The autistic brain: birth through adulthood.

We discuss evidence of brain maldevelopment in the first years of life in autism and new neuroanatomical and functional evidence from later ages of development. Head circumference, an accurate indicator of brain size in children, was reported to jump from normal or below normal size in the first postnatal months in autistic infants to the 84 th percentile by about 1 year of age; this abnormally accelerated growth was concluded by 2 years of age. Infants with extreme head (and therefore brain) growth fell into the severe end of the clinical spectrum and had more extreme neuroanatomical abnormalities. In the frontal and temporal lobes in autism, there have been reports of abnormal increases in gray and white matter at 2 to 4 years; reduced metabolic measures; deviant diffusion tensor imaging results in white matter; underdeveloped cortical minicolumns; and reduced functional activation during socio-emotional, cognitive and attention tasks. Cerebellar abnormalities included abnormal volumes, reduced number and size of Purkinje neurons in the vermis and hemispheres, molecular defects, and reduced functional activation in posterior regions. A new neurobiological phenomenon in autism has been described that precedes the onset of clinical behavioral symptoms, and is brief and age-delimited to the first two years of life. The neurobiological defects that precede, trigger, and underlie it may form part of the developmental precursors of some of the anatomical, functional, and behavioral manifestations of autism. Future studies of the first years of life may help elucidate the factors and processes that bring about the unfolding of autistic behavior.