The study, published October 11, 2004, in the online edition of the Annals of Neurology, found that a language center of the brain -- Broca's area -- is apparently normal in autistic boys who have normal language capabilities.
By contrast, autistic boys with language problems have brain changes that match those seen in non-autistic boys who suffer from a rare disorder called Specific Language Impairment (SLI).
"The variability of these brain difference measures within the study groups is probably too high to allow this to be used as a diagnostic test for individual subjects," said study leader Gordon J. Harris, Ph.D., of Massachusetts General Hospital in Boston. "However, this study strongly supports viewing the deficits in language functioning of autism and SLI as disruptions in neurodevelopment and neurobiology."
Although autism affects many aspects of communication and social interaction, language difficulties are among the core impairments of the disease. Researchers have noted that the set of language deficits in autism is very similar to that seen in SLI.
Children with SLI have delayed language development, but their cognitive and social-emotional development proceeds normally. There is evidence that genetic abnormalities may underlie the two disorders. For example, children with SLI are more likely to have siblings with autism.
But there is another significant observation about children with autism, which is that some of them have normal language abilities. This supports the idea that autism is less a single disorder, and more a grouping of related disorders with overlapping symptoms.
"It is well-known that individuals with autism are often not responsive to treatment," said Anne L. Foundas, MD, an expert in cognitive and behavioral neurology of Tulane University in New Orleans, Louisiana. "Perhaps, this lack of response is not that these treatments do not work, but rather that these treatments only work in a specific biological subgroup."
Harris and his colleagues had previously used MRI to compare the brain language areas of boys with autism and normally developing boys. They had noted that there appeared to be a shift in the importance of the language areas from the left side of the brain to the right side in autism.
Although the two hemispheres of the brain are largely symmetrical, with similar groups of nerve cells and patterns of connections on each side, there are some differences. In most right-handed people, the left hemisphere of the brain dominates the understanding and production of language. Language centers such as Broca's area -- named for the 19th century French neurologist Paul Broca -- are correspondingly larger on the left side of the brain in most right-handed people.
In a study published several years ago, Harris's group found that the situation was reversed in a group of right-handed boys with autism -- Broca's area was still asymmetrical between brain hemispheres, but the right side was larger, on average, instead of the left.
In the current study, the researchers divided a new group of autistic boys into those with and those without language impairment. They also looked at boys with SLI and normally developing boys.
"This study is the first to show a direct, brain-based link between autism, SLI, and language ability," said Harris. "Both groups of boys with language impairment -- both the autism and SLI groups -- showed the reversal of language area asymmetry, while both groups of language normal boys -- both the autism and control language normal groups -- had typical asymmetry."
Furthermore, Harris noted, the degree of language impairment was related to the degree of Broca's area asymmetry.
"This study represents a major contribution to the field," commented Foundas. "Ultimately, if we can subdivide individuals based on some objective measures -- for example neurocognitive, genetic, or neuroimaging tests -- then we may be able to more selectively and effectively treat people with debilitating neurodevelopmental disorders like autism and SLI."
Article: "Language-Association Cortex Asymmetry in Autism and Specific Language Impairment," Lies De Fossé, Steven M. Hodge, Nikos Makris, David N. Kennedy, Verne S. Caviness, Jr., Lauren McGrath, Shelly Steele, David A. Ziegler, Martha R. Herbert, Jean A. Frazier, Helen Tager-Flusberg, and Gordon J. Harris, Annals of Neurology; Published Online: October 11, 2004 (DOI: 10.1002/ana.20275).
The Annals of Neurology, the preeminent neurological journal worldwide, is published by the American Neurological Association, the world's oldest and most prestigious neurological association. The 1,400 members of the ANA--selected from among the most respected academic neurologists and neuroscientists in North America and other countries--are devoted to furthering the understanding and treatment of nervous system disorders. For more information, visit www.aneuroa.org.