How does left-brain asymmetry relate to the ability to read

Summary: Left-largest brain asymmetry can predict better and average performance at the foundational level of reading ability.

source: Plus

Researchers led by Mark Eckert at the Medical University of South Carolina in the US report that two seemingly opposing theories of language processing are both correct.

Publishing in an open access journal Biology Plus On April 5th, the study showed that left-largest brain asymmetry can predict better and average performance on a basic measure of reading ability, depending on whether the analysis is performed on the whole brain or in specific regions.

The ability to convert fluent written symbols into speech sounds is an essential aspect of reading that varies from person to person and is difficult for individuals with conditions such as dyslexia. While a structural asymmetry between the left and right sides of the brain appears to be related to this ability, how that remains remains a mystery.

Using structural magnetic resonance imaging (MRI) of more than 700 children and adults, along with a reading test of pseudo-words and a mathematical method called continuous homology, the new study tested two opposing theories of how brain asymmetry affects vocal processing.

Researchers have developed a method for determining levels of brain asymmetry from MRI images using continuous symmetry. They found that when looking at the location of each individual’s most asymmetric region, greater left-brain asymmetry was associated with a better ability to read pseudo-words. This supports the cerebral tiling hypothesis.

This indicates asymmetric structures of the brain
Brain structure asymmetry is shown, as defined using a new topological approach that identifies relevant features (left) from noise within asymmetric structures (right). Credit: Federico Juric

At the same time, they found that greater left asymmetry in certain regions — including a motor planning region called Brodmann’s area 8, and a performance control region called the dorsal cingulate — was associated with average reading ability, supporting the channel hypothesis.

Notably, the ability to read pseudo-words was not consistently associated with asymmetry in brain regions known to be important for specific language functions. How left/right structural asymmetries affect other types of reading abilities and influence left language network functionality remains to be studied.

“Our findings suggest that at the population level, the brain’s structural asymmetries are associated with the normal development of speech sound processing ability that is important for establishing reading proficiency,” Eckert adds.

About this research in Neuroscience News

author: press office
source: Plus
call: Press office – PLOS
picture: The image is attributed to Federico Iuricich

original search: open access.
Cortical asymmetry in different spatial hierarchies relates to phonological processing ability by Eckert MA, Vaden KI Jr, Iuricich F, Dyslexia Data Consortium (2022). Biology Plus


Cortical asymmetries in different spatial hierarchies relate to phonological processing ability

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The ability to assign speech sounds to the corresponding letters is critical to creating a fluent reading. People differ in this sound processing ability, which is presumably due to variation in hemispheric asymmetry within brain regions that support language.

The lateral cerebral generalization hypothesis predicts that more asymmetric brain structures facilitate the development of basic reading skills such as phonological processing. That is, structural variances are expected to increase linearly with power. In contrast, the channel hypothesis predicts that asymmetry constrains behavioral performance within the normal range. That is, structural asymmetries are expected to be associated with phonological processing, with average acoustic processing occurring in people with asymmetric structures.

These expectations have been examined in relatively large samples of children (N = 424) and adults (n = 300), using topological asymmetry analysis of T1-weighted brain images and measurement of audio processing decoding. There was limited evidence for structural asymmetry and correlates of phonemic decoding in brain regions relevant to classical language.

However, in modest support for the cerebral tiling hypothesis, small to medium effect volumes where audio decoding accuracy increased with greater volume structural asymmetry across left hemisphere cortical areas, but not right hemisphere cortical areas, for both Adult and pediatric samples.

In support of the channel hypothesis, small to medium effect volumes were observed where phonemic decoding in the normal range was associated with increased asymmetry in specific cortical regions of both adult and pediatric samples, which included performance monitoring and motor planning regions in the brain that contribute to oral and written language functions.

Thus, the significance of each hypothesis in phonemic decoding may depend on the magnitude of brain organization.