Acta Neurobiologiae Experimentalis recently published Influence of pediatric vaccines on amygdala growth and opiod ligand binding in rhesus macaque infants: A pilot study. Attention to this study is gaining in momentum since it delves into the issue of vaccinations. The focus of Acta Neurobiologiae Experimentalis' recent study was to ascertain a connection between vaccinations and autism onset.
In his own Focus on Autism, Kris Turlejski, Editor-in-Chief of Acta Neurobiologiae Experimentalis, writes that "...poorly understood differences in genetic make-up of some children, may be the cause of their exaggerated reaction to some factors that are harmless to other children." He also writes that "...a genetically-conditioned difference in susceptibility of a small fraction of children may make them vulnerable to the process of vaccination that is innocuous for the vast majority."
Aschner and Ceccatelli conclude there is "...no reliable data indicating that administration of vaccines containing thimerosal is a primary cause of autism. However, one cannot rule out the possibility that the individual gene profile and/or gene–environment interactions may play a role in modulating the response to acquired risk by modifying the individual susceptibility." (From Monkeys & Mice)
Some sort of predisposition seems contributory even before vaccinations are given, when it comes to autism.
MRI study at the University of Wisconsin found that siblings of autism affected individuals showed a similar pattern with regard to amygdala size, and eye contact feature in autism - but there was difference in activation of the fusiform gyrus between the autism group and their well siblings; because of that difference, an intermediate pattern is suggested due to the fact that the well siblings used circuitry very similar to healthy controls, and only slightly like their autistic siblings - even while sharing many of the same biological similarities to their autistic siblings. "Finding many of the same differences, albeit more moderate, in well siblings helps to confirm that autism is likely the most severe expression of a broad spectrum of genetically-influenced characteristics. (Richard Davidson, Ph.D.)
When and how the amygdala develops for some within the autism spectrum is a question that has been finding answers. For some there is atypical enlargement of the amygdala during earliest development; hyperarousal is proposed to cause neurons in the amygdala to fire more frequently.
Dysregulation of the amygdala has been evidenced with regard to anxiety and depression.
Perhaps the intermediate pattern suggested as the result of the University of Wisconsin's study is a pattern of depression, from biological vulnerability (from Mom and Dad), that manifests in minds that are not yet close to being fully developed. If depression onset occurred during earliest stages of development that would certainly result in hyperarousal and would affect overall development in a mind not yet fully developed.
Amygdala Volume Marks the Acute State in the Early Course of Depression concludes that a... state related increase of amygdala volume can be detected early in the course of MDD (Manic Depressive Disorder)
Seasonal presentation of depression might explain why, many times, only one child in a family ends up within the autism spectrum - and the other children are only moderately affected. Even while having many biological elements in common with their autistic sibling.