The neurological bases of reading: characteristics and discoveries

The neurological bases of reading have been of great interest to the scientific community, especially with regard to literacy disorders.

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

Next we do a brief review on the neurological bases of reading, what data has been compiled over the years and what effects would they have at the brain level in people who present dyslexias

• Related article: "Parts of the human brain (and functions)"

What are the neurological bases of reading?

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

Unsurprisingly, scientists have focused on the brain, trying to see which areas are involved in language and specifically 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 disorders of literacy. These studies were conducted in the late 19th century and Dejerine is credited with identifying the alexia syndrome with and without agraphia in 1892, studying patients who presented vascular lesions due to infarcts in the parietal and left occipital areas.

Thanks to Dejerine's work, it was possible to the first systematizations of brain areas and, following a connectionist model, over time the implication of various areas would be suggested.

These include the participation of the left occipital cortex, involved in visual processing; the left temporo-parieto-occipital area, which would be in charge of the visual-verbal integration; the angular turn, with the skill of linguistic coding; the posterior temporal cortex, responsible for semantic access; and the lower left frontal cortex, responsible for motor speech.

But not all the credit goes to Dejerine. Many other researchers over the last two centuries have helped to clarify the neurological bases of reading. Among them we have Norman Geschwind, who studied the convergence of bihemispheric visual input in the left occipital cortex from which would project the information to the ipsilateral angular gyrus, a region that is considered the center of the visual images of the words. This information would then 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 spelling characteristics of letters and words would be recognized, that is, it is here where the lexical-orthographic or visual component of the language would be found.

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

But in addition to this route for reading, called classical, lexical, translexic or semantic, other alternative paths have been proposed that would be activated depending on different circumstances. One of these situations would be reading without accessing the meaning of the words, where the lexical-phonological or direct route would be involved, directly activating the lexical-phonological component from the visual. We would also have the phonological or sublexic route, activated when pseudowords or unknown words are read.

It is normal that, every day, when reading any text, we use one way or another 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 way, in which all the aforementioned regions would apply. Now, 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 mother tongue or if it is foreign ...

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Pathologies associated with impaired reading

As we mentioned, the study of the neurological bases of reading drinks much from the study of people who have suffered some type 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 type of brain injury, while in the cases in which there seem to be problems since the beginning of the writing, the origin of the problem would be in some inherited alteration.

Reading and writing disorders are known as dysgraphia and dyslexia, preferred terms before those of agrafia and alexia, used more in classical texts. These disorders can be classified according to whether the problem is in a purely linguistic alteration, as would be the case of dyslexias and dysgraphias. central, or it is found in the afferent or efferent subsystems involved in the reading-writing process, speaking in this case of dyslexia and dysgraphia peripheral.

Next we are going to talk in depth about peripheral and central dyslexias, what areas are involved and some varieties within these two large groups.

Peripheral dyslexias

Peripheral dyslexias are alterations in reading that occur due to damage to the areas involved in the process that goes from the analysis of linguistic information to the process lexical-visual. A classic example of this type of alteration is found in 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 body callous.

It is produced the loss in the connection between the visual processing of the image and the lexical-orthographic component, in such a way that the reading process does not have lexicological recognition. This results in the process being done letter by letter, making the reading process somewhat slower and less functional. The affected person can read, but he does so using only the sublexic path, for which he accesses semantics once he has reached the phonological stage. That is, first read the sounds of the letters, and then grasp their meaning.

There are other cases where it happens that the visual analysis of words and letters is damaged, so that strange phenomena, such as omitting letters at the beginning of words, as in dyslexia due to negligence. Other cases are that of attentional dyslexia, in which letters are exchanged between neighboring words, and then We also have what is known as visual dyslexia, in which words are substituted for others when they look like similar.

Central dyslexias

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

In case of finding the damage in the phonological pathway, normally caused by a lesion in the left perisylvian cortex, it is given phonological dyslexia. This modality of central dyslexia is characterized by a difficulty in reading unknown words or pseudowords and, also in To some extent, there is difficulty in reading the functional words (articles, determiners, pronouns, prepositions, conjunctions, or links). Those affected by this condition find it difficult to convert the graphological (written) into phonological (pronounced).

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

Finally, we can comment on the case of deep dyslexia. In this reading alteration, dependence on the grammatical and semantic category can be observed, being affected especially reading words like verbs, adjectives, and functional particles compared to reading nouns. There are also problems reading abstract words versus concrete ones. Deep dyslexia has been associated with multiple lesions at different levels in the pathways involved in reading in the left hemisphere.