One of the key skills for becoming a competent reader is our ability to look at abstract symbols and map them onto sounds. In the academic world, this skill can vary from person to person, with conditions like dyslexia making it more difficult for some.
A team of researchers tested two different hypotheses on how brain structures relate to reading skills, and found some support for both of them.
The human brain is asymmetrical, and some of the structures on the left side are thought to be involved in language processing.
It is possible that the presence of asymmetries in the left part of the brain reinforces the qualities needed to read the canalization hypothesis. If you have more asymmetry, you will be in the average reading skill range.
Both hypotheses have merit, depending on the level of analysis, whether across the entire brain hemisphere or in specific regions.
When looking at the most asymmetric structure, the team found that reading ability increased with greater asymmetry in the left hemisphere.
The Medical University of South Carolina used previous research to identify asymmetrical structural differences in the brains of over 700 children and adults.
Similar to fMRI, which measures metabolism, the scans draw a map of the brain by revealing structures.
People were asked to perform reading ability tests that required them to sound out pseudowords because our brains are not conditioned to them.
When researchers determined levels of brain asymmetry, they found that the more asymmetric the left hemisphere brain region, the better the performance in the pseudoword reading task.
The team wrote in the paper that leftward asymmetry in the size of the superior temporal gyrus is classically thought to reflect leftward hemispheric organization for language, which contributes to poor reading skills.
The brain's ability to do heavy lifting in specific cognitive tasks is determined by the cerebralization hypothesis. The left side is associated with processes related to language, but previous research has suffered from having small sample sizes, and findings show activity in the right hemisphere when people perform language-based tasks.
It is not clear if the functional differences between the hemispheres depend on their structural differences, but certain regions on the left side of the brain are reliably larger than those on the right.
The team found that if certain brain regions were more exaggerated in their structural differences between the two lobes, the person was more likely to fall within the average reading ability range.
The canalization hypothesis states that a needle moving along a record's grooves can be visualized as a trajectory. In the case of language processing and reading, protective genetic mechanisms would kick in.
Due to the reliably expressed mechanisms, phonological processing is constrained. Unconstrained expression of impaired or elevated abilities would be possible with the absence of these asymmetries.
The findings show that brain asymmetries are related to the normal development of a speech sound processing ability that is important for establishing proficient reading.
The performance of the pseudoword reading task was not related to the regions of the left hemisphere that are important for specific language functions. This leaves an open question about how these structural asymmetries affect people's reading abilities.
Cerebralization and canalization hypotheses may have validity but at different scales of cerebral organization and function.
A greater degree of asymmetry within the left hemisphere may allow for more efficient processing.
The study was published in a journal.
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