If a single, consistent voice speaks a stream of words, brains get used to the voice right away and adapt. But if every word spoken is in a different voice, the brain does not adapt. The difference in adaptation, as seen on the left, is large. Essentially, brains are working hard to process different voices, and much less hard to process a single voice. However, those with dyslexia adapt much less. In this image, the amount of adaptation, as seen on the right, is small. Dyslexic brains are working hard to process speech no matter what. Credit: Perrachione et al.
A distinctive neural signature found in the brains of people with dyslexia may explain why these individuals have difficulty learning to read, according to a new study from MIT neuroscientists.
The researchers discovered 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.
This suggests that the brain’s plasticity, which underpins its ability to learn new things, is reduced, says John Gabrieli, the Grover M. Hermann Professor in Health Sciences and Technology, a professor of brain and cognitive sciences, and a member of MIT’s McGovern Institute for Brain Research.
“It’s a difference in the brain that’s not about reading per se, but it’s a difference in perceptual learning that’s pretty broad,” says Gabrieli, who is the study’s senior author. “This is a path by which a brain difference could influence learning to read, which involves so many demands on plasticity.”
Former MIT graduate student Tyler Perrachione, who is now an assistant professor at Boston University, is the lead author of the study, which appears in the Dec. 21 issue of Neuron.
The MIT team 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 revealed 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.
Neuron, Perrachione, et al.: “Dysfunction of rapid neural adaptation in dyslexia.” DOI: 10.1016/j.neuron.2016.11.020 , www.cell.com/neuron/fulltext/S0896-6273(16)30858-3