Summary: A new study suggests brain plasticity may be reduced in people with dyslexia.
Study suggests reduced plasticity could account for reading difficulties.
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.
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 says. 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,” Gabrieli says. “Because you’ve done something before, it’s easier to do it again.”
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 brain location changed depending on the nature of the content that was being perceived, but the reduced adaptation was consistent across very different domains,” Gabrieli says.
He was surprised to see that this effect was so widespread, appearing even during tasks that have nothing to do with reading; people with dyslexia have no documented difficulties in recognizing objects or faces.
He hypothesizes that the impairment shows up primarily in reading because deciphering letters and mapping them to sounds is such a demanding cognitive task. “There are probably few tasks people undertake that require as much plasticity as reading,” Gabrieli says.
In their final experiment, the researchers tested first and second graders with and without reading difficulties, and they found the same disparity in neural adaptation.
“We got almost the identical reduction in plasticity, which suggests that this is occurring quite early in learning to read,” Gabrieli says. “It’s not a consequence of a different learning experience over the years in struggling to read.”
Guinevere Eden, a professor of pediatrics and director of the Center for the Study of Learning at Georgetown University Medical Center, described the study as “groundbreaking.”
“For children with dyslexia, we know that the brain looks different in terms of anatomy and function, but we have not been able to establish why,” says Eden, who was not involved in the research. “This study makes an important step in that direction: It gets to the true characteristics of the properties of the neurons in these brain regions, not just their outward appearance.”
Gabrieli’s lab now plans to study younger children to see if these differences might be apparent even before children begin to learn to read. They also hope to use other types of brain measurements such as magnetoencephalography (MEG) to follow the time course of the neural adaptation more closely.
Funding: The research was funded by the Ellison Medical Foundation, the National Institutes of Health, and a National Science Foundation Graduate Research Fellowship.
Source: Anne Trafton – MIT
Image Source: NeuroscienceNews.com image is credited to MIT.
Original Research: Full open access research for “Dysfunction of Rapid Neural Adaptation in Dyslexia” by Tyler K. Perrachione, Stephanie N. Del Tufo, Rebecca Winter, Jack Murtagh, Abigail Cyr, Patricia Chang, Kelly Halverson, Satrajit S. Ghosh, Joanna A. Christodoulou, and John D.E. Gabrieli in Neuron. Published online December 21 2016 doi:10.1016/j.neuron.2016.11.020
Dysfunction of Rapid Neural Adaptation in Dyslexia
•We found reduced neurophysiological adaptation in adults and children with dyslexia
•In dyslexia, adaptation to speech from a consistent voice was significantly reduced
•Repetition of words, objects, and faces also elicited less adaptation in dyslexia
•Reading skills in dyslexia were related to the degree of neural adaptation
Identification of specific neurophysiological dysfunctions resulting in selective reading difficulty (dyslexia) has remained elusive. In addition to impaired reading development, individuals with dyslexia frequently exhibit behavioral deficits in perceptual adaptation. Here, we assessed neurophysiological adaptation to stimulus repetition in adults and children with dyslexia for a wide variety of stimuli, spoken words, written words, visual objects, and faces. For every stimulus type, individuals with dyslexia exhibited significantly diminished neural adaptation compared to controls in stimulus-specific cortical areas. Better reading skills in adults and children with dyslexia were associated with greater repetition-induced neural adaptation. These results highlight a dysfunction of rapid neural adaptation as a core neurophysiological difference in dyslexia that may underlie impaired reading development. Reduced neurophysiological adaptation may relate to prior reports of reduced behavioral adaptation in dyslexia and may reveal a difference in brain functions that ultimately results in a specific reading impairment.
“Dysfunction of Rapid Neural Adaptation in Dyslexia” by Tyler K. Perrachione, Stephanie N. Del Tufo, Rebecca Winter, Jack Murtagh, Abigail Cyr, Patricia Chang, Kelly Halverson, Satrajit S. Ghosh, Joanna A. Christodoulou, and John D.E. Gabrieli in Neuron. Published online December 21 2016 doi:10.1016/j.neuron.2016.11.020