Developmental dyslexia is a specific learning difficulty with a neurobiological origin affecting the deciphering of written language in children without sensory disorders and normal intelligence and schooling [1-3]. This is a disorder with a range of different difficulties: letter sequencing, omissions, inversions, and phoneme or syllable substitutions in words. These errors entail difficulties in word recognition, reduced reading fluidity, and reduced reading comprehension.

Globally, it is estimated to affect between 5% and 17% of the school aged population [5]. Incidence is higher in languages in which the correlation between graphemes and phonemes is more arbitrary, i.e., opaque languages like English, where, for example, the word yacht is read /’jɒt/. Depending on a language’s regularity in transforming graphic symbols into sounds (orthographic transparency), there is variability in the characteristics of dyslexia [4].

In less transparent languages such as English [5] an incidence of 4-10% is estimated in the school aged population. In Italian, which is a more transparent language like Spanish, a prevalence of 3-5% [6] is estimated, i.e., approximately one student per class. There is often a history of learning difficulties in families of children with dyslexia [8-14], especially in the case of phonological or central dyslexia [15].

Developmental dyslexia is usually diagnosed at around 7-8 years old (second or third grade), when the school system becomes more demanding in terms of literacy, and the child is considered to have had sufficient time to develop their basic reading skills. At this stage, the educational system begins to require that the fusion word’s sounds becomes an automatic process, moving from a more local decoding method (letter by letter or analytical) to a more fluid and global one, i.e., the word in its entirety. This allows children with average development to begin to read more fluidly, without focusing so much on the letters and more on the meaning of the word itself.

Another of the criteria necessary to diagnosing dyslexia is that reading difficulties remain, despite being able to be reduced, after a minimum of six months of specific intervention. This criterion was recently introduced to be able to discriminate between specific learning disorders of neurobiological origin and other learning difficulties that may be related to environmental or motivational aspects [1, 2].

Although dyslexia becomes more evident in the middle of primary education, difficulties with pre-literacy skills and sound and word awareness can often already be detected in preschool [10, 16, 17]. From a prevention standpoint, some early indicators to detect dyslexia in preschool are potential difficulties fusing and segmenting sound phonetics, language difficulties, visual and spatial coordination, visual attention, and particularly phonological skills [8, 18, 19].

There are currently considered to be two main types of developmental dyslexia: one with a phonological pattern, based on grapheme to phoneme correspondence and another with a surface pattern associated with the lexical representation of the word in the memory [20-22]. In both types of dyslexia, there are inaccuracies when reading, although the difficulties in the phonological type tend to be more marked compared to the surface one.

Given the different profiles, it has been considered that there may be different causes for both types of dyslexia. The first, phonological dyslexia, is mainly related to difficulty with the language’s phonology [23-34]. The second, surface dyslexia, is also related to aspects of attention in the visual and spatial exploration of words [20]. In both cases, there are factors associated with dyslexia that contribute to the type and intensity of the difficulty, and they are indicated in some multifactorial models [35-39].

Thus, there are different models to explain dyslexia that attempt to address its patterns and manifestations: the hypothesis of phonological language deficit, the multifactorial models, the visual spatial attention process anomaly, the visual perception process anomaly (or magnocellular model), and the motor coordination process anomaly (or cerebellar model) theories.

One of the hypotheses most widely accepted by scientists is that developmental dyslexia is related to a phonological language deficit, i.e., it comes from a difficulty in transforming letters into sounds and vice versa, as well as manipulating or reflecting on the elements of language [23, 25-34, 40].

Phonological deficits appear separate from other cognitive abilities and effective educational instruction. They may appear in mainly three areas: phonological awareness [30, 41-45], phonological short-term memory [46, 47], and phonological recoding in lexical access [48, 49].

Depending on the severity of the phonological deficit and the individual’s general processing resources, their reading experience, and strategies, different profiles of dyslexia can be found [50]. There are dyslexias characterized by a prominent difficulty in recognizing words that are uncommon, irregular, or borrowed from other languages (i.e., cul-de-sac /ˈkʌldəˌsak/) [15, 51-53]. There are other more superficial types of dyslexia in which reading difficulties can be associated with immaturity and reduced automation of the reading process, which causes their reading fluidity and accuracy to be comparable to the level of a younger child [31, 52, 53].

Other studies that are more focused on the analysis of reading development and pre-literacy skills have indicated the influence of different factors, including phonological ones, to explain both the reading acquisition process and potential disorders [54, 55].

In this case, multifactorial models [36, 57-59] are discussed. These models highlight the role of linguistic [12, 37, 60-63], visual and spatial attention [64], working memory [65], and naming speed [66] difficulties in dyslexia, in addition to potential phonological deficits.

In some cases, therefore, light phonological deficits can be found without developing a significant reading difficulty [37, 56], because there are other factors that influence reading performance, the level of severity of the dyslexia, and the kinds of errors made [36, 56]. These factors can sometimes be considered early indicators of risk (if they lead to weaknesses) or protective factors (if they lead to strengths) in preschool.

There are findings that consider attentional, visual, and spatial aspects in certain types of dyslexia related to anomalies in finding, grouping, and changing attentional focus [67-70], speed of parallel letter processing [67,78], and the distribution of attention resources in the visual field [71-77]. All of this could explain the letter sequencing and localization errors in people with dyslexia who do not have difficulties with phonological awareness [79, 80].

Some studies, such as the one proposed by Ans et al. (1998) in their connectionist multiple-trace memory model for polysyllabic word reading [81], have attempted to establish a theoretical relationship between attentional processes, the different ways to recognize words (phonologically and superficially), the type of processing, and the resources involved in each of them. This model has been adapted solely to acquired dyslexia, and it has yet to be applied to reading acquisition and developmental dyslexia. Despite this, it provides ideas about the way in which selective visual attention and phonological deficits combine or separate in reading processes, generating phonological and surface dyslexia. This model would explain, for example, the errors in syllable recognition of students who do not have difficulties recognizing words overall.

Other studies have proposed the possibility of a deficit in the transient visual system, which allows ocular movements to be synchronized and brought to parts of the text not identified by the fovea. This deficit would manifest itself in difficulties in contrast sensitivity [82-85] and movement perception [86, 87]. This hypothesis is currently not well-supported, as there is evidence that people with dyslexia are more likely to be distracted by words nearby when reading [88].

At the level of visual perception, potential deficits have also been identified and associated with the magnocellular visual subsystem [89-91], an interconnection pathway between the retina and the areas of the brain dedicated to processing visual information. There are studies that indicate some involvement of this system in reading pseudo words [92, 93] in phonological dyslexias [93, 94], but not in surface dyslexias [95, 96]. Despite this, in this case there is no evidence clearly demonstrating a relationship between visual processing difficulties and reading results for dyslexic children [67].

Other studies have proposed a potential correlation between motor difficulties and automation at the cerebellar level and phonological awareness difficulties in reading development [97-99]. Given the cerebellum’s role in motor control and process automation, anomalies in this cerebral area may feed back difficulties in speech, phonological representation formation in spoken language, and, as a result, alterations in phonological awareness. This hypothesis would provide an explanation for the concurrence of phonological difficulties and motor deficits in some cases of developmental dyslexia. This theory is currently still being debated, given that there is insufficient information about direct relationships between cerebellar dysfunctions and reading difficulties [100-102].

In conclusion, there are currently multiple models that attempt to provide an explanation of the origin, prognosis, and types of dyslexia without a clear consensus. Nevertheless, from the clinical and educational standpoint, the number of models boosts the need to consider all factors that may be related to dyslexia in preparing the diagnosis and planning effective intervention.

The diagnosis must take on linguistic, phonological, attentional, mnemonic, and visual motor aspects as well as those related to differentiation, reading comprehension, orthography, and intellectual competencies, etc. Neurocognitive and educational intervention must be based on these results without forgetting the affective, motivational, and social component that is involved in any learning process.

Finally, collaboration between specialists, families, and schools will result in the support needed by the student, as it will allow common goals to be set, results to be understood, and compensation and exoneration measures to be established. Providing the appropriate tools in class may make the difference for the student, giving them a basis on which to learn to develop their own strategies, feel like part of the group, and obtain benefits for their self-esteem and self-reliance.

 

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