For a considerable time scientists were searching for one clear brain difference that may be lead to autistic behaviours. However, this hope has yet to be fulfilled, with few studies identifying brain characteristics that are shared by different individuals diagnosed with autism.
This may be a further indication that autism has many different causes, but it may also be a reflection of the difficulties in studying the brain.
Currently, scientists use a variety of clever techniques to understand the structure and function of the brain, such as magnetic fields, X-rays and radioactive chemicals. As ingenious as these methods are, they are unable to provide a full measure of the tremendous complexity of how the brain operates.It is also unlikely that autism affects just one area of the brain alone. The complex behaviours of individuals with autism, which include cognitive, language and sensory difficulties, make it difficult to pinpoint just one brain region that may be affected. Nevertheless, some promising leads have shown how different brain pathways may lead to autistic behaviours.
There is increasing evidence that differences in brain development may begin prenatally in some individuals with autism. Several studies of prenatal ultrasound measurements have found evidence for differences in the growth patterns of the brain in fetuses later diagnosed with autism. Newborns later diagnosed with autism are often also reported to have large heads at birth (“macrocephaly”).
Another research technique has been to dissect the brains of individuals with autism who have prematurely died, so-called post-mortem studies. A recent study that examined the brains of 11 autistic individuals at the microscopic level found changes in the structure and organisation of the brain cells that form during fetal life, indicating differences in brain development that begin very soon after conception.
Another well-studied area in autism is head circumference growth in the first years of life. This research dates back to 1943 and Leo Kanner’s original study that found five of the 11 children with autism he examined had large heads.
Several small studies throughout the 1990s and 2000s searched the medical records of relatively small groups of children with autism. These found that a key period was the first two years of life, in which a minority of children later diagnosed with autism had a marked increase in the rate of growth of their head.
During the first two years of life, the size of an infant’s head is a reasonable indicator of total brain size, and for many years “brain overgrowth” during very early development was seen as a risk factor for a later diagnosis of autism.
However, more recently, this view has been challenged by the release of the largest ever study in this area, which found no link between infant head circumference growth and autism.
However, the only consistent finding is just how much inconsistency there is. Not every individual with autism has differences in the size or pattern of growth of different brain regions. For those individuals who do, it is unclear how this may relate to their autistic behaviours.
A great deal of brain imaging research has examined the connections within the brain of individuals with autism. Connectivity is a measure of how well and how much two brain areas communicate with each other. In the study of autism, scientists distinguish between short-range connections (between neighbouring brain areas) and long-range connections (between brain areas further apart).
One prominent theory that has emerged from brain imaging studies is that some individuals with autism may have under-connectivity in long-range connections, but over-connectivity in short-range connections.
If found to be accurate, these brain differences may be able to explain why some individuals with autism have difficulties with complex tasks that require the integration of information from multiple brain regions (such as cognitive and social abilities), but have no difficulties, or even enhanced abilities, for tasks that require less integration across brain areas (such as sensory processing).