In 1902, George Still, the father of British paediatrics, gave one of the earliest descriptions of ADHD (attention deficit hyperactivity disorder), calling it a "moral defect without general impairment of intellect" characterised by an "abnormal incapacity for sustained attention".
While the second part remains largely true, causal theories have moved on from descriptions of a "moral defect". Brain imaging studies in particular have shown that there are structural and functional changes that underpin ADHD symptoms, and in a paper published last month in the journal Biological Psychiatry, a group at the Institute for Disorders of Impulse and Attention in the University of Southampton assess one of the newest ways of studying ADHD – by coupling brain imaging with neuroeconomics. They speculate that the condition may be associated with "suboptimal" economic decisions.
At its core, neuroeconomics attempts to understand the neural basis of economic decision making. All economic decisions involve an infinitely complicated set of interacting networks in the brain that adjust and implement plans that are aimed at securing a desired outcome. In children with ADHD, decision making is often compromised because they cannot wait for a reward – a phenomenon known as "delay aversion".
This means that a smaller reward, received now, is often preferred over a more substantial reward in the future. Brain imaging studies have found that ADHD children have a hypersensitivity to delay that often translates into a desire to receive a reward as quickly as possible.
An attention network that lies in the frontal and parietal lobes of the brain also seems to be underactive in ADHD children. Treatment with the drug Ritalin can increase activity in a structure that lies at the heart of this network – the dorsal anterior midcingulate cortex. This can ramp up activity in the attention network and increase the brain's ability to tolerate a delay before it receives a reward.
In a resting state, when we let our minds wander, another very interesting network called the default mode network appears to motor along in the background. First discovered in 1995 by researchers at the Medical College of Wisconsin, the default mode network is not only active during introspection and day-dreaming, but also deactivates as soon as we begin to attend to a task that requires a bit of serious thinking.
In ADHD children, however, the default mode network does not seem to cut out when it should, so when they switch to a task that requires focused thought, the default mode network's activity can interfere with concentration.
Money, attractive faces and appetising foods consistently activate a third brain network that deals with rewards: the frontostriatal network. This network weighs up the pros and cons of different choices and when it malfunctions there is a reduced ability to link rewards with actions. Memory is also important here as the brain must be able to hold different alternatives in mind so that it can fully explore the implications of different choices.
Children with ADHD have difficulty holding different choices in mind, which in turn leads to poorer decision making. Research has shown that children with ADHD have a reduced concentration of dopamine receptors in this reward network. Again, drug treatment can help to boost its activity and improve the brain's ability to consider different choices before it makes a decision.
But the story of ADHD is not all negative – there must be good evolutionary reasons for retaining ADHD traits in the population. Some of the positive aspects of ADHD include increased creativity and novel problem-solving abilities. Evolution needs variability and new avenues, and it is possible that those with ADHD will seek directions that would otherwise remain unexplored.
In an interesting genetic study of a Kenyan tribe, researchers at Northwestern University in the US looked at the DRD4 7R gene, which has been linked with ADHD and novelty seeking. Only people with this ADHD-related gene were studied, but one group had recently settled into a community while a second group remained nomadic.
The nomads with the ADHD-related gene were found to have a higher body mass index than their settled counterparts with the gene. Novelty-seeking and increased impulsivity may have helped the nomads to obtain food. The researchers also suggested that the shorter attention span conferred by the DRD4 7R gene might help nomadic children to learn more efficiently in a rapidly changing outdoor environment.
In contrast, a similar attention span in the settled children would interfere with their learning in school. So the DRD4 7R gene may be more suitable for a nomadic lifestyle and less suitable for the sustained attention needed to make a living buying and selling goods in a village or town market place.
All of which raises some interesting prospects for expanding these kinds of studies into the field of trading and banking for a more precise look at the neural goings-on behind some of our more spectacular financial failures.
So what was once vaguely described in a fit of hand-waving as "a moral defect" is now being understood in ever more sophisticated ways as a dysfunction of many different brain networks that control decision making, memory and goal setting. As neuroeconomic and brain imaging tools shed more light on ADHD, a more subtle understanding of its symptomatology will also be vital for developing new treatments.
