Abnormal activity in specific brain regions has been associated with autism spectrum disorders.
Autism spectrum disorder (ASD), a grouping that includes autism and disorders similar to it, is a pervasive developmental disorder manifested primarily in greatly diminished social interaction and communication skills. No one knows exactly what causes ASD, but scientists have identified many neurological abnormalities that might contribute to the social and cognitive deficits typically observed in ASD.
Abnormal activity in specific brain regions has been associated with ASD. For example, a part of the brain called the inferior frontal gyrus was markedly less active in children with ASD during the performance of certain tasks related to social interaction, according to a research team led by Marco Iacoboni, a neuroscientist at the University of California, Los Angeles.1
In a study reported in Nature Neuroscience, Iacoboni’s team used functional magnetic resonance imaging to investigate neural activity of 10 high-functioning children with ASD and 10 normally developing children as the children observed and imitated facial emotional expressions. The degree of reduced activity correlated with the severity of their symptoms.
The inferior frontal gyrus is believed to be part of the so-called mirror neuron system, which plays an important role in the perception and expression of emotions and enables individuals to experience empathy. The findings indicate that a dysfunctional mirror neuron system may underlie the social deficits observed in autism.
Abnormalities in brain size also have been associated with ASD. In a study published in the American Journal of Psychiatry, a group of researchers led by Antonio Hardan, a Stanford University psychiatrist, used magnetic resonance imaging scans to compare the size of the cortex (the outer layer of the brain) between 17 children with autism and 14 children without the disorder.2 Cortical thickness is a sensitive index of normal brain development.
Although the meaning of cortical thickness at the level of individual cells is unknown, the researchers believe it may indicate the degree of "arborization," the branching of brain cell connections. During normal brain development, a massive overproduction of cells and these connections (synapses) to other brain cells occurs. A competitive elimination, or "pruning," of neurons and these connections follows. The scientists hypothesize that this pruning results in cortical thinning.
When the investigators analyzed the brain images, they found increased cortical thickness in the brain’s temporal and parietal lobes of children with autism. They suggest that these anatomical differences are partly responsible for the increased brain size in ASD.
These findings will lead the scientists to look at what normally controls the thinning of the cortex—including genetic influences. They plan to investigate the different genes that are involved in this process, with the hope of finding an association that will help them better understand what causes thicker brain structures in autism, which might lead to new treatments.
An enlarged amygdala also appears to contribute to the increased brain size observed in autism, according to a study published in Archives of General Psychiatry.3 The amygdala is a part of the brain that plays an important role in socio-emotional functioning. In this study, researchers led by Stephen Dager of the University of Washington used magnetic resonance imaging to measure amygdala size in 45 children with ASD between the ages of 3 and 4.
The investigators found that an enlarged amygdala—particularly its right side—was associated with symptom severity in these children. Moreover, when the researchers tested the same children about 3 years later, they found that the children with a greater degree of right amygdala enlargement had poorer development of language and social skills.
These results strongly implicate amygdala abnormalities in the behavioral impairments found in autism, and they also suggest that the size of the right amygdala might be used to predict the clinical course of the disease.
In a more recent paper, published in Neurology, researchers from the same laboratory reported that the disabilities found in children with autism, compared to children with developmental delay, may be attributable to increased "transverse relaxation" of brain cells (gray matter) in the brain.4 Transverse relaxation, as measured by magnetic resonance imaging (MRI), is a measure of how tightly bound brain cells are, as measured by the extent to which they displace water in the brain. This technique is used to measure brain maturation over time.
The investigators compared 60 children with autism to 16 with developmental delay and 10 who were developing normally. All of the children were between 2 and 4 years old. They found that cells were significantly more tightly bound in normally developing children, compared with children with autism. Children with developmental delay fell between the normally developing children and those with autism in terms of how tightly the cells were bound.
Autism remains a mysterious disease. These imaging studies suggest developmental abnormalities in the formation of neuronal structures, perhaps early in gestation.