#968 What causes dyslexia?

What causes dyslexia? It is caused by a difference in the brain parts that process reading, but nobody knows exactly what causes that difference.

As an aside, there is a common joke that the word for people who cannot read or spell very easily, dyslexia, is one of the hardest words to read or spell. Dyslexia actually comes from Greek. “Dys” means bad, abnormal, or difficult, and “lexis” means word. The disorder was named in 1885 by a German ophthalmologist called Rudolf Berlin.

What is dyslexia? It is commonly associated with not being able to read very well. Somebody with dyslexia has trouble working out how groups of letters are connected to different speech sounds. This process is called decoding. People with dyslexia might do poorly at school because they have trouble reading, and they have difficulty understanding grammar. This difficulty to make sense of words can sometimes be found in regular conversation as well and people with dyslexia might have trouble understanding jokes or repeating what they have heard. Dyslexia is the most common learning disorder in the world and is said to affect about 20% of people.

What happens when we read? The first thing to note is that we are not born with the ability to read. We are born with the ability to talk, and studies into the language gene have shown that. We don’t need to be taught how to talk. We just mimic the people talking around us and slowly pick it up. We don’t do that with reading. If you are not taught how to read, you won’t be able to read. You can’t just pick it up naturally. You could probably teach yourself using various study aids, but it is easier to be taught by someone. We need someone with experience to show us what sounds to associate with what groups of letters. Because the ability to read isn’t hardwired into the brain, as we learn to read our brain makes connections between parts that were not intended for reading, and repurposes them. The connections become so strong that we can read easily without even thinking about it.

So, how do we read? At its simplest, we look at the words on the page, our brain recognizes then, and assigns sounds and meaning to them. It is more complicated than that, though, and it requires four different parts of our brain to work together. The frontal lobe controls our speech, reasoning, planning, emotions, and consciousness. We need it to organize and produce speech, as well as read silently. People who have had this part of their brain damaged have difficulty speaking and reading. Then there is the parietal lobe which controls sensory perceptions. This part of the brain links the words you hear or see to your memory so that you can find meanings for them. Then there is the occipital lobe where the visual cortex is found. This is the part that receives the visual signals of the words and letters you are looking at. Lastly, the temporal lobe is involved with language processing. There are also two areas between these other areas that are important. The left parietotemporal system matches the groups of letters to their sounds. Also, the left occipitotemporal area, which stores whole words. When we start to read, the we put individual sounds to the letters and sound out the words, but as we get more proficient at reading, the left occipitotemporal area starts to recognize whole words and doesn’t need to go through the sounding out process. This enables us to read much more quickly. When people are fitted with an MRI, scientists can see each part of the brain light up and it doesn’t matter what language you are reading, always the same part lights up.

So, what happens to these parts of the brain when someone has dyslexia? Dyslexic people often have trouble recognizing words easily and they cannot connect the phonemes to the right groups of letters. MRI scans of the brains of people with dyslexia show that there are very slight differences. The biggest difference seems to be that the brains of dyslexic people have a lot of difficulty passing information from one part that is concerned with reading to another part. The brain of someone with dyslexia is structurally different and has less neuroplasticity, especially in the left hemisphere, which is concerned with language and reading. They have less white matter in their brains as well. White matter is made of the connective fibers that nerves use to communicate with each other. They also have less grey matter in the part of the brain used for processing the sound structure of language.

Experts can see the differences between the brains of people with dyslexia and those without, but they are not exactly sure what causes it in the first place. If your parents have dyslexia, you have a 40 to 60% chance of developing it yourself, which means that it is probably genetic. Geneticists have experimented and they think they may have found the gene that is responsible, but they are not yet sure why. It is a mutation of the KIAA0319 gene. This gene is involved in early brain development and in this particular mutation it is reduced by about 40%. And that reduction could be what leads to the changes in the brain that cause dyslexia. And that is what I learned today.

Photo by Vincenzo Malagoli: https://www.pexels.com/photo/person-reading-book-and-holding-coffee-1550648/

Sources

https://www.lexialearning.com/blog/what-is-the-science-of-reading-how-the-human-brain-learns-to-read

https://www.readingrockets.org/reading-101/how-children-learn-read/reading-brain

https://www.americanbrainfoundation.org/diseases/dyslexia

https://www.mayoclinic.org/diseases-conditions/dyslexia/symptoms-causes/syc-20353552