Autistic symptoms begin in the first years of life, and recent
magnetic resonance imaging studies have discovered brain growth
abnormalities that precede and overlap with the onset of these symptoms.
Recent postmortem studies of the autistic brain provide evidence of
cellular abnormalities and processes that may underlie the recently
discovered early brain overgrowth and arrest of growth that marks the
first years of life in autism. Alternative origins and time tables for
these cellular defects and processes are discussed. These cellular and
growth abnormalities are most pronounced in frontal, cerebellar, and
temporal structures that normally mediate the development of those same
higher order social, emotional, speech, language, speech, attention, and
cognitive functions that characterize autism. Cellular and growth
pathologies are milder and perhaps nonexistent in other structures (e.g.,
occipital cortex), which are known to mediate functions that are often
either mildly affected or entirely unaffected in autistic patients. It is
argued that in autism, higher order functions largely fail to develop
normally in the first place because frontal, cerebellar, and temporal
cellular and growth pathologies occur prior to and during the critical
period when these higher order neural systems first begin to form their
circuitry. It is hypothesized that microstructural maldevelopment results
in local and short distance overconnectivity in frontal cortex that is
largely ineffective and in a failure of long-distance
cortical–cortical coupling, and thus a reduction in
frontal–posterior reciprocal connectivity. This altered circuitry
impairs the essential role of frontal cortex in integrating information
from diverse functional systems (emotional, sensory, autonomic, memory,
etc.) and providing context-based and goal-directed feedback to lower
level systems.The authors were supported by
funds from the National Institute of Mental Health (2-ROI-MH36840) and
National Institute of Neurological Disorders and Stroke (2-ROI-NS19855)
awarded to Eric Courchesne.