The Autistic Brain

Despite the efforts of researchers and medical practitioners, autism is still only partially understood. Boys are more susceptible than girls by a factor of about four, which may be due to the way that sex hormones interact with a gene called RORA (Sarachana et al), which is one of the many genes implicated in autism (Nguyen et al). Still, the complete genetic profile of autism isn’t known, and it’s clear that environmental factors also affect whether and to what degree an individual with a genetic predisposition expresses autism.

One of the environmental factors cited anecdotally and by some popular media sources as a contributor to autism is the measles, mumps and rubella (MMR) vaccine. However, over 20 scientific studies of vaccines and their side effects (Poland) have shown that there is no link whatsoever.

Interestingly enough, the environmental factors that help to influence development of autism may be prenatal ones, according to a new study in the Journal of the American Medical Association (Courchesne et al). This study examined the size of the brains of young autistic boys, as compared to the size of the brains of young non-autistic boys. With data adjusted for age, the autistic boys had 67% more neurons (brain cells) in an area of the brain called the prefrontal cortex — this part of the brain deals with things like communication and social interaction — than the non-autistic boys had. The autistic boys’ brains were also about 17% heavier than the brains of same-aged non-autistic boys, despite the fact that normally, brains in same-age children don’t vary significantly in weight. Interestingly enough, however, the researchers found that the increased number of neurons was greater than would be suggested by the increased brain weight. This rules out the possibility that autistic children simply have larger brains than non-autistic children, and instead makes it clear that the autistic brain has too many neurons packed into a given space.

What makes this study important in understanding autism is that the neurons of the prefrontal cortex develop and multiply during the prenatal period. Specifically, these neurons develop between approximately the 10th and 20th week of gestation. Once a baby is born, he has all the prefrontal cortex neurons he will ever have. The brain then begins a process that takes place through babyhood and toddlerhood called apoptosis, or programmed cell death. During apoptosis, the brain kills off those neurons that don’t improve brain function or form meaningful connections. This helps the brain to function more efficiently, and is an important part of neural development. The researchers in the Courchesne study did not attempt to determine whether autistic children had more neurons in the prefrontal cortex because they developed more neurons initially, or because their brains failed to perform apoptosis appropriately. Regardless, the study results help to rule out the notion that a single environmental factor or exposure (such as an MMR vaccine) could cause autism. If the prefrontal cortex of autistic children contains more neurons because they overproliferated during the prenatal period, postnatal environmental factors (such as the MMR vaccine) aren’t causative. If the prefrontal cortex contains more neurons because of a failure of apoptosis, postnatal environmental factors could influence the development of autism, but couldn’t cause it to emerge “all of a sudden” (as some parents have described in response to the MMR vaccine), because apoptosis takes place over a long period of time — many years, to be precise.

This study doesn’t fully explain autism — no single study is likely to do so — but it does help move us toward an increased understanding of the disease. Further, by making it clear that autistic brains are physically different than those of non-autistic children, which rules out vaccines as a possible cause of autism, parents can make more accurate risk-to-benefit decisions regarding health care.


What factors do you think researchers will find are implicated in autism?



Courchesne et al. Neuron number and size in prefrontal cortex of children with autism. JAMA. 2011 Nov 9;306(18):2001-10.

Nguyen et al. Global methylation profiling of lymphoblastoid cell lines reveals epigenetic contributions to autism spectrum disorders and a novel autism candidate gene, RORA, whose protein product is reduced in autistic brain. FASEB J. 2010 Aug;24(8):3036-51. Epub 2010 Apr 7.

Poland. MMR Vaccine and Autism: Vaccine Nihilism and Postmodern Science. Mayo Clin Proc. 2011 Sep;86(9):869-71.

Sarachana et al. Sex Hormones in Autism: Androgens and Estrogens Differentially and Reciprocally Regulate RORA, a Novel Candidate Gene for Autism. PLoS One. 2011 Feb 16;6(2):e17116.


5 Comments (+add yours?)

  1. Jennifer @GradBaby
    Dec 20, 2011 @ 20:18:07

    Thank you for posting this – it is a balanced summary of recent Autism research. Really interesting and nice to hear updates in research without fear-mongering & judgement.


  2. Megyn @Minimalist Mommi
    Dec 20, 2011 @ 21:53:35

    Such an intriguing finding! It makes you wonder if there environmental factors from the pregnant mother’s end. Is it diet related? Exposure to something? Or could it just be genetic? This is all so fascinating, and I’m grateful that you point out how autism doesn’t just happen over night. I get so frustrated when people try and find a simple cause for a psychological ailment, like autism. There are very few psych. problems that occur from a single event. I just wish more people were informed-so thank you for doing just that 🙂


  3. Ashley @ C is for Cockerham
    Dec 29, 2011 @ 19:30:34

    I was just talking to my SIL, who is a speech pathologist and has worked with autistic children, about this topic last week. This was a good read, and I’ll be curious to see what future research finds.


  4. Joanne Manaster (@sciencegoddess)
    Jan 07, 2012 @ 04:36:45

    What a great concept for a blog! Thanks for putting this together. I gravitated toward this post as I keep up with autism related studies since I have a child on the spectrum.

    I am paying “blog calls” to each @scio12 attendee to say “Hi” and give your blog a shoutout on twitter (I’m @sciencegoddess). I look forward to meeting you in January!


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