ADHD mechanisms probed

Monday April 19 2010

Children with ADHD may respond to instant rewards “in the same way as they do to medication”, says the BBC.

The news is based on a study in which children with attention deficit hyperactivity disorder (ADHD) were assessed through a computer-based task that offered them extra points for less impulsive behaviour. This important study, albeit small, furthers our understanding of how ADHD affects particular brain activity and the way that interventions such as medication and motivational conditions can alter that response. The increased incentive offered in the task improved areas of brain activity that are usually affected by the disorder, having an effect similar to that of medication. However, there are some limitations, including that the behavioural response of the child does not appear to have been assessed, and that the reward scenario used may not be easily transferable to everyday life.

Given the nature of the study and that the researchers say that their tasks were not designed “to replicate behavioural modification programs used in clinical practice”, the direct implications of these findings are unclear and require further research. Parents should not alter their child’s medication without consulting their doctors.

Where did the story come from?

The study was carried out by Dr Madeleine Groom and colleagues from the University of Nottingham, the University of Oxford and Simon Fraser University in Canada. The study was funded by the Wellcome Trust and was published in the peer-reviewed medical journal, Biological Psychiatry.

The study was described accurately by BBC News, although it cannot be said at the current time that these findings warrant a reduction in the doses of drugs such as Ritalin.

What kind of research was this?

The researchers say that ADHD is thought to be caused by executive deficits (deficiencies in the part of the mind that controls attention and functioning) and/or by changes in motivational style and reward processing. They say that some of the effects of motivational incentives have not been studied. In this observational study, the researchers enrolled children with ADHD and a similar group of normally developing children, and compared their performance in various tasks.

What did the research involve?

Twenty-eight children aged 9 to 15 years with ADHD were referred by child psychiatrists and paediatricians. The study only included those with a diagnosis of ADHD-combined (a particular subtype of the condition) who had an established response to methylphenidate (Ritalin). Children with comorbid tic disorder, pervasive developmental disorder, a neurological disorder or an IQ below 70 were not included in the research. A separate group of 28 “normally developing” children were recruited from schools and were matched with these children in terms of their age, gender and socioeconomic status.

The groups were exposed to a task described as a modified version of the ‘visual go/no-go task’. This is described as a computer-based task where children were asked to catch as many green aliens as possible (the ‘go stimulus’), but to avoid catching any black aliens (the ‘no-go stimulus’). During the go trials children gained points for timely responses and lost points for slow responses. The go trials and no-go trials were presented separately. In total there were 600 trials performed, of which 25% were no-go trials.

The alien-catching task was performed under three different motivational scoring systems: low motivation, reward and response cost. These systems were designed to place the children under different motivational conditions. Under the low motivation conditions the children gained one point for each successful catch and lost one point for each failed catch. Under the reward conditions they gained five points for each correct catch. Under the response cost conditions a penalty of five points was deducted for each incorrect catch. Children with ADHD performed the go/no-go task once while they were taking their usual medication (methylphenidate) and once without it (medication was stopped 36 hours before the task).

Electrophysiological data (i.e. brain activity) were recorded using electrodes attached to the head and near to the eye to record eye movements. The performances of the two groups of children (ADHD group versus control group) under different motivational conditions were then compared in terms of their event-related potential (ERP) scores. An ERP score is a measure of the brain’s response to the stimulus that the children were receiving through the task. The researchers were particularly interested in two ERPs, called N2 and P3. They say that in healthy individuals these increase when motor inhibition or conflict resolution is required, but that this is impaired in ADHD brains. The differences seen when ADHD children took medication and stopped medication were also compared.

What were the basic results?

The study found that diagnosis, medication and motivational condition all affected the ‘amplitudes’ of N2 and P3 responses. This means that the control children performed differently to those with ADHD who were not medicated (greater amplitude) and that those taking medication performed differently to those not taking medication. It seemed that increasing incentives to perform correctly in the tasks improved the ERPs seen in ADHD children.

How did the researchers interpret the results?

The researchers conclude that motivational incentives increase the ERPs relating to conflict response and attention in children with ADHD, bringing them to the same level as healthy control children in the low-motivation task. The study also found that stimulant medication further increased the benefits of motivational incentives.


This observational study has used methods that are quite complex and particular to this field of study. It is important research, although there are limitations that the researchers note, including the following:

  • They say that their sample sizes were small, which means they may have missed some important interactions between factors.
  • They also note that it is important to determine whether the effects seen in their study are similar in children whose ADHD is not so strictly defined, and whether it applies to children with inattentive ADHD (another subtype of the disorder).

The children in this study were asked to withhold medication for 36 hours in order to compare the effects of the task during medicated and un-medicated stages. It is unclear whether this was a sufficient ‘wash-out’ period or how the withdrawal of medication was monitored.

This study has demonstrated that motivation and reward can affect certain brain responses in children with ADHD. It has made efforts to quantify these responses and to compare them to the responses seen with medication. However, the rewards given, i.e. extra points in the task, cannot easily be transferred to everyday situations nor can they be taken to imply that other forms of rewards given by parents or teachers would have similar results. Also, although the study measured the effects of the motivation and reward situation on the electrical impulses of the child’s brain, the actual feelings and behavioural inclinations of the child do not appear to have been monitored, either in the short or long term.

Given the nature of the study and the researchers’ own caution that their tasks were not designed “to replicate behavioural modification programs used in clinical practice”, the direct implications of these findings for the treatment of children with ADHD are unclear.

Analysis by Bazian
Edited by NHS Choices