This study provides quantitative evidence linking autism to an abnormality of the amygdala, especially the lateral nucleus—a major emotion-processing area with connections to parts of the brain responsible for higher cognitive functions.
"These new findings, based on cell counting, complement other independent studies that suggest amygdala abnormalities likely contribute significantly to the primary core deficit in social function that defines this disorder," says Emanuel DiCicco-Bloom, MD, professor of pediatric neurology at the University of Medicine and Dentistry of New Jersey.
Authors Cynthia Schumann, PhD, of the University of California, San Diego, and David Amaral, PhD, director of the M.I.N.D. Institute at the University of California, Davis, counted and measured neurons in the amygdala of nine postmortem autistic male brains and 10 age-matched male postmortem non-autistic brains. Ages ranged from 10 to 44 years old. Unlike previous postmortem studies, the sample excluded brains of individuals with epilepsy or similar disorders associated with cell loss in the amygdala.
Paradoxically, past research using magnetic resonance imaging with children has shown that the amygdala in young males with autism is abnormally large in volume due to precocious maturation. "It is somewhat surprising, therefore, that there are ultimately fewer neurons in the autistic amygdala," says Schumann.
Schumann suggests different explanations: the amygdala was always that way from birth, a degenerative process later in life may cause the neuron loss, or the heightened level of stress and anxiety commonly observed in patients with autism could, over time, lead to a loss of neurons. More studies are needed to pinpo int exactly why autistic brains have fewer neurons in the amygdala, she adds.
"We're in the very early stages of understanding autism and its neurological pathologies," says Amaral. "It's clearly a process with many steps, and at least we are now one step further."
Autism is a lifelong neurodevelopmental disorder characterized by social and communication deficits. It affects as many as 1 in every 166 children, primarily males.