New insights into dyslexia are revealing structural differences in the brain that researchers say are linked to genetic predispositions for this neurodiverse condition, challenging past research.
Dyslexia is the most common learning disorder and affects somewhere between 5 to 10 percent of the population, with some estimates as high as 17 percent. The Mayo Clinic classifies Dyslexia as a learning disorder with reading and writing but is not due to problems related to intelligence, hearing, or vision.
The recent analysis comprised 35,000 adults’ genetic predisposition to dyslexia, acquired from UK Biobank and 23andMe data. Clyde Francks and Sourena Soheili-Nezhad of Max Planck Institute for Psycholinguistics and Radboud University Medical Center analyzed the data to find a link to brain-specific structures and 35 gene variants associated with other conditions.
“Thirty-five genetic variants that influence the chance of having dyslexia were already known from a very large study by the company 23andMe in the USA, carried out in over 1 million people,” Francks said in an email to The Debrief. “However, that study did not include brain MRI data.”
The findings indicate that dyslexia has diverse brain origins and is a complex, multifaceted condition with various genetic and neurological underpinnings. This could also help explain why dyslexia manifests differently in each individual. Some people struggle with reading and writing, others have difficulty processing, and others have additional strengths.
“Our sample size was more than two orders of magnitude larger than any neuroimaging case-control study of dyslexia published to date, which is likely to have aided in the robustness of our findings,” says Sourena Soheili-Nezhad in a statement.
Past neurological studies on Dyslexia have resulted in findings of structural brain differences in the left hemisphere that correlate with genetic predispositions to dyslexia. This means one or more parents or family members would also have markers of Dyslexia. However, these studies were done in the past with only hundreds of people and were merely based on a hypothesis but were less data-driven.
“The genetic contribution to dyslexia involves many thousands of genetic variants all across the genome, each with a small effect on the chance of having dyslexia, but in combination, they add up to a measurable contribution,” states Francks in an email to The Debrief.
“There are various traits that are partly associated with dyslexia in the population, including educational attainment and attention-deficit/hyperactivity disorder. There is evidence that certain genetic variants impact dyslexia as well as these other traits. This is why we looked at how genetic effects on these other traits are related to brain structure,” says Francks.
“In this way, we could assess which aspects of brain structure are linked relatively specifically to the genetic chance of having dyslexia, as opposed to more generally to genetic effects on other related traits,” Francks said. “For example, lower motor cortex volume was relatively specific to the genetic chance of having dyslexia, whereas lower nerve fiber density in the internal capsule was found more generally in relation to genetic effects on dyslexia, ADHD, educational attainment and fluid intelligence.”
The findings pointed to reduced volumes in the internal capsule and motor cortex areas of the brain, which were associated with a genetic predisposition to dyslexia, with the brain’s internal capsule also connected to ADHD and language performance challenges.
Francks added, “For the motor cortex, our finding is consistent with previous studies that found an association between lower motor proficiency and poorer reading skills. This association is probably complex and comes about in different ways.”
Fundamentally, infants with advanced motor skills may develop language-related abilities more rapidly due to improved coordination in producing speech sounds, tracking letters, or keeping rhythm during songs. However, motor skills and reading development might not be directly connected; instead, they are influenced by shared genetic factors affecting brain development.
“For example, infants with better motor skills might be able to learn language-related skills more quickly, through better coordination when producing speech sounds, pointing to track letters and words, or keeping time during chants and songs,” Francks told The Debrief.
Specifically, the internal capsule contains nerve fibers crucial for motor coordination and other cognitive functions. Genetic factors linked to traits like educational achievement and ADHD also appear to influence this brain region.
“For the internal capsule, some nerve fiber tracts pass through here that are important for motor coordination, but also numerous other cognitive domains,” Francks said.
“We found that genetic effects on various other traits such as educational attainment or attention-deficit/hyperactivity disorder also impact the internal capsule.”
The recent study, “Distinct impact modes of polygenic disposition to dyslexia in the adult brain,” was published in Science Advances on December 18, 2024.
Chrissy Newton is a PR professional and founder of VOCAB Communications. She currently appears on The Discovery Channel and Max and hosts the Rebelliously Curious podcast, which can be found on The Debrief’s YouTube Channel on all audio podcast streaming platforms. Follow her on X: @ChrissyNewton and at chrissynewton.com.