The line between the physical and the psychological has grown increasingly blurry, as researchers have identified physical changes in the brains of people who suffer from mental illness, extreme stress and other conditions.

Now a new study has found that people with autism have fewer neurons in the amygdala, a part of the brain involved in emotion and memory.

Using a computer-aided microscope system, David Amaral of California's UC Davis M.I.N.D. Institute, counted neurons - cells responsible for creating and transmitting electrical impulses - in the amygdala of nine postmortem brains of males who had autism and ten males who did not. There were significantly fewer neurons in the brains of people with autism.

The study appears in the July 19, 2006 issue of the Journal of Neuroscience.

"While we have known that autism is a developmental brain disorder, where, how and when the autistic brain develops abnormally has been a mystery," said Thomas R. Insel, a physician and director of the National Institute of Mental Health. "This new finding is important because it demonstrates that the structure of the amygdala is abnormal in autism. Along with other findings on the abnormal function of the amygdala, research is beginning to narrow the search for the brain basis of autism."

Autism, a lifelong disorder characterized by social and communication difficulties, affects 1 in every 166 children, primarily boys.

"A better understanding of the neurobiology of the amygdala is crucial to advance autism research, and this study helps answer many important questions about the fundamental basis of autism," said Andy Shih, chief science officer for the National Alliance for Autism Research.