A Northwestern University study is the first to suggest that delayed brain development and its interaction with puberty may be key factors contributing to language-based learning disabilities such as dyslexia. The article appears in the online edition of the Proceedings of the National Academy of Sciences (PNAS).
In “Learning Problems, Delayed Development and Puberty,” co-authors Beverly A. Wright and Steven G. Zecker provide a new and overarching developmental hypothesis that could change the way that these disabilities, that affect one out of 12 children with normal intelligence, are studied, understood and treated. The authors are associate professors of communication sciences and disorders at Northwestern.
“Approaching learning disabilities from the perspective of delayed brain development could potentially unite many seemingly disparate deficits observed in adults with learning problems -- from evidence that their white brain matter is abnormally distributed to findings that they have difficulty distinguishing and manipulating language sounds,” said Wright.
The idea of brain delay also could help explain anecdotal evidence from clinicians who work with learning disabled children that those children toilet train late, have difficulty learning to ride a bicycle, talk later and generally appear less developmentally mature than their unaffected counterparts.
The Northwestern researchers found that the brains of individuals with learning problems not only appear to develop more slowly than those of their unaffected counterparts but also actually may stop developing around the time of puberty's onset. Combined, these two findings could help to account for an array of existing scientific data documenting similarities and differences between individuals with and without learning problems.
Wright and Zecker focused on the hearing ability of 115 participants who ranged in age from 6 years to adult. Of these individuals, 54 had been diagnosed with dyslexia, specific language impairment or central auditory processing disorder while the remaining 61 had no suspected learning difficulties. The participants completed five auditory detection tasks that measured their ability to hear a tone in the presence of background noise.
On all five tasks, the children with learning problems performed like unaffected children who were 2 to 4 years younger. On three of the tasks -- tasks for which performance was found to be “adultlike” at or before age 10 in children without learning problems -- the performance of the individuals with learning problems caught up with that of the other participants by adulthood.
However, on the two tasks in which the performance of unimpaired participants was found to continue to improve during the teenage years, the adults with learning problems performed more poorly than unimpaired adults and thus had failed to make up for their delays.
“We found that the children with impairments started out about three years behind, but after that, their rate of improvement was very similar to that of the children without impairments,” said Wright. “At around 10 years, however -- right around puberty’s onset -- we saw a halt in further development in the children with learning problems.”
In attempting to understand the causes of learning disabilities, scientists including Wright have tended to identify and study differences between children with learning problems and their same-aged counterparts. In a study published in Nature several years ago, for example, Wright concluded that children with specific language impairment had difficulties hearing sounds only in particular sound contexts.
“Back then I thought that the hearing problem was really contributing to their learning disability, as though some unique, particular, detailed characteristic were responsible. Today I'm thinking more broadly, trying to keep in mind the wide array of characteristics that have been observed to be abnormal in individuals with learning problems,” said Wright.
What she and Zecker are proposing is a very testable hypothesis that can be applied to a wide range of existing data. “If people start finding more evidence consistent with this hypothesis it will dramatically change the way we study learning problems,” Wright said. “Scientists will design experiments that examine subjects of varied ages in order to determine the developmental course of the characteristic they are studying.”
Wright and Zecker’s research also lends credence to what scientists using MRI and other techniques have discovered about the activity in the teenage brain. Until recently, it was thought that the brain was fully developed relatively early in childhood. Today it is clear that the teenage brain is a formidable work-in-progress undergoing myriad changes.
“If our hypothesis is correct, it suggests a strong need for early intervention and a potential for improving the abilities of individuals with learning difficulties,” said Wright. “With early identification of children with language-related learning disabilities, we may be able to remediate many of these problems by ‘training’ a child’s brain very early in life.”
Wright and Zecker's research was supported by the National Institutes of Health.