Using a new form of brain imaging known as diffusion tensor imaging (DTI), researchers in the Center for Cognitive Brain Imaging at Carnegie Mellon University have discovered that the so-called white matter in the brains of people with autism has lower structural integrity than in the brains of normal individuals. This provides further evidence that the anatomical differences characterizing the brains of people with autism are related to the way those brains process information.
The results of this latest study were published in the journal NeuroReport. The scientists used DTI — which tracks the movement of water through brain tissue — to measure the structural integrity of the white matter that acts as cables to wire the parts of the brain together. Normally, water molecules move, or diffuse, in a direction parallel to the orientation of the nerve fibers of the white matter. They're aided by the coherent structure of the fibers and a process called myelination, in which a sheath is formed around the fibers that speeds nerve impulses. The movement of water is more dispersed if the structural integrity of the tissue is low — i.e., if the fibers are less dense, less coherently organized, or less myelinated — as it was with the participants with autism in the Carnegie Mellon study. Researchers found this dispersed pattern particularly in areas in and around the corpus callosum, the large band of nerve fibers that connects the two hemispheres of the brain.
"These reductions in white matter integrity may underlie the behavioral pattern observed in autism of narrowly focused thought and weak coherence of different streams of thought," said Marcel Just, director of the Center for Cognitive Brain Imaging and a co-author of the latest study. "The new findings also provide supporting evidence for a new theory of autism that attributes the disorder to underconnectivity among brain regions," Just said.
In 2004, Just and his colleagues proposed the underconnectivity theory based on a groundbreaking study in which they discovered abnormalities in the white matter that suggested a lack of coordination among brain areas in people with autism. This theory helps explain a paradox of autism: Some people with autism have normal or even superior skills in some areas, while many other types of thinking are disordered.
Last summer, Just led a team of researchers that found for the first time that the abnormality in synchronization among brain areas is related to the abnormality in the white matter. They discovered that key portions of the corpus callosum seem to play a role in the limitation on synchronization. In people with autism, anatomical connectivity — based on the size of the white matter — was found to be positively correlated with functional connectivity, which is the synchronization of the active brain regions. They also found that the functional connectivity was lower in those participants in whom the autism was more severe.
These studies, along with the latest paper, are providing a comprehensive picture of the autistic brain, whose components operate with less coordination than is normally the case, and which is less reliant on frontal components and more reliant on posterior components. The latest DTI finding shows that some of the frontal-posterior communication fiber tracts are abnormal, consistent with the lower degree of frontal-posterior coordination.
"The brain components in autism function more like a jam session and less like a symphony," Just said.
The latest study was co-authored by Rajesh K. Kana and Timothy A. Keller of the Center for Cognitive Brain Imaging. This research was supported by the National Institute of Child Health and Human Development.
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