The neurological bases of reading: characteristics and findings

The neurological basis of reading has aroused great interest in the scientific community, especially with regard to reading and writing disorders.

Since the first neuroanatomical studies were conducted on the areas involved in this remarkable ability of the human species, it has been suggested that there are many areas and pathways involved in the identification, encoding and pronunciation of letters and words.

Below is a brief overview of the neurological basis of reading, data collected over the years, and the effect it would have on the brain level of people with dyslexia.

    What are the neurological bases of reading?

    Reading and writing are skills that set us apart from the rest of animal species. These two are very interesting components of language, a skill that has always been studied within the scientific community trying to see if in other species could be reproduced a capacity that in our species is so standardized.

    As might be expected, scientists have focused their attention on the brain, trying to see which areas are involved in language and particularly in literacy.

    Neurological hypotheses of reading

    One of the first researchers to describe the neurological processes behind writing and reading was Joseph Jules Dejerine, who obtained valuable data through the study of clinical cases of people with literacy disorders. These studies were carried out at the end of the 19th century and Dejerine is credited with identifying allergic syndromes with and without spelling in 1892, studying patients with vascular lesions due to heart attacks in the left parietal and occipital regions.

    Thanks to Dejerine’s work it was possible to realize the first systematizations of cerebral areas and, following a connectionist model, the involvement of several domains would finally be suggested.

    Among these would be the participation of the left occipital cortex involved in visual processing; the left temporal-wall-occipital area, which would be in charge of visuo-verbal integration; angular turn with linguistic coding capability; the posterior temporal cortex, in charge of semantic access; and the left lower frontal cortex, in charge of motor speech.

    But all the credit does not go to Dejerine. Many other researchers over the past two centuries have helped to clarify the neurological basis of reading. Among them we have Norman Geschwind, who studied the convergence of bihemispheric visual afference in the left occipital cortex from which information would be projected at ipsilateral angular rotation, a region believed to be the center of visual images of words. This information would then pass to the posterior temporal cortex and, therefore, to Broca’s region through the arched fascicle.

    The left occipital cortex would do the visual analysis. In this area the orthographic characteristics of letters and words would be recognized, that is to say that it is here that the lexical-orthographic or visual component of the language would be recognized.

    Thanks to this analysis, the semantics, that is to say the meaning of words, is accessible and the corresponding phonology is also activated. In other words, this whole process allows us to express what has been read by speaking, to understand what is read.

    But in addition to this so-called classical, lexical, translexical or semantic reading route, other alternative routes have been proposed which would be activated depending on the circumstances. One of these situations would be to read without accessing the meaning of words, where the lexico-phonological or direct path would be involved, activating the lexico-phonological component directly from the visual. We would also have the phonological or sublexical path, activated when reading pseudo-words or unknown words.

    It is normal that every day when reading a text we use it in one way or another depending on the needs we have at that time.. If we were to read a text in depth, understand what we are reading and study it, we would use the classic route, in which all of the regions mentioned above would apply. However, these routes may vary depending on whether it is a known or unknown word, whether one needs to read quickly or accurately, whether the word is part of our native language or whether it is foreign. ..

      Pathologies associated with reading disorders

      As we have mentioned, the study of the neurological basis of reading draws heavily on the study of people who have experienced some kind of impairment in their ability to read and write. In people who could read and write before the problem manifested itself, it usually happens that the disorder is the result of some type of brain injury, while in cases where it seems that there are already problems since the start of writing. inherited alteration.

      Reading and writing disorders are called dysgraphia and dyslexia, terms preferred before those of spelling and alexia, more used in classical texts. These disorders can be classified according to whether the problem is in a purely linguistic impairment, as would be the case with central dyslexia and dysgraphia, or is in the afferent or efferent subsystems involved in the literacy process, speaking in this case of dyslexia. and peripheral dysgraphia.

      Below, we’ll talk in depth about peripheral and central dyslexia, the areas involved, and some varieties within these two major groups.

      Peripheral dyslexia

      Peripheral dyslexia are impairments in reading that occur due to some damage to areas involved in the process ranging from the analysis of linguistic information to the lexical-visual process. We have a classic example of this type of alteration in pure or non-stapled alexia, originally described in cases of left occipital lesion or the afference of the contralateral occipital cortex to the splenus of the corpus callosum. .

      it happens the loss of the connection between the visual processing of the image and the lexical-orthographic component, so that the reading process does not have lexical recognition. As a result, the process is done letter by letter, which makes the reading process a bit slower and less functional. The affected person can read, but he does so using only the sublexical route, so he accesses the semantics once he has reached the phonological stage. That is, he first reads the sounds of the letters, then grasps their meaning.

      There are other cases in which it happens that the visual analysis of words and letters is impaired, so that strange phenomena occur such as, for example, the omission of letters at the beginning of words, as happens in dyslexia due to neglect. Other cases are that of attentional dyslexia, in which the letters are changed between neighboring words, and then we also have what is called visual dyslexia, in which the words are replaced by others to have an appearance. similar.

      Central dyslexia

      In central dyslexia, impaired reading is usually due to a failure in the stages of lexical recognition, semantic access and phonological processing prior to speech production. These are syndromes that can affect any of the different reading pathways after visual analysis of the text read.

      In case of damage to the phonological pathway, usually caused by damage to the left perisilvic cortex, phonological dyslexia. This mode of central dyslexia is characterized by difficulty reading unfamiliar words or pseudo-words and also to some extent difficulty reading functional words (articles, determinants, pronouns, prepositions, conjunctions or nexuses). People affected by this condition find it difficult to convert handwriting (written) to phonological (pronounced).

      Superficial dyslexia is dyslexia that can be observed in people who have difficulty reading irregular words. To this condition is added a certain tendency to regularize graphemes of ambiguous phonology (regularization error) and generating words which do not exist. This results in many errors of omission, addition, replacement or transfer. Some researchers place the problem in the inability to access the lexicon through a comprehensive word recognition strategy.

      Finally, we can comment on the case of deep dyslexia. In this reading impairment, a grammatical and semantic category dependency can be observed, and the reading of words like verbs, adjectives and functional particles is mainly affected compared to the reading of nouns. There are also problems reading abstract words versus concrete words. Profound dyslexia has been linked to multiple injuries at different levels in the pathways involved in reading in the left hemisphere.

      Bibliographical references

      • Dejerine J (1892). Contribution to the anatomopathological and clinical study of the different varieties of verbal blindness. Memories Biological Society 4: 61-90.
      • Benson DF, Ardila A (1996). Aphasia. A clinical point of view. Oxford University Press, New York.
      • Berthier ML (2001). Review. Unexpected relations between the brain and language in aphasia. Evidence of transcortical sensory aphasia associated with frontal lobe damage. Aphasia 15: 99-130.
      • Catani M, Mesulam M (2008). The arched question and theme of disconnection in language and aphasia: history and current state. Cortex 44 (8): 953-961.
      • Caplan D (1999). Tongue. Structure, treatment and disorders. MIT Press, Massachusetts.
      • Damasio H, Damasio AR (1980). The anatomical basis of conduction aphasia. Brain 103: 337-350. Jódar VM (2005). Language and memory disorders. Editorial UOC, Barcelona.

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