The neurological basis of reading: characteristics and discoveries.

Summary of the neurological basis of reading: the parts of the brain behind the ability to read.

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

Since the first neuroanatomical studies 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, coding and pronunciation of letters and words.

The following is a brief review of the neurological basis of reading, what data has been collected over the years, and what effects there may be at the brain level in people with dyslexia.

What are the neurological bases of reading?

Reading and writing are abilities that distinguish us from the rest of the animal species. These two are very interesting components of language, an ability that has always been the subject of study within the scientific community in trying to see if other species could replicate an ability that is so standardized in our species.

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

Neurological hypotheses of reading

One of the first researchers to make descriptions of the neurological processes behind writing and reading was Joseph Jules Dejerine, who obtained valuable data through the study of clinical cases of people with reading and writing disorders. These studies were carried out at the end of the XIX century and Dejerine is credited with having identified the syndrome of alexia with and without agraphia in 1892, studying patients with vascular lesions due to infarcts in the left parietal and occipital areas.by studying patients with vascular lesions due to infarcts in the left parietal and occipital areas.

Thanks to Dejerine's work, it was possible to make the first systematizations of brain areas and, following a connectionist model and, following a connectionist model, over time the involvement of several areas was suggested.

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

But not all the credit goes to Dejerine. Many other researchers over the last two centuries have contributed to elucidating the neurological basis of reading. Among them we have Norman Geschwindwho studied the convergence of the bihemispheric visual afference in the left occipital cortex from which the information would be projected to the ipsilateral angular gyrus, a region considered to be the center of visual images of words. Then, this information would pass to the posterior temporal cortex and, from there, to Broca's area through the arcuate fasciculus.

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

Thanks to this analysis, the semantics, that is, the meaning of the words, would be accessed and the corresponding phonology would also be activated. In other words, this whole process makes it possible to express what is read through speech, understanding what is read.

But in addition to this route for reading, known as classical, lexical, translexical or semantic, other alternative paths have been proposed that would be activated depending on different circumstances. One of these situations would be to read without accessing the meaning of the words.In this case, the lexical-phonological or direct route would be involved, directly activating the lexical-phonological component from the visual one. We would also have the phonological or sub-lexical route, activated when reading pseudo-words or unknown words.

It is normal that, every day, when reading any text, we use one or the other route depending on the needs we have at that moment.. If we have to read a text in depth, understanding what we read and studying it, we would use the classical route, in which all the above-mentioned regions would be applied. However, these routes may vary depending on whether we are dealing with a known or unknown word, if we have to read quickly or accurately, if the word is part of our native language or if it is a foreign one...

Pathologies associated with reading disorders.

As we mentioned before, the study of the neurological bases of reading draws heavily on the study of people who have suffered some kind of alteration in their ability to read and write. In people who knew how to read and write before manifesting the problem it usually happens that the disorder arises from some kind of brain injury, while in cases where there seem to be problems since the beginning of writing, the origin of the problem would be in some inherited alteration.

Reading and writing disorders are known as dysgraphia and dyslexia.These disorders can be classified according to the type of reading and writing disorder. These disorders can be classified according to whether the problem is found in a purely linguistic alteration, as would be the case of central dyslexias and dysgraphias, or whether it is found in the afferent or efferent subsystems involved in the reading and writing process, in this case peripheral dyslexias and dysgraphias.

We will now talk in depth about peripheral and central dyslexias, which areas are involved and some varieties within these two large groups.

Peripheral dyslexias

Peripheral dyslexias are reading disorders that occur due to some damage in the areas involved in the process that goes from the analysis of linguistic information to the lexical-visual process. A classic example of this type of alterations is pure alexia or without agraphia, originally described in cases of left occipital lesion or afference from the contralateral occipital cortex, at the level of the splenium of the corpus callosum.

There is a loss in the connection loss in the connection between the visual image processing and the lexical-orthographic component, so that the visual processing of the image and the lexical-orthographic component is lost.The reading process does not have lexicological recognition. This results in a letter-by-letter process, making the reading process slower and not very functional. The affected person can read, but does so using only the sublexical pathway, so he/she accesses semantics only after reaching the phonological stage. That is, he/she first reads the sounds of the letters, and then grasps their meaning.

There are other cases in which the visual analysis of words and letters is impaired, so that strange phenomena occur, for example, omitting letters at the beginning of words, as happens in neglect dyslexia. Other cases are that of attentional dyslexia, in which letters are changed between neighboring words, and then we also have what is known as visual dyslexia, in which words are substituted for others because they have a similar appearance.

Central dyslexias

In central dyslexias, the alteration in reading is generally due to a failure at the stages of lexicological recognition, semantic access and phonological processing prior to speech production.. These are syndromes whose cause may affect any of the different pathways for reading once the visual analysis of the text being read has been overcome.

If the damage is found in the phonological pathway, usually caused by a lesion in the left perisylvian cortex, the following occurs phonological dyslexia. This form of central dyslexia is characterized by difficulty in reading unknown words or pseudowords and, to some extent, difficulty in reading function words (articles, determiners, pronouns, prepositions, conjunctions or nexuses). The person affected by this condition finds it difficult to convert the graphological (written) into phonological (pronounced).

Superficial dyslexia is that which can be observed in people who have problems reading irregular words.. In this condition is added a certain tendency to regularize the graphemes of ambiguous phonology (regularization error) and generating words that do not exist. This gives rise to many errors of omission, addition, substitution or translocation. Some researchers place the problem in the inability to access the lexicon by means of a whole word recognition strategy.

Finally, we can comment on the case of profound dyslexia. In this reading impairment, grammatical and semantic category dependence can be observed, with the reading of words such as verbs, adjectives and functional particles being affected in comparison with the reading of nouns. There are also problems in reading abstract words as opposed to concrete ones. Profound dyslexia has been related to multiple lesions at different levels in the pathways involved in reading in the left hemisphere.