(ORDO NEWS) — Our ability to look at abstract symbols and translate them into sounds is one of the key skills to become a literate reader. In the scientific world, this skill is known as phonological processing, and it can vary from person to person, with conditions like dyslexia making it harder for some.
In a new study, a team of scientists tested two supposedly conflicting hypotheses about how brain structures are related to reading, and surprisingly found support for both.
The human brain is naturally asymmetrical and some structures on the left side are thought to be involved in language processing; according to a hypothesis called “cerebral lateralization”, the greater the asymmetry, the better the ability to read.
On the other hand, it could turn out that the presence of an asymmetry in the left side of the brain simply reinforces the traits required for reading – what the team calls the “canalization hypothesis.” So having more asymmetry just puts you in the middle range of reading skills.
Now it turns out that depending on the level of analysis – across the entire hemisphere of the brain or in individual areas – both hypotheses are justified.
In particular, the research team found that reading ability does increase with increased asymmetry in the left hemisphere, but only when considering the most asymmetric structure, essentially taking into account the hemisphere as a whole.
Researchers at the Medical University of South Carolina used magnetic resonance imaging (MRI) data from previous studies to identify asymmetric structural differences in the brains of more than 700 children and adults.
Although MRI is similar to functional magnetic resonance imaging (fMRI), which measures metabolic activity, it maps the brain by revealing anatomical structures.
Participants were also asked to complete reading ability tests, including tests that required them to say pseudo-words: made-up words that require an increased degree of processing because our brains are not designed for them – this is called phonological decoding.
By determining the level of brain asymmetry, the researchers found that when looking at the most asymmetric region of the human left hemisphere, greater asymmetry was associated with better performance on the pseudoword reading task.
“Left-sided asymmetry in the size of the superior temporal gyrus, in particular, is classically thought to be a reflection of a left-brained organization of language, which, when impaired, contributes to reading impairment, consistent with the cerebral lateralization hypothesis,” the group writes in their paper.
According to the cerebral lateralization hypothesis, each lobe of the brain is specialized in its ability to perform hard work in certain cognitive tasks.
The left side is commonly associated with language-related processes, but previous studies have suffered from small samples and results showing right hemisphere activity when people perform language-related tasks.
If this is true, then it is not yet clear whether the functional differences between the hemispheres depend on their structural differences, but specific areas on the left side of the brain are significantly larger than the same areas on the right.
Meanwhile, the research team also found that if certain specific areas of the brain had more pronounced structural differences between the two lobes, then the person was more likely to fall into the average range of reading ability.
This is consistent with the canalization hypothesis, which can be represented as the movement of the needle along the groove of the record, in which it maintains a given trajectory. In the case of language processing and reading, protective genetic mechanisms are activated to develop the necessary brain asymmetry.
Because these mechanisms are robustly expressed, phonological processing is usually limited within the normal range. The absence of these asymmetries can lead to an unlimited manifestation of impaired or enhanced abilities.
“Our results show that, at a population level, structural brain asymmetries are associated with the normal development of speech sound processing ability, which is important for the formation of reading skills,” said Mark Eckert, lead author of the study from the Medical University of South Carolina.
Strangely, performing the pseudoword reading task was not associated with asymmetries in areas of the left hemisphere known to be important for specific language functions. This leaves open the question of exactly how these structural asymmetries on a larger scale affect people’s ability to read.
“The hypotheses of cerebral lateralization and canalization may be valid, but at different scales of brain organization and function,” the team concludes.
“A greater degree of asymmetry in the left hemisphere may contribute to more efficient phonological processing, possibly due to greater specialization of the hemispheres.”
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