"This study further strengthens the biological basis of the language problems seen in both autism and SLI," says Gordon Harris, PhD, director of the Radiology Computer-Aided Diagnostics Laboratory (RAD CADx Lab) in the MGH Department of Radiology, the paper's senior author.
Autism is a serious developmental disorder characterized by a lack of normal social interaction, language abnormalities and repetitive, ritualistic behavior. In previous imaging studies of autistic boys, Harris's group identified an alteration in language-associated areas of the brain. In most right-handed individuals, what is referred to as Broca's area is larger on the left side of the brain. The 2002 MGH study showed that most right-handed autistic boys had a reversal of this pattern, with the larger area on the right side.
Other researchers have reported similar reversal of the normal brain asymmetry in people with SLI, a condition of delayed language development without other impairments, and in other language disorders. A genetic connection may exist between SLI and autism, since relatives of those with one disorder have an increased risk of developing the other. Because their earlier study did not distinguish between autistic boys with and without language problems, the MGH team looked more closely at whether the altered brain asymmetry might be related to language difficulties specifically, or to autism in general.
The study enrolled 16 boys with autism, six of whom had normal language development; 9 boys with SLI; and 11 without any developmental disabilities. As in the earlier study, only boys participated because autism predominantly affects males. The boys were ages 6 through 13 and were all right-handed. Detailed magnetic resonance imaging (MRI) studies were taken of participants' brains, and the data was analyzed using advanced techniques that provide detailed information about specific anatomical regions.
The images showed that the reversal of the normal left/right asymmetry of language-associated brain structures was strongly associated with language impairment but not necessarily with autism. In both the participants with SLI and the autistic boys with language problems, Broca's area was larger on the right side. The right side was largest in those with the most serious impairments. In the autistic boys with normal language development, Broca's area was larger on the left, as was seen in normal controls.
"There seem to be clear differences in developmental neuroanatomy in both these disorders," says Harris. "In boys with normal language skills, handedness and language tend to be dominant in the same hemisphere, but for those with language impairment, there appears to be a split. A key question to pursue is whether the language dominance dichotomy results from the language impairments or whether development of these characteristics in opposing hemispheres leads to language disorganization." Harris adds that future studies should look into whether these findings are also seen in autistic adults and, if enough participants can be enrolled, in females with autism. He is an assistant professor of Radiology at Harvard Medical School.
The report's co-authors are first author Lies De Fosse of the MGH RAD CADx Lab and Center for Morphometric Analysis; Steven Hodge, Nikos Makris, MD, PhD, David Kennedy, PhD, Verne Caviness, MD, DPhil, David Ziegler, and Martha Herbert, MD, PhD, of MGH; Lauren McGrath, Shelley Steele, and Helen Tager-Flusberg, PhD, Boston University Medical Center; and Jean Frazier, MD, McLean Hospital. The research was supported by grants from the National Institutes of Health.
Massachusetts General Hospital, established in 1811, is the original and largest teaching hospital of Harvard Medical School. The MGH conducts the largest hospital-based research program in the United States, with an annual research budget of more than $400 million and major research centers in AIDS, cardiovascular research, cancer, cutaneous biology, medical imaging, neurodegenerative disorders, transplantation biology and photomedicine. In 1994, MGH and Brigham and Women's Hospital joined to form Partners HealthCare System, an integrated health care delivery system comprising the two academic medical centers, specialty and community hospitals, a network of physician groups, and nonacute and home health services.