"This is the first quantitative evidence of an abnormal number of neurons in the autistic amygdala," Amaral notes. "We were able to analyze more than double the number of previously examined postmortem brains, none of which had seizure disorders or any major neurological disorder other than autism."
Previous studies had relied on measures of the density of neurons as well as the brains of autistic males who had also suffered from epileptic seizures--a condition known to produce defects in the amygdala. Amaral and Schumann counted neurons with a three-dimensional probe at high magnification. They found that although there was no variance among amygdala volumes in all the brains, the autistic males as a group had roughly 1.5 million fewer neurons than their peers.
Other brain imaging studies have shown that autistic boys develop adult-size amygdala by around the age of eight, compared to late adolescence for other young males. And it remains unclear whether other brain regions in autistics might face a neuron deficit as well. "One possibility is that there are always fewer neurons in the amygdala of people with autism. Another possibility is that a degenerative process occurs later in life and leads to neuron loss," Schumann says. "More studies are needed to refine our findings." Their results appear in a paper published today in The Journal of Neuroscience.