This public law defined learning disability as a "disorder in one or more of the basic psychological processes involved in understanding or in using spoken or written language, which may manifest itself in an imperfect ability to listen, think, speak, write, spell or to do mathematical calculations. In , Galaburda and Kemper  and Galaburda et al. Observed anatomical differences in the language center in a dyslexic brain, showing microscopic cortical malformations known as ectopias and more rarely vascular micro-malformations and in some instances these cortical malformations appeared as a microgyrus.
These studies and those of Cohen et al. In , Castles and Coltheart describe developmental dyslexia as two prevalent and distinct varieties using the subtypes of Alexia acquired dyslexia , Surface and Phonological Dyslexia. Cestnick and Coltheart demonstrated what these underlying deficits are in part, through unveiling different profiles of phonological versus surface dyslexics.
Phonological processing tasks predict reading accuracy and comprehension. Manis et al.
In , from post autopsy specimens Galaburda et al. They measured cross-sectional neuronal areas in the medial geniculate nuclei MGNs of five dyslexic and seven control brains. In contrast to controls, which showed no asymmetry, the left-side medial geniculate nucleus MGN neurons were significantly smaller than the right in the dyslexic sample.
Also, as compared with controls, there were more small neurons and fewer large neurons in the left dyslexic MGN. These findings are consistent with reported behavioral findings of a left hemisphere-based phonological defect in dyslexic individuals. The development of neuroimaging technologies during the s and s enabled dyslexia research to make significant advances.
Positron emission tomography PET and functional magnetic resonance imaging fMRI studies have revealed the neural signature of adult normal reading e. Brain imaging studies have also characterized the anomalous patterns of neuronal activation associated with reading and phonological processing in adults with persistent or compensated developmental dyslexia e. Differences in task-related signal change in the left temporoparietal and occipitotemporal cortices have emerged as the most consistent findings in studies of dyslexia in the alphabetic writing system Paulesu et al.
In , Wydell and Butterworth reported the case study of an English-Japanese bilingual with monolingual dyslexia. In , Temple et al. Suggest that dyslexia may be characterized in childhood by disruptions in the neural bases of both phonological and orthographic processes important for reading. In , Talcott et al. In , Turkeltaub et al. Using a task that isolates reading-related brain activity and minimizes confounding performance effects, we carried out a cross-sectional functional magnetic resonance imaging fMRI study using subjects whose ages ranged from 6 to 22 years.
We found that learning to read is associated with two patterns of change in brain activity: increased activity in left-hemisphere middle temporal and Inferior frontal gyrus and decreased activity in right inferotemporal cortica areas. Activity in the left-posterior superior temporal sulcus of the youngest readers was associated with the maturation of their phonological processing abilities.
These findings inform current reading models and provide strong support for Orton's theory of reading development. A guide to the areas of the brain List of regions in the human brain , Cerebral hemisphere. In , Ziegler and colleagues claimed that the dyslexia suffered by German or Italian dyslexics is very similar to the one suffered by English dyslexics readers of different—shallow versus deep orthographic systems , supporting the idea that the origin of dyslexia is mostly biological.
As of , current models of the relation between the brain and dyslexia generally focus on some form of defective or delayed brain maturation. More recently, genetic research has provided increasing evidence supporting a genetic origin of dyslexia. In , a University of Hong Kong study argues that dyslexia affects different structural parts of children's brains depending on the language which the children read.
As of , researchers Lyytinen et al. There are many previous and current theories of dyslexia, but one that has much support from research is that, whatever the biological cause, dyslexia is a matter of reduced phonological awareness, the ability to analyze and link the units of spoken and written languages. In , S Heim et al. Different theories conceptualise dyslexia as either a phonological, attentional, auditory, magnocellular, or automatisation deficit. Such heterogeneity suggests the existence of yet unrecognised subtypes of dyslexics suffering from distinguishable deficits.
The purpose of the study was to identify cognitive subtypes of dyslexia. Out of children screened for reading ability 49 dyslexics and 48 controls were tested for phonological awareness, auditory discrimination, motion detection, visual attention, and rhythm imitation.
What is dyslexia?
A combined cluster and discriminant analysis approach revealed three clusters of dyslexics with different cognitive deficits. Compared to reading-unimpaired children cluster no. These results indicate that dyslexia may result from distinct cognitive impairments. As a consequence, prevention and remediation programmes should be specifically targeted for the individual child's deficit pattern. Also in , Wai Ting Siok et al. In , KK Chung et al. From Wikipedia, the free encyclopedia.
Main articles: Dyslexia and Dyslexia research. Malatesha; Quatroche, Diana Becoming a Professional Reading Teacher. Chelsea, Michigan: Paul H. Brookes Publishing Co. Neurologisches Centralblatt. Annals of Dyslexia.
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Congenital Word-blindness. London: H. Archives of Neurology and Psychiatry. December Journal of the American Medical Association. Ritchey Journal of Special Education. American Journal of Orthopsychiatry. Journal of Research in Reading. In recent years, however, the focus on phonological abilities as relevant for reading and writing disorders has somewhat expanded to now include also visual and auditory processing abilities, visual and auditory attention, or automatization Nicolson et al.
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Ramus summarizes that many studies have already confirmed the link between auditory processing and dyslexia. In his review, he discusses the connection between auditory processing and phonological deficit. He concluded that both a phonological deficit and a deficit in auditory processing can appear independently in dyslexic children. It needs to be kept in mind that a severe auditory impairment nonetheless can have a negative influence on phonological skills and therefore affect reading and writing Ramus, ; Ramus et al.
Steinbrink et al. Their study also demonstrated that substandard skills in auditory processing may explain deficits in phonological processing and might therefore, in some cases, be regarded as causal for phonological deficits. Importantly, Steinbrink et al. In most cases, however, if one of those three dimensions was affected, the deficit co-occurred with at least one other, or even all deficits.
Christmann et al. Mirror writing and kinetic reversals e. Orton established the theory that the two sides of the brain code spatial information oppositely. Some mistakes, such as confused consonants, e. This phonological aspect in connection with dyslexia led to discussions about multi- and unicausal models for a comprehensive summary see Corballis and Beale, ; Lachmann, still being discussed today see the following sections.
Until now, many studies have confirmed the further hypothesis in connection with the visual aspect that dyslexia can be based on a deficit in the magnocellular system cf. Tholen et al. An impaired magnocellular system causes blurred visual representation of letters, for example.
As a result, letters are more difficulty to distinguish. Dyslexics therefore have difficulties with detecting fast movements and misperceive seemingly moving letters Stein, The connection between dysgraphia and an impaired magnocellular system has not been proven until now. But the problem of not clearly seeing letters in their correct orders, as a consequence of a magnocellular deficit Stein, , can cause individuals to write letters in a wrong order. This error can be transferred to dysgraphic children. Facoetti et al. Since phonological and orthographic representations are also accessed during writing, this implies that attention has an impact on the acquisition of writing skills as well.
Indirect evidence for this notion comes from the study by Rosenblum et al. These authors assessed handwriting of dysgraphic children. They summarize research and describe the problem of dysgraphics having not fully automatized letter production. Therefore, dysgraphics have an increased demand on their memory and attention while writing. Consequently, higher-level cognitive processes are constrained. As a further limitation, these authors describe the fact that children may forget plans held in memory before they are able to write them down on paper because of slow handwriting.
Rosenblum et al. This includes inconsistent letter size, acute turns in letters, uneven and unsteady writing as well as sudden changes in size and direction of letter writing.
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Finally, Adi-Japha et al. In summary, they determined that clinically relevant attention problems cause problems in writing. They suggested that the deficits in writing do not necessarily have underlying linguistic problems but an impaired graphemic buffer and impairments in kinematic motor production. Compared to children without ADHD, their test subjects with ADHD made a higher amount of spelling errors in the morphological categories function words, free morphemes, and derivational words.
History of dyslexia research - Wikipedia
Yet the reading abilities of the ADHD-test subjects were similar to those of the comparison group. Children with ADHD scored worse in letter insertion, substitutions, omissions, and transpositions graphemic buffer tasks. They changed similarly shaped letters. However, the results in fluency were similar for both groups. Furthermore, tests on motor kinematic production showed poor time utilization which means that children with ADHD take longer for writing.
The test subjects displayed inconsistent and disproportionate handwriting so that their handwriting was difficult to read. A high amount of corrections and writing with high levels of pressure on the pen for subjects with ADHD was pointed out as well Adi-Japha et al. These findings refer back again to the Rosenblum et al. Proficient readers master GPC without problems because of their normally developed basic articulatory and auditory skills. It is assumed that the cerebellum supports the automatization of these basic abilities. Consequently, learning and automatization of, e.
The cerebellum processes motor skills and coordination as well as linguistic and cognitive skills Ito, Nicolson et al. That means that a dysfunctional cerebellum can lead to reading as well as writing disorders Nicolson et al. Nicolson and Fawcett developed a possible causal chain of the influence of cerebellar impairment on reading and writing skills see Figure 1.
Writing problems, thus, ought to be caused by motor skill difficulties. Reading problems arise from problems in phonology, based on deficient articulatory skills that stem from cerebellar impairment. In addition, spelling problems result from a deficit in skill automatization.
The authors point out that the activated regions of the cerebellum differ for each of the three routes. A causal chain of the cerebellar-deficit-hypothesis cf. Nicolson and Fawcett, Nevertheless, the current state of research showed diverse outcomes with no significantly worse results between dyslexics and controls with respect to automatization tasks Ramus et al. The group of test subjects with the worst scores in automatization tasks rhythm imitation paradigm , tested by Heim et al. Therefore, a larger multiple cognitive deficit was concluded. By contrast, earlier studies demonstrated a connection between automatization and dyslexia Fawcett et al.
Nicolson and Fawcett proposed a neural-system approach. They stated that dyslexics have an intact declarative but impaired procedural learning system with activity in diverse brain areas such as the prefrontal language system, basal ganglia and cerebellar structures for a detailed overview of the neural activation, see Nicolson and Fawcett, The authors base their theory on the fact that dyslexic children have problems to automatize knowledge so that explicit attentive control is no longer necessary.
They developed a three-stage automaticity-deficit-framework of skill learning to explain potential additional motor skills also shown in Figure 1 in dyslexic children: in an initial declarative stage, a person learns what to do; in an intermediate stage the person learns how to do it; and in a final autonomous stage, the skill becomes fluent and automatic. In a response-blending study Nicolson and Fawcett, the authors found out that dyslexic children have problems in the initial stage and take longer for intermediate proceduralization. The authors declare that these findings are consistent with the known role of the cerebellum regarding skill automatization or point to a communication problem Nicolson and Fawcett, These aforementioned authors describe five stages of learning: fast learning minutes , slow learning hours , consolidation overnight , automatization hundreds of trials , and retention weeks.
The authors worked out that dyslexics can have problems in one or more of these stages, e. Consequently, the authors summarize that because of an impairment of procedural learning caused by an impaired cerebellum, children with dyslexia and dysgraphia have problems to automatize skills Nicolson and Fawcett, Specific Speech Disorder, LI and developmental dyslexia are multifactorial disorders with respect to genetic, environmental and cognitive etiology; all include deficits in phonological processing Pennington and Bishop, Pennington and Bishop found comorbidity of LI and later developmental dyslexia but a lower risk for SSD and later dyslexia.
No grammatical limitations were found in children with dyslexia in combination with LI Bishop and Snowling, These authors created a two-dimensional model that demonstrates the relationship between dyslexia and LI on the basis of phonological deficits. This model also allows for possible additional disordered domains of oral non-phonological language skills.
This confirms the importance of diagnosing the different underlying disorders as explained above. With respect to writing skill deficits, oral language difficulties seem to be a predictor as well. Children with LI perform worse in tasks like writing names, letters, and spelling words in comparison to a control group Puranik and Lonigan, Olson et al. By contrast, orthographic coding was not significantly inheritable.
They assume that a deficit in orthographic coding can be a result of environmental factors. Parent education and parent reading history seem to have no effect on reading skills Pennington et al. Thompson et al. Given this wide range of abilities other than phonological skills, which seem to be relevant for the successful acquisition of reading skills, Heim et al.
Dyslexic children in Cluster 1 were characterized by phonological, visual, and auditory deficits. The children categorized in Cluster 2 performed worse only in PA whereas those in Cluster 3 performed worse in terms of visuospatial attention. These results confirm the heterogeneity of dyslexia and the need to diagnose individually, considering the possibility of deficits in these domains which might influence poor reading skills.
As indicated in Heim et al. In most cases, dyslexia seems to accompany multiple deficits or may represent a single deficit with variation, e. Even so, the phenomenon of dysgraphia requires further investigation. As discussed above, it can be safely assumed that diverse underlying disorders must be regarded for diagnosing developmental dysgraphia as well. First evidence for this hypothesis was given with respect to ADHD and hand-eye coordination.
The risk factor of LI and PA seems to be similar for developmental dyslexia and dysgraphia, whereas the difference between skills in RAN-tasks underscores the hypothesis that developmental dyslexia and dysgraphia are not symmetric disorders Winkes, Even if a detailed description of intervention of developmental dyslexia and dysgraphia 2 goes beyond the scope of this article, it is noteworthy that these new insights about comorbidities and predictors ought to have an important influence on future research on intervention.
This guideline, which targets an interdisciplinary audience physicians, therapists, pedagogs , gives a survey of current research regarding diagnosis of developmental reading and writing disorders and corresponding intervention methods.
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The authors of the German S3 guideline recommend the expert to regard a discrepancy of at least 1. The performance in particular tests of those different learning domains should be 1 SD below the arithmetic mean. The authors also suggest a diagnosis of impaired vision as well as peripheral hearing disorder as exclusion criteria. The same criteria were used to evaluate tests for underlying disorders e.
In concluding, it can be said that, in addition to knowledge about discrepancy criteria and so on, information is given about different recommendable tests for reading and writing. This can be a helpful support for the practitioner. The German guideline clearly worked out that no adequate materials for young adults as well as terminally appropriate tests for underlying disorders exist.
In the current literature many different models, especially of subtyping dyslexia, have been published. As already mentioned, an individual diagnosis of reading, writing and especially of underlying skills is inevitable. Yet only one underlying deficit, e. Since there are many cases where more than one function is impaired, it is challenging to describe dyslexia with a theoretical model. In the literature the debate concerns choosing the right type of model — single vs.
Connectionist models show the importance of phonology and semantics for speech, language, and reading development. A problem in phonology or semantics can affect one or more of the developmental skills Pennington and Bishop, These authors criticize the problem of models only dealing with single deficits instead of multiple deficits on comorbidities. They found that these models partially encompass one another.
PA is included in some models and is seen as a prerequisite for the diagnosis of dyslexia. The authors conclude that one single phonological model probably is not adequate because of the high amount of predictive reading deficits Pennington et al. A model should allow a single underlying deficit as well as multiple impaired skills. Finally their study revealed that a hybrid model that encompasses all four possibilities of cognitive single- and multiple deficit models Table 1 best depicts the causes of dyslexia.
For the sake of comparison, it should be noted that nearly all dyslexics tested by Pennington et al. TABLE 1. Four cognitive models of dyslexia Pennington et al. The question arises whether developmental dysgraphia can be depicted with such a hybrid model as well. As discussed above, developmental dysgraphia also seems to show diverse underlying deficits. A single deficit in PA is possible Moll et al. Analogously to the Pennington et al.
Yet a model that allows multiple cognitive deficits such as impaired PA, working memory as well as an underlying LI is theoretically feasible, too. Therefore, it can be speculated that the findings explained above can be transferred to developmental dysgraphia: a hybrid model that allows a single or multiple cognitive deficits might be a good foundation to explain developmental dysgraphia. To this end, studies of individual profiles of dysgraphia e.
In line with this argumentation, an earlier investigation by Ziegler et al. Analyzing dyslexics by testing the different representational levels letter level, phoneme system, orthographic and phonological lexicon of the DRC; Coltheart et al. Ziegler et al. Because of the high importance for reading and writing performance and for better understanding, the dual-route-model Figure 2 will be explained briefly in the following part. In the first version, the model only explained the reading of single words; since writing was added.
Since then it had been further developed many times, inter alios by Coltheart , as the DRC. Reading and writing are built up as mirror-inverted processes. Moreover, the box-and-arrow model explains different possibilities of reading and writing by using different routes and components. Words acquire their meaning when one activates the semantic system. The dual route model of reading and writing cf. Winkes, The transferability of those models to diagnose developmental reading and writing disabilities was shown by different authors e.
However, one should bear in mind that the theory of the models is based on research with patients with acquired disorders. Assets and drawbacks are discussed later on. The developmental framework of Frith serves as the basis for later developmental models. Written language development is subdivided in different phases: logographic, alphabetic, and orthographic strategy. Other authors, e. The following table gives an overview of the main phases of reading and writing acquisition of a normally developing child in a language with rather transparent orthography Table 2.
The regularity of a writing system, i. Thus, transparent alphabetic orthographies such as German, Italian, Czech, and Finnish are easier and faster to learn than English or French, for instance. Less is known about the alphasyllabaries e. The psycholinguistic grain size theory Goswami, ; Ziegler and Goswami, describes the connection between PA and reading acquisition across languages.
Depending on the particular language, the orthography varies in its phonological representation and its phonological consistency. The authors assume that the size of the units, used for reading according to the non-lexical route, varies between children and adults, depending on the orthographic consistency of the different languages. Therefore, there are developmental differences across languages. In orthographic consistent languages, a faster development of phoneme-level skills can be observed e.
It may be useful to differentiate between alphabetic languages e. The first type varies in the consistency of letter-sound correspondence from a high consistency e. Landerl et al. PA remains the main predictor of reading across languages and plays a greater role in highly consistent orthographies Ziegler et al. Perhaps contrary to expectations, PA plays an important role in learning to read Chinese as well. Less surprisingly and in contrast to alphabetic languages, morphological and syllabic awareness are highly important for learning to read Chinese McBride-Chang et al.
As for reading acquisition, PA also serves as a core component for spelling development. Furthermore, letter knowledge plays an important role for writing acquisition across alphabetic languages Caravolas, Caravolas and Bruck compared English and Czech children with respect to their phonologically accurate pseudoword spellings in first grade. They discovered that children who learn orthographically consistent languages like Czech show faster spelling development than children who learn the less orthographically consistent language of English.
Furthermore, children learning more orthographically consistent languages seem to learn not only the basic spelling skills like pseudoword spelling more easily but also more advanced spelling skills cf. Caravolas, Bialystok et al. Children learning two alphabetic languages especially benefit while learning to read. The authors state that bilingual children who learn alphabetic languages like Hebrew or Spanish as a second language and English as the first language performed better in PA tasks than monolingual and bilingual children who spoke English and Chinese as a non-alphabetic-language.
The link between the advantage gained in PA tasks for bilingual children learning two alphabetic languages implies that bilingual children may somehow benefit in writing acquisition as well —depending on the similarity or distinctness of the individual languages a child grows up with. To the best of our knowledge, no data are currently available which shed light onto this question. We now return to the initial question about to which extent do developmental dyslexia and developmental dysgraphia share common features.
In this paper, we have discussed the following core facts. PA plays an important role for dyslexia and dysgraphia as well. Moreover, it can be assumed that further predictors and comorbidities such as working memory, auditory processing, visual attention, and LI play an important role in reading and writing as well.
Future research on this topic is needed to prove this assumption. However, the relevance of those comorbidities and predictors seems somewhat different for dyslexia and dysgraphia. For instance, even though PA is important for reading and writing skills, if we take a deeper look at the reading and writing process, we can see crucial differences between the two.
Moreover, whereas on the one hand performance in RAN-tasks is only predictive for developmental dyslexia, on the other hand hand-eye coordination is relevant only for writing performance. Finally, the roles of other domains such as attention or working memory clearly require further research. Altogether, the present state of the art seems to suggest that developmental dyslexia and developmental dysgraphia have a rather broad common underlying basis, in particular, in the domain of phonological processing, and a high amount of co-occurrence.
It also becomes clear that still too little is known about the relationship between reading and writing, or dyslexia and dysgraphia with respect to other cognitive, sensory, or motor domains. Developmental dyslexia has often been a focal point of research in the past years. Peterson and Pennington , p.
Two lines of research open up here. It may prove helpful to include the notion of individual profiles, or even subtypes, of dyslexia to the domain of dysgraphia. The second is related to this topic and focusses on applying these novel models and concepts for individualized diagnosis and therapy. Yet more robust evidence, in particular from RCTs randomized controlled trials , are needed to demonstrate the relevance and external validity of these novel concepts by reducing the number of non- or low-responders to dysgraphia intervention and by increasing the overall success rate.
The present overview has aimed to promote this endeavor by indicating what aspects of developmental dysgraphia we may learn from research on developmental dyslexia. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Bishop, D. Developmental dyslexia and specific language impairment: same or different? Bosse, M. Developmental dyslexia: the visual attention span deficit hypothesis. Cognition , — Caravolas, M. Spelling development in alphabetic writing systems: a cross-linguistic perspective. Child Psychol. Christmann, C. Evidence for a general auditory processing deficit in developmental dyslexia from a discrimination paradigm using speech versus nonspeech sounds matched in complexity.
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