How old were you when you first learned to read?
Reading is a complex skill that involves connecting symbols (letters) to specific sounds and then connecting these symbols together into words and sentences. Although children as young as two to three years old may take an interest in reading, reading skills are generally taught in the first years of elementary school. There is great variability in the timing of when kids learn to read, but it’s clear that some children struggle more than others.
One reason that children may find reading challenging is if they have dyslexia,1 a learning disability that affects how the brain processes written information including reading, writing, and spelling. New research by Dr. Clyde Francks, Head of the Imaging Genomics Research Group at Max Planck Institute in the Netherlands and Professor of Brain Imaging Genomics at Radboud University Medical Centre in the Netherlands, increases our understanding of the genetic causes contributing to dyslexia, which might eventually help to improve diagnosis and educational interventions for dyslexia in the future.
The Genetics of Dyslexia
Dyslexia affects an estimated 3-7% of school-age children around the world. Scientists have limited understanding of how and why dyslexia develops in some people. Researchers have hypothesized that challenges with associating sounds with letters, difficulty identifying patterns to connect letters into words, and problems with short term memory or attention might play a role. To date, studies of brain structure in people with dyslexia have not clearly supported any particular hypothesis.
Dr. Francks is a geneticist who specializes in the relationship between genetics and language. His team sought to take a genetics approach to understanding dyslexia without any specific hypothesis in mind regarding which parts of the brain might be affected.
Geneticists have an interest in dyslexia because research has shown it can be inherited, with studies estimating that it is 40%-70% heritable from one generation to the next. Recently, a large study of over one million people with dyslexia identified at least 35 genes whose variations might impact the development of dyslexia.
“There are actually many more than 35 genes that contribute to the development of dyslexia, likely thousands,” explained Dr. Francks. While each gene may have a relatively minor effect on its own, added together with other genes, the effect might be enough to lead to dyslexia. “Understanding the genes involved in dyslexia might point us towards certain areas of the brain that are affected in people with this learning difficulty,” Dr. Francks added.
Connecting Genetics to Brain Regions
To identify brain regions involved in dyslexia, Dr. Francks and colleagues used data from the UK Biobank. The UK Biobank is a large-scale database of medical and health information that scientists can access to conduct specific research projects. Dr. Francks and his team used data from over 30,000 individuals whose genetic information and brain imaging scans were available in the UK Biobank database.
The researchers did not know whether the individuals whose information was in the UK Biobank had dyslexia or not. Instead, they used the available genetic information to develop a score for the genetic likelihood of developing dyslexia, called the polygenic score (PGS). The brain imaging studies were used to assess brain volume and nerve fiber2 density. The researchers then analyzed the relationship between PGS, brain volume, and nerve fiber density.
The results showed that higher PGS was associated with lower total brain volume in the motor cortex3 and increased volume in the visual cortex.4 The motor cortex is a region of the brain responsible for voluntary movements, and the visual cortex is responsible for processing visual information received from the eyes.
Figure 1.
Regions of the brain including the motor cortex (primary motor area) in the frontal lobe (orange) and the visual cortex (primary visual area) in the occipital lobe (green).
[Source: https://encyclopedia.lubopitko-bg.com/The_Brain1.html]
A reduced volume of gray matter5 within the motor cortex supports findings from previous studies linking reduced motor skills to poorer reading skills. It is still unclear whether motor skills directly contribute to reading proficiency or whether genetic changes impact both motor skills and reading skills separately.
In addition, the results from this study showed that higher dyslexia PGS was associated with lower nerve fiber density in the internal capsule6 of the brain. The internal capsule is a small region of the brain responsible for sharing information between the spinal cord and brain regions involved in higher order functions, such as the motor cortex. The nerve fibers that pass through the internal capsule play an important role in motor coordination and other cognitive functions.
Figure 2.
Nerve fiber tracts of the human brain including the internal capsule.
[Source: https://commons.wikimedia.org/wiki/File:Schematic_illustration_of_projection_fibers.jpg]
The researchers don’t know when the brain changes they observed take place. It is possible that some changes occur early in development or when these individuals were children. It is also possible that some of these changes are responses to altered behavior over the course of decades in people who are more genetically susceptible to dyslexia.
Importantly, some of Dr. Francks’ findings in the brain were more specific to dyslexia PGS than others. For example, reduced volume in the motor cortex was specific to dyslexia PGS. On the other hand, lower nerve fiber density in the internal capsule was found in relation to PGS for various other traits in addition to dyslexia, such as attention deficit/hyperactivity disorder (ADHD) .7 ADHD is another common condition affecting children and can include symptoms such as inattention, hyperactivity, and impulsivity.
Taken together, these results point to specific areas of the brain, especially the motor cortex and internal capsule, that are affected by genetic changes that make dyslexia more likely. “These results improve our understanding of which brain regions can be affected in dyslexia,” concluded Dr. Francks.
For future research, Dr. Francks suggests that similar studies using data from children and adolescents, rather than adults, would help to understand changes in the brain involved in dyslexia as they happen in younger people. This could help lead to earlier diagnoses and additional educational interventions for children who need them.
Dr. Clyde Francks is Head of the Imaging Genomics Research Group at the Max Planck Institute in the Netherlands and Professor of Brain Imaging Genomics at Radboud University Medical Center in the Netherlands. His research focuses on the genetics of language and brain disorders. When not in the laboratory, Dr. Francks enjoys road trips, hiking with his wife and kids, reading non-fiction, and watching TV and movies.
For More Information:
- Soheili-Nezhad S, Schijven D, Mars RB, Fisher SE, Francks C. Distinct impact modes of polygenic disposition to dyslexia in the adult brain. Sci Adv. 2024;10(51):eadq2754. https://www.science.org/doi/10.1126/sciadv.adq2754
To Learn More:
- American Brain Foundation. https://www.americanbrainfoundation.org/diseases/dyslexia/
- Understood. https://www.understood.org/en/articles/what-is-dyslexia
- International Dyslexia Association. https://dyslexiaida.org/dyslexia-basics/
Written by Rebecca Kranz with Andrea Gwosdow, PhD at www.gwosdow.com
