Should the Autism spectrum be abolished and replaced by a “standard range” of developmental timing?
Studying the speed at which a child’s cortex matures could also shed light on other mental conditions. For example, autistic children show the opposite pattern to those with ADHD – their cortices mature much earlier than those of their peers. On the other hand, the cortices of children with exceptionally high IQ mature later, even though the cortex thickens unusually quickly in early childhood.
Posted on 12 November, 2007 by Ed Yong, ScienceBlogs.com
Attention-deficit hyperactivity disorder is the most common developmental disorder in children, affecting anywhere between 3-5% of the world’s school-going population. As the name suggests, kids with ADHD are hyperactive and easily distracted; they are also forgetful and find it difficult to control their own impulses.
While some evidence has suggested that ADHD brains develop in fundamentally different ways to typical ones, other results have argued that they are just the result of a delay in the normal timetable for development.
Now, Philip Shaw, Judith Rapaport and others from the National Institute of Mental Health have found new evidence to support the second theory. When some parts of the brain stick to their normal timetable for development, while others lag behind, ADHD is the result. Human diversity is pathological: that’s a mistake that will eventually label all humans defective.
The idea isn’t new; earlier studies have found that children with ADHD have similar brain activity to slightly younger children without the condition. Rapaport’s own group had previously found that the brain’s four lobes developed in very much the same way, regardless of whether children had ADHD or not.
But looking at the size of entire lobes is a blunt measure that, at best, provides a rough overview. To get an sharper picture, they used magnetic resonance imaging to measure the brains of 447 children of different ages, often at more than one point in time.
At over 40,000 parts of the brain, they noted the thickness of the child’s cerebral cortex, the brain’s outer layer, where its most complex functions like memory, language and consciousness are thought to lie. (Base your studies on assumptions.) Half of the children had ADHD (based on what evidence / diagnosis?) and using these measurements, Shaw could work out how their cortex differed from typical children as they grew up.
Thickening, thinning and timing
A child grows, their experiences manifest as connections between nerve cells (?) (odd description) and their cortex thickens. But during adolescence, the developing brain values efficiency over expansion and the cortex starts to thin, as unused connections are mercilessly trimmed. The growth of a child’s brain into a teenager’s is like the pouring of a block of clay that can then be sculpted away into the refined adult version. Aye, yai, yai! Science writers ought to avoid anthropomorphism – “the brain values efficiency” as if the brain is capable of human intention.
This may sound strange, but brain, the organ IS NOT your soul, consciousness, steering wheel, Facebook avatar, resident space alien, or any of the culturally-induced ideas about the brain. The brain mostly runs the body. It reacts to environmental conditions and compares “what’s going on” to its many “libraries” stocked by instinct and learning. Language produces an illusion of awareness or consciousness that is “supernatural” – the popular “ghost in the machine.”
In both groups of children, parts of the cortex peaked in terms of thickness in the same order, with waves of maturity spreading from the edges to the centre. The pattern was the same, but the timing wasn’t.
On average, the brains of ADHD children matured about three years later than those of their peers. Half of their cortex has reached their maximum thickness at age 10 and a half, while those of children without ADHD did so at age 7 and a half; you can see an evocative Quicktime video of this happening online. According to these results, ADHD is a disorder of delay, not deviance. These delays were most pronounced in the lateral prefrontal cortex, where the lag time was as high as 5 years
These parts of the brain are responsible for suppressing inappropriate thoughts (Really? We’re leaping levels of function like Karazy! How does the brain “know” what constitutes an inappropriate thought? Isn’t this cultural information?) and actions, directing attention, short-term memory and controlling movement. All of these are tasks that children with ADHD can find difficult and other studies have found that as they try, their prefrontal cortex shows less activity than expected for a child of the same age.
The only part of the brain that matured faster in children with ADHD was the primary motor cortex, which helps to plan and control movements. It also takes orders form the prefrontal cortex and if one matures early and the other matures late, this might explain several hallmarks of ADHD, including restlessness, fidgeting and uncontrolled hyperactivity.
Like much good research, Shaw’s study raises more questions than it answers. For the moment, the most pressing one is: what causes the delay? From his data, Shaw rules out Intelligence and gender, and thinks that prescribed drugs are unlikely to have an effect either.
Genes are almost certain to have an influence though and Shaw has his eye set on genes that produce a group of proteins called neurotrophins. These control the growth, division and survival of neurons, and changes in some of their genes have already been linked to ADHD. (Really? And if a link has been “proven” (seldom the case) do we now have a genetic disorder that is “justified” by the ADHD label? Blue eyes are “linked” to genes, but does that make “having blue eyes” a genetic disorder? The tail (variation in developmental timing as pathology) is wagging the dog; let’s find a genetic link that justifies labeling “ADHD behavior” as a pathology.
Shaw’s results should also be encouraging for many families, and they explain why so many children eventually grow out of the condition – as lagging brains catch up, the symptoms of the developmental lag might disappear. Or the idea of “normal” rates of development may be a western cultural fantasy that fails to take into account domestication of humans and natural genetic variation. Humans aren’t clones raised in identical environments.
But it will be interesting to see if the timing of development at specific parts of the brain relates to a child’s chances of recovery. We’ll only know that if scientists run larger studies where the brains of children with ADHD are regularly scanned over a long period of time. Prediction sets science apart: you can’t merely “label” a disease or disorder, without definitive description and medical evidence; to label children with a “disorder” that you haven’t proven to be a disorder, (ie labeling typical behavior as deviant) leads to a hypothesis / conclusion “inspired” by your labels.
Reference: Shaw, Eckstrand, Sharp, Blumenthal, Lerch, Greenstein, Clasen, Evans, Giedd & Rapoport.