For over 60 years, the causes of autism have eluded researchers. Most recently, the hunt focused on the developmental disorder's neurological roots, from attention-directing cells in the cerebellum to the emotion-charged amygdala and executive centers in the frontal lobes.

Now, scientists propose that autism is a systemwide lack of communication among otherwise functional brain areas.

Marcel Just, a psychology professor at Carnegie Mellon University in Pittsburgh, has put forth the new theory of "underconnectivity" after studying how the brains of autistic subjects differ from normal brains when processing sentences. Using functional magnetic resonance imaging (fMRI), Just found that autistics rely heavily on parts of the brain that specialize in working with the meanings of individual words.

In contrast, normal subjects showed greater activity in areas that integrate the words of a sentence into a conceptual and structural whole. Brain regions which work in synchrony in normal people were slow to respond in autistic subjects, Just found. "It's like a company with a brilliant CEO and hardworking employees, but the phone system is down," says Just, who published the studies in Brain. "Complex work can't get done."

Underconnectivity may also explain why autistic individuals sometimes display better than average ability on certain tasks: When there's a problem with communication, each part does its best to work autonomously—sometimes extraordinarily well.

"Underconnectivity is an extremely interesting idea," says Helen Tager-Flusberg, director of autism research at Boston University. "Until now, I don't think we had a comprehensive neurobiological theory of autism." She cautions, however, that although Just's theory can explain a broad range of symptoms, it doesn't address developmental mechanisms, such as atypical timing of growth in different brain areas.

Uta Frith, a researcher at University College London who has also defined autism as an inability to integrate information, takes a guarded stance. Since other conditions like dyslexia also show weak connectivity among brain regions, says Frith, "the finding by itself does not explain that much about autism."