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A new study finds that men and boys with autism have fewer neurons in the part of the brain involved in memory and emotion.

NPR's Jon Hamilton reports on the latest evidence that this area of the brain may be one of the keys to understanding autism.

JON HAMILTON reporting:

For a long time, it wasn't clear how the brains of people with autism differed from other brains. That began to change in the 1980s, when a group of scientists examined the entire brain of a man with autism. He died at age 29 and his brain had been sliced into thousands of thin sections.

Dr. Margaret Bauman of Harvard University was part of the group that did the research. She says scientists used microscopes to compare slices from the autistic brain with matching slices from the brain of a typical man who died at age 25.

Dr. MARGARET BAUMAN (Harvard University): We could look at identical sections in the autistic brain and in the control brain. And we could look at them side by side, which is basically what we did. We went through this entire brain.

HAMILTON: One slice at a time. Bowman says one place they found a difference was in the amygdala. It's an almond-shaped clump of cells deep in the brain. Actually, there are two amygdalae, one on each side. And they're critical to processing certain emotional reactions, particularly fear. Bauman says it looked like the brain cells from the man with autism were smaller and more densely packed.

Dr. BAUMAN: We were pretty excited about it at that time because this was really the first time anybody had shown any definite abnormalities at a microscopic level about brains and autism.

HAMILTON: Ever since then, scientists have been trying to get a better understanding of exactly what's different about the amygdala of a person with autism.

The latest effort comes from scientists at UC San Diego and the MIND Institute at UC Davis. They compared the brains from nine people with autism with the brains from ten typical people. The people ranged in age from 10 to 44 and had died from a variety of causes.

Dr. David Amaral of the MIND Institute says the team used automated techniques to estimate the number of brain cells in each amygdala. Amaral says the brains of people with autism were clearly different.

Dr. DAVID AMARAL (MIND Institute): Overall, there was a decreased number of neurons in the amygdala, and then particularly in one subdivision of it called the lateral nucleus.

HAMILTON: Which communicates with a part of the brain that controls perception. Typical brains had about 12 million neurons. Brains from people with autism averaged about 1 million fewer. Amaral says it makes sense that there would be a difference in the amygdala because people with autism tend to be very anxious.

Dr. AMARAL: The amygdala's involved in appreciating dangerous situations in the environment and generating an appropriate response to danger, which is fear and escape. And probably in a pathological state, leads to anxiety.

HAMILTON: Brain-imaging studies have already shown that in boys with autism, the amygdala develops early and stops growing around the age of eight. In typical boys, it continues to grow until age 18.

Amaral says it's possible that this early development accounts for the reduced number of neurons later in life. He says too much early activity in the amygdala also could produce abnormal fears, which might be an explanation for some of the other problems associated with autism.

Dr. AMARAL: If you have abnormal fears, how would that contribute, for example, to the development of normal social behavior or to the development of other, you know, learning and memory and other areas?

HAMILTON: Amaral says scientists are a long way from knowing, in part because the number of brains they've studied from people with autism is still only in the dozens.

Dr. AMARAL: By contrast, if you think of something like Alzheimer's Disease, probably tens of thousands of brains have been looked at. And it's pretty clear now where the pathology starts in Alzheimer's and what it looks like and what brain regions are involved. We're still in pretty early stages of trying to establish the pathology of autism.

HAMILTON: That may be changing. An autism research group has recently set up a system to encourage the donation of brain tissue from people with autism. Amaral's study is among the first to take advantage of that system.

Jon Hamilton, NPR News.

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