Subclassification of Autism
A review of the most carefully controlled neuroscientific studies (Bachevalier, 1994) using various methods such as autopsy material, brain scanning and imaging (MRI, CAT, PET, SPECT, rCBF) suggest that it may be helpful to divide autism into two distinct subclasses: Type1, where there are distinct neurologic signs and varying ranges of mental retardation (encompassing approximately 60-70% of the autistic population); Type 2, where the CNS is anatomically intact and there is no mental retardation (encompassing approximately 30-40% of the autistic population). Obviously a continuum from severe functioning deficits to very minor exists in the autism population. DeLong (DeLong and Nohria, 1994) has appropriately described this phenomenon as the "spectrum" of disorders in autism. The two sub-types should be seen as fitting into such a spectrum or continuum.
It makes greater sense to subclassify autism in the above manner when the neurodevelopmental evidence is taken into consideration. For example, we know from the earlier discussion about critical periods, particularly during 8-16 weeks of gestation, that mental retardation is most likely to occur if any event interferes with the process of neuronal migration. This earlier onset disruption of the developing fetus, regardless of the cause, would produce more severe consequences. This type of disruption would affect many brain areas and have definite abnormalities such as those seen by Kemper and Bauman (1993). Cells in the limbic system (amygdala, hippocampus, cingulate, septum) would most likely be affected. The development of the Purkinje cells and deep cerebellar nuclei would also undoubtedly be affected. In this case, many neural circuits would not be properly formed and brain functioning would therefore be compromised later in life. The blend with mental retardation would make it difficult to reverse the brain functioning difficulties found with such people. Level of improvement would obviously be influenced to a very large extent by the degree of brain impairment.

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