Wikipedia has a surprisingly thorough (as of this posting) entry on autism. Despite the sections that are flagged as needing citation someone (or several someones) has done a very good job of bringing together some of the more reliable information available to the public about autism. It lists the DSM criteria but not the ICD criteria for diagnosing autism.
The entry mentions (but does not use) the terms high functioning and low functioning that are sometimes applied to people with autism. I confess that I have come to detest these terms and avoid them like the plague. They don't really convey any reliable information about the abilities of the individuals they are supposed to describe.
The entry also has information on how autism results from changes to nearly every part of the brain (resulting from heritable changes at the genetic level). Since this is my preferred theory on how autism works I'll post the information here for your ease of perusal.
Physiology and neurology
Autism appears to involve a greater amount of the brain than previously thought. A study of 112 children (56 with autism and 56 without), published in the Journal of Child Neuropsychology, found those with autism to have more problems with complex tasks, such as tying their shoelaces or writing, which suggests that many areas of the brain are involved. Children with autism performed simple tasks as well as or better than those without. In tests of visual and spatial skills, autistic children did well at finding small objects in complex pictures (e.g., finding the character Waldo in "Where's Waldo" pictures). However, they found it difficult to tell the difference between similar-looking people. Children with autism tended to do well in spelling and grammar, but found it much more difficult to understand complex speech, such as idioms or similes when the meaning of the phrase is figurative. They would, for example, not understand that "He kicked the bucket" meant someone had died, or were likely to actually hop if told to "hop to it".
The inference from this research, according to researchers at the Pittsburgh School of Medicine, is that "These findings show that you cannot compartmentalize autism. It's much more complex.”
The research from this perspective has a number of implications:
- Autism is more than likely a global disorder which affects how the brain processes the information it receives, while complex information tends to make this more readily apparent.
- Neurological ‘wiring’ in people with autism manifest abnormalities in the areas of the brain that communicate with each other.
- Observed abnormalities provide a reasonable explanation for why children with autism have problems with complex tasks which require multiple areas of the brain to work together; autistic people tend to do better in tasks that only require one region of the brain.
- The causes of autism are possibly more pervasive than previously believed; for example, more areas of the brain are affected than just those involving social interaction, communication, interests, and imagination.
- Autism may not be primarily a disorder of social interaction; research must now take into account non-social aspects.
A possible explanation for the characteristics of the syndrome is a variation in the way the brain itself reacts to sensory input and how parts of the brain then handle the information. An electroencephalographic (EEG) study of 36 adults (half of whom had autism) at Washington University in St. Louis found that adults with autism show differences in the manner in which neural activity is coordinated. The implication seems to be that there is poor internal communication between different areas of the brain. (Electroencephalographs, or EEGs, measure the activity of brain cells.)
The study indicated that there were abnormal patterns in the way the brain cells were connected in the temporal lobe of the brain. (The temporal lobe deals with language.) These abnormal patterns would seem to indicate inefficient and inconsistent communication inside the brain of autistic people.
- Autistic brains are slightly larger and heavier and a larger than normal head circumference is commonly noted.
- In the limbic system, there is an excess of cells and they are too small. The neurons themselves appear to be underdeveloped. Dendritic trees which provide the basis for connections between neurons are truncated (i.e. shortened).
- In the cerebellum, purkinje cells are widely affected. The anatomic differences correlate to the curtailment of development earlier than 30 weeks gestation. In other words, the development of the cells appears to have stopped at some time before the 30th week in utero
- An enlarged third ventricle of the brain appears to accompany autism in those who are non-mentally retarded, but the reasons for this and its effects are still unknown.
Research has not yet established exactly what is specific to autism and what may be seen in other disorders however.