How the natural asymmetries of the brain affect the reading ability of each person

The ability to assign letters to the corresponding speech sounds, or auditory processing, is crucial to becoming a competent reader. There is substantial variation in phonological processing capabilities due, in part, to genetic influences. Individual differences in processing have also been linked to varied brain structures, even within a well-established set of brain regions that support such processing. A group of scientists proposed that the degree to which these regions show hemispheric asymmetries in structure and/or function may explain individual differences in language skills.

The human brain exhibits pronounced asymmetries. In the accuracy of phonological processing, these regions exhibit more activity of the left hemisphere than in the right hemisphere. Specifically, these tasks, for example, the choice of a rhyme, cause increased activity to the left in the supramarginal, inferior frontal, superior temporal and dorsal cingulate gyri.

Recently, a group of researchers led by Mark Eckert at the Medical University of South Carolina, United States, reported that two seemingly opposing theories about language processing are correct.

The study, published in the open access journal PLOS Biology, shows that greater asymmetry of the left brain can predict both average or exceeding performance in reading ability, depending on whether the analysis is performed on the entire brain or in specific regions.

Being able to fluently convert written symbols into speech sounds is a basic aspect of reading that varies from person to person and is difficult for those with differential conditions, such as dyslexia. While the structural asymmetries between the right and left sides of the brain seem to be related to this ability, exactly how it remains a mystery.

Using structural MRI scans in more than 700 children and adults, along with a pseudo-word reading test and a mathematical method called persistent homology, new research by specialists at the University of South Carolina proved how two opposing theories about how brain asymmetries should affect phonological processing could coincide in the results.

Researchers developed a way to determine levels of brain asymmetry from magnetic resonance imaging using persistent homology. They found that when considering the location of the most asymmetric region of each individual, greater asymmetry of the left brain was related to better reading ability of pseudowords. This supports a hypothesis of brain lateralization.

At the same time, they found that greater left asymmetry in specific regions, including a motor planning region called Brodmann Area 8 and a performance monitoring region called the dorsal cingulate, were associated with an average reading capacity, supporting a hypothesis of channeling. It should be noted that the ability to read pseudowords was not consistently related to asymmetries in brain regions that, however, are known to be important for specific language functions.

However, for specialists it remains to study how the left/right structural asymmetries of the brain affect other types of reading skills and influence the functions of a left language network. Eckert suggests that his findings indicate that, “at the population level, structural brain asymmetries are related to the normal development of a speech sound processing capacity that is important for establishing competent reading.”

To the extent that structural asymmetries play a causal role in the development of phonological processing skills rather than reflecting the consequence of atypical language development, “the results of this study suggest the need to rethink explanations about how such asymmetries underlie the expression of oral and written language skills”, the specialists concluded.

“The hypotheses of brain lateralization and channeling may be valid, but at different scales of brain organization and function,” Eckert warns. A greater degree of asymmetry within the left hemisphere may allow for more efficient phonological processing, perhaps due to greater specialization of that hemisphere. People at risk of impaired phonological processing may have a relatively preserved function due to the asymmetric development of the regions of the brain that serve to monitor performance.”

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