In people with autism, the brain areas that perform complex analysis appear less likely to work together during problem solving tasks than in people who do not have the disorder, report researchers working in a network funded by the National Institutes of Health.
The researchers found that communications between these higher-order centers in the brains of people with autism appear to be directly related to the thickness of the anatomical connections between them.
In a separate report, the same research team found that, in people with autism, brain areas normally associated with visual tasks also appear to be active during language-related tasks, providing evidence to explain a bias towards visual thinking common in autism.
"These findings provide support to a new theory that views autism as a failure of brain regions to communicate with each other," said Duane Alexander, M.D., Director of NIH's National Institute of Child Health and Human Development. "The findings may one day provide the basis for improved treatments for autism that stimulate communication between brain areas."
The studies and the theory are the work of Marcel Just, Ph.D., D.O. Hebb Professor of Psychology at Carnegie Mellon University in Pittsburgh, Pennsylvania, and Nancy Minshew, M.D., Professor of Psychiatry and Neurology at the University of Pittsburgh School of Medicine and their colleagues.
The research was conducted by the Collaborative Program of Excellence in Autism, a research network funded by the NICHD and the National Institute on Deafness and Other Communication Disorders.
People with autism often have difficulty communicating and interacting socially with other people. The saying "unable to see the forest for the trees" describes how people with autism frequently excel at details, yet struggle to comprehend the larger picture. For example, some children with autism may become spelling bee champions, but have difficulty understanding the meaning of a sentence or a story.
An earlier finding by these researchers described how a group of people with autism tended to use parts of the brain typically associated with processing shapes to remember letters of the alphabet. A news release detailing that finding appears at http://www.nichd.nih.gov/new/releases/final_autism.cfm.
Participants with autism in both current studies had normal I.Q. There were no significant differences between the participants with and without autism in age or I.Q.
The first of the two new studies recently was published online in the journal Cerebral Cortex. In that study, the researchers used a brain imaging technique known as functional magnetic resonance imaging, or fMRI, to view the brains of people with autism as well as a comparison group of people who do not have autism. All of the study participants were asked to complete the Tower of London test. The task involves moving three balls into a specified arrangement in an array of three receptacles. The Tower of London is used to gauge the functioning of the prefrontal cortex.
This brain area, located in the front, upper part of the brain, deals with strategic planning and problem-solving. The prefrontal cortex is the executive area of the brain, in which decision making, judgment, and impulse control reside.
A little further back is the parietal cortex, which controls high-level visual thinking and visual imagery, supporting the visual aspects of the problem-solving. Both the prefrontal and parietal cortex play a critical part in performing the Tower of London test.
In the normal participants, the prefrontal cortex and the parietal cortex tended to function in synchrony (increasing and decreasing their activity at the same time) while solving the Tower of London task. This suggests that the two brain areas were working together to solve the problem.
In the participants with autism, however, the two brain areas, prefrontal and parietal, were less likely to function in synchrony while working on the task.
The researchers made another discovery, for the first time finding a relationship between this lower level of synchrony and the properties of some of the neurological "cables" or white matter fiber tracts that connect brain areas.
White matter consists of fibers that, like cabling, connect brain areas. The largest of the white matter tracts is known as the corpus callosum, which allows communication between the two hemispheres (halves) of the brain.
"The size of the corpus callosum was smaller in the group with autism, suggesting that inter-regional brain cabling is disrupted in autism," Dr. Just said.
In essence, the extent to which the two key brain areas (prefrontal and parietal) of the autistic participants worked in synchrony was correlated with the size of the corpus callosum. The smaller the corpus callosum, the less likely the two areas were to function in synchrony. In the normal participants, however, the size of the corpus callosum did not appear to be correlated with the ability of the two areas to work in synchrony.
"This finding provides strong evidence that autism is a disorder involving the biological connections and the coordination of processing between brain areas," Dr. Just said.
He added, however, that the thickness, or extent, of connections between brain areas may not be the basis for the disorder. Although the neurological connections between the prefrontal cortex appear to be reduced in autism, the brains of people with autism have thicker connections between certain brain regions within each hemisphere.
"At this point, we can say that autism appears to be a disorder of abnormal neurological and informational connections of the brain, but we can't yet explain the nature of that abnormality," Dr. Just said.
In the second study, published online in the journal Brain, the researchers examined the extent to which brain areas involved in language interact with brain regions that process images. Dr. Just explained that earlier studies, as well as anecdotal accounts, suggest that people with autism rely more heavily on visual and spatial areas of the brain than do other people.
In this study, the researchers used fMRI to examine brain functioning in participants with autism and in normal participants during a true-false test involving reading sentences with low imagery content and high imagery content. A typical low imagery sentence consisted of constructions like "Addition, subtraction, and multiplication are all math skills." A high imagery sentence, "The number eight when rotated 90 degrees looks like a pair of eyeglasses," would first activate left prefrontal brain areas involved with language, and then would involve parietal areas dealing with vision and imagery as the study participant mentally manipulated the number eight.
As the researchers expected, the visual brain areas of the normal participants were active only when evaluating sentences with imagery content. In contrast, the visual centers in the brains of participants with autism were active when evaluating both high imagery and low imagery sentences.
"The heavy reliance on visualization in people with autism may be an adaptation to compensate for a diminished ability to call on prefrontal regions of the brain," Dr. Just said.
The second study also confirmed the observations in the first study - that the prefrontal and parietal brain regions of the cortex in people with autism were less likely to work in synchrony than were the brains of normal volunteers. The second study also confirmed that the extent to which the two parts of the cortex could work together was correlated with the size of the corpus callosum that connected them.
Dr. Just and his colleagues are conducting additional studies to ascertain the nature of the abnormality of the connections in the brains of people with autism.