For a lump of fat, the human brain is surprisingly flexible. As you learn new words, skills or patterns, your brain adapts to access that information more easily by making new or strengthened connections between neurons. By practicing piano or studying math, you are paving these pathways that allow you to recall what you learned quickly and sometimes even unconsciously. The brain’s remarkable ability to rewire itself throughout a person's life is known as plasticity—and neuroscientists consider it an invaluable cognitive asset.
Yet some people have more of this trait than others, which can have profound effects on their learning abilities. Now, it appears that individuals with dyslexia exhibit far less plasticity in their brains than those without, researchers report this week in the journal Neuron. By using MRI scans to observe the brains of people with and without dyslexia as they completed learning tasks, the researchers have pinpointed how the rigidity of dyslexic brains may be behind the reading difficulties that are often caused by the disorder.
Though the disorder can take many forms, dyslexic people generally struggle with reading comprehension and other tasks related to processing language, such as memorization or learning a foreign language. Scientists have long suspected that dyslexia may be due to a problem in the brain’s language processing centers. But after a century of probing the world's most common learning disability—which affects at least one in 10 people worldwide—researchers are still mostly in the dark about the mechanisms behind it.
To shed light on how the dyslexic brain learns, a team of researchers led by MIT neuroscientist John Gabrieli decided to look beyond language processing. Instead, Gabrieli’s team put dozens of adults and children as young as 6 years old in MRI machines and had them listen to speech, read words and look at different objects and faces while doing tasks requiring them to think about what they were seeing or hearing. The experimenters repeatedly exposed all participants to the same words or faces or objects while measuring how their brains responded to this stimuli; about half had dyslexia, and half did not.
In all of the tests, researchers could see the brains of the children and adults without dyslexia adapting readily to the repeated stimuli. Their brains showed much less activity after they saw or heard a word or object or face that they'd been exposed to multiple times, suggesting that they had adapted already to process this information more efficiently, says Gabrieli.
But the brains of the participants with dyslexia displayed much less of this adaptation. It appeared as if the dyslexic brain had to fully reprocess the stimuli each time they were exposed to it, without the benefit of neural shortcuts that would make things more efficient, Gabrieli says. "There's a fairly broad difference in adults and children who struggle to read and how quickly their brain learns to respond to repeated information," he says. "That rapid change in efficiency seems to be starkly diminished in these individuals."
Outside of the lab, it's known that people with dyslexia don't struggle as much to recognize faces or objects or spoken language as they do to read. Gabrieli suspects that their lack of neural plasticity may manifest most when it comes to reading because of the amount of thinking and learning it requires. "Learning to read is one of the hardest things that people ever do," he says. "Maybe that just stresses the system so much that if you don't have this capacity available, it becomes really burdensome."
Because reading difficulties are one of the main symptoms of dyslexia, researchers have long focused on studying and treating this phenomenon as a language processing disorder. That has meant focusing on the language processing part of the brain, rather than overall neural flexibility. The new study gives a new perspective: Seeing dyslexia more as the brain struggling to adapt could help explain the other learning difficulties it can cause, and why symptoms can vary so much from person to person, says Guinevere Eden, a neuroscientist at Georgetown University who directs the school's Center for the Study of Learning.
Eden, who was not involved in the research, calls the new study "groundbreaking." "However, it also speaks to the gravity of the disorder,” she says, “and explains why it is such a terrible struggle for children and adults with dyslexia to learn.”
Gabrieli is now interested in looking at the brains of younger children, including infants, to see if he can glimpse the earliest stages of dyslexia. By better understanding how and when the disorder manifests, he hopes to pave the way for potential treatments—which might include behavioral exercises for young children to help their brains learn to adapt, or possibly medicines that could help increase brain plasticity. For her part, Eden says she would like to see research into how the brains of dyslexic people could possibly be changed at the level of neurons, though she acknowledges that "this will take time and ultimately may or may not be feasible."