that can affect wellbeing. Early intervention is critical to minimize the long-term difficulties of these individuals. However, the behavioral and neural correlates which predict dyslexia are challenging to depict before reading is acquired. One of the precursors for language and reading acquisition is executive functions (EF). The present review aims to highlight the current atypicality found in individuals with dyslexia in the domain of EF using behavioral measures, brain mapping, functional connectivity, and diffusion tensor imaging along development. Individuals with dyslexia show EF abnormalities in both behavioral and neurobiological domains, starting in early childhood that persist into adulthood. EF impairment precedes reading disability, therefore adding an EF assessment to the neuropsychological testing is recommended for early intervention. EF training should also be considered for the most comprehensive outcomes.
Morken F, Helland T, Hugdahl K, Specht K.
NeuroImage. 2017;144(Pt A):92-100. doi:10.1016/j.neuroimage.2016.09.060.
Dyslexia is a literacy disorder affecting the efficient acquisition of reading and writing skills. The disorder is neurobiological in origin. Due to its developmental nature, longitudinal studies of dyslexia are of essence. They are, however, relatively scarce. The present study took a longitudinal approach to cortical connectivity of brain imaging data in reading tasks in children with dyslexia and children with typical reading development. The participants were followed with repeated measurements through Pre-literacy (6 years old), Emergent Literacy (8 years old) and Literacy (12 years old) stages, using Dynamic Causal Modelling (DCM) when analysing functional magnetic resonance imaging (fMRI) data. Even though there are a few longitudinal studies on effective connectivity in typical reading, to our knowledge, no studies have previously investigated these issues in relation to dyslexia. We set up a model of a brain reading network involving five cortical regions (inferior frontal gyrus, precentral gyrus, superior temporal gyrus, inferior parietal lobule, and occipito-temporal cortex). Using DCM, connectivity measures were calculated for each connection in the model. These measures were further analysed using factorial ANOVA. The results showed that the difference between groups centred on connections going to and from the inferior frontal gyrus (two connections) and the occipito-temporal cortex (three connections). For all five connections, the typical group showed stable or decreasing connectivity measures. The dyslexia group, on the other hand, showed a marked up-regulation (occipito-temporal connections) or down-regulation (inferior frontal gyrus connections) from 6 years to 8 years, followed by normalization from 8 years to 12 years. We interpret this as a delay in the dyslexia group in developing into the Pre-literacy and Emergent literacy stages. This delay could possibly be detrimental to literacy development. By age 12, there was no statistically significant difference in connectivity between the groups, but differences in literacy skills were still present, and were in fact larger than when measured at younger ages.