MIT scientists have identified a distinctive neural signature found in the brains of people with dyslexia which may explain why these individuals have difficulty learning to read.
Researchers have found that in people with dyslexia, the brain has a diminished ability to acclimate to a repeated input - a trait known as neural adaptation.
For example, when dyslexic students see the same word repeatedly, brain regions involved in reading do not show the same adaptation seen in typical readers, researchers said.
This suggests that the brain's plasticity - which underpins its ability to learn new things - is reduced, said John Gabrieli from Massachusetts Institute of Technology (MIT) in the US.
"It is a difference in the brain that is not about reading per se, but it is a difference in perceptual learning that is pretty broad," said Gabrieli.
"This is a path by which a brain difference could influence learning to read, which involves so many demands on plasticity," he added.
Researchers used magnetic resonance imaging (MRI) to scan the brains of young adults with and without reading difficulties as they performed a variety of tasks.
In the first experiment, the subjects listened to a series of words read by either four different speakers or a single speaker. The MRI scans showed distinctive patterns of activity in each group of subjects.
In nondyslexic people, areas of the brain that are involved in language showed neural adaption after hearing words said by the same speaker, but not when different speakers said the words.
However, the dyslexic subjects showed much less adaptation to hearing words said by a single speaker.
Neurons that respond to a particular sensory input usually react strongly at first, but their response becomes muted as the input continues.
This neural adaptation reflects chemical changes in neurons that make it easier for them to respond to a familiar stimulus, Gabrieli said. This phenomenon, known as plasticity, is key to learning new skills.
"You learn something upon the initial presentation that makes you better able to do it the second time, and the ease is marked by reduced neural activity. Because you have done something before, it is easier to do it again," said Gabrieli.
The researchers then ran a series of experiments to test how broad this effect might be.
They asked subjects to look at series of the same word or different words; pictures of the same object or different objects; and pictures of the same face or different faces.
In each case, they found that in people with dyslexia, brain regions devoted to interpreting words, objects, and faces, respectively, did not show neural adaptation when the same stimuli were repeated multiple times.
The study appears in the journal Neuron.
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