According to Dinstein (207), the early brain overgrowth hypothesis states that autism spectrum disorder may be hallmarked by abnormally large brain size caused by the premature acceleration of cell proliferation, migration, and or differentiation early on in development. The period of overgrowth is thought to be a result of both genetic predispositions and environmental insults. Additionally, the overgrowth is generally only distinguishable in children with ASD for the first few years of their lives as it is followed by a period of arrested growth that realigns their brain size back to normal dimensions (Dinstein, 207). While most researchers agree such is the case, they have failed to unanimously pinpoint one region of the brain that is most affected by overgrowth. Rather, the research generally points to sizable differences early on in head circumference that is not indicative of any specific brain region. For this reason, generalizable head circumference measurements may be a superior biomarker for early detection of autism spectrum disorder, even prior to the onset of behavioral abnormalities.
Research from Maier (205) sought to bring consistency to previous studies that dealt with the amygdala and hippocampal volume among individuals with autism spectrum disorder, specifically those who are high functioning. In fact, this study was the first manual morphometric study that focused on ASD adults with an IQ greater than 00 (Maier 205). Prior research ci
ted had reported findings of larger, while others reported smaller, amygdala and hippocampal volumes; Yet some studies had reported no differences at all. The exclusion of individuals with an IQ below 00 increased the likelihood of examining non-syndromic forms of autism spectrum disorder, which lead the researchers to hypothesize that there would be minimal differences in brain volume observed (Maier 205). This would support the early overgrowth hypothesis in that differences in brain size are not distinguishable in adults with autism spectrum disorder due to the period of arrested growth. 30 ASD individuals were matched by age, gender, and IQ to 30 healthy individuals, and all were subjected to anatomical MRI scans focusing on amygdala and hippocampal volume (Maier 205). The results showed that the ASD group generally maintained bilaterally larger hippocampal volumes with a slight decrease in amygdala volume (Maier 205). The researchers also indicate that the hippocampal enlargement may play an active role in a more intensive experience of the world and therefore hyperactivity and hypersensitivity are commonly associated with autism spectrum disorder (Maier 205). While this data refutes the argument that the period of arrested growth realigns the brain size of individuals with ASD back to typical development, it supports the theory that there is a sizable and distinguishable difference in brain size, and therefore head circumference, of ASD individuals.