Our ability to look at abstract symbols and map them onto sounds is one of the key skills for becoming a competent reader. In the academic world, this is known as phonological processing, and this skill can vary from person to person, with conditions like dyslexia making it more difficult for some. 

In a new study, a team of researchers tested two supposedly opposing hypotheses on how brain structures relate to reading skill, and rather strangely, found some support for both. 

The human brain is naturally asymmetrical, and some of the structures on the left side are thought to be involved in language processing; according to the hypothesis called 'cerebral lateralization', the more asymmetry here, the better one's reading ability.

On the other hand, it might also be that the presence of asymmetries in the left part of the brain simply reinforces the traits needed to read – what the team calls the 'canalization hypothesis'. By that token, having more asymmetry just puts you in the average reading skill range.

It now appears that depending on the level of analysis – whether across the entire brain hemisphere or in specific regions – both hypotheses have merit. 

Specifically, the team found that reading ability did indeed increase with greater asymmetry in the left hemisphere, but only when looking at 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 research to identify asymmetrical structural differences in the brains of over 700 children and adults.

While similar to functional magnetic resonance imaging (fMRI), which measures metabolic activity, MRI scans draw a map of the brain by revealing anatomical structures. 

Participants had also been asked to perform reading ability tests, including ones that required them to sound out pseudowords: made-up words which involve a heightened degree of processing because our brains are not conditioned to them – this is called phonological decoding.

Once researchers had determined levels of brain asymmetry, they found that when a person's most asymmetric left hemisphere brain region was considered, greater asymmetry was associated with better performance in the pseudoword reading task. 

"Leftward asymmetry in the size of the superior temporal gyrus, in particular, is classically thought to reflect leftward hemispheric organization for language, which, when disrupted, contributes to poor reading skills according to a cerebral lateralization hypothesis," the team wrote in the paper. 

Under the cerebral lateralization hypothesis, each lobe of the brain is specialized in its ability to do the heavy lifting in specific cognitive tasks. The left side is generally associated with processes related to language, but previous research has suffered from having small sample sizes, and findings that also show activity in the right hemisphere when people perform language-based tasks. 

If true, it's still not clear that the functional differences between the hemispheres depend on their structural differences, but specific regions on the left side of the brain are reliably larger than those same regions on the right.

Meanwhile, the team also found that if certain specific 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. 

This fits in line with the canalization hypothesis, which can be visualized as a needle moving along the groove of a record, whereby it maintains a predetermined trajectory. In the case of language processing and reading, protective genetic mechanisms would kick in to develop the necessary brain asymmetries.

Because these mechanisms are reliably expressed, phonological processing is typically constrained within a normal range. The absence of these asymmetries would allow for the unconstrained expression of impaired or elevated abilities.

"Our findings indicate that, at a population level, structural brain asymmetries are related to the normal development of a speech sound processing ability that is important for establishing proficient reading," says lead author Mark Eckert from the Medical University of South Carolina. 

Strangely, performance in the pseudoword reading task was not related to asymmetries in regions of the left hemisphere that are known to be important for specific language functions. This leaves an open question for exactly how these structural asymmetries, at larger scales, affect people's reading abilities. 

"Cerebral lateralization and canalization hypotheses may both have validity but at different scales of cerebral organization and function," the team concludes.

"A greater degree of asymmetry within the left hemisphere may allow for more efficient phonological processing, perhaps due to greater hemispheric specialization."

The study was published in the journal PLOS Biology.