A new study by the University of Cambridge in the UK questions the widely held view that dysfunction in dopamine – a chemical that controls the brain’s reward and pleasure centers – is the main cause of ADHD, a condition characterized by inattentiveness, hyperactivity and impulsiveness.

Writing in the latest issue of the journal Brain, the researchers, from the University’s Medical Research Council (MRC)/Wellcome Trust Behavioural and Clinical Neuroscience Institute (BCNI), suggest instead that the main cause of attention deficit hyperactivity disorder (ADHD) is to be found in structural differences in the brain’s grey matter.

They hope their findings will significantly improve our understanding of the cause of ADHD and help improve future treatments.

Dopamine is a chemical naturally produced in the brain that is important for concentration or sustained attention, working memory and motivation. It helps carry signals between brain cells by attaching to dopamine cell receptors – special entry-points in cell membranes that can only be opened by that particular molecule.

Ritalin – a drug approved for the treatment of ADHD – works by raising levels of dopamine, causing more to bind to the cells and thus increase the communication between them.

In their landmark study, the researchers used a combination of positron emission tomography (PET) and magnetic resonance imaging (MRI) to measure grey matter and dopamine receptors and determine how the drug methylphenidate (Ritalin) affected dopamine in patients with ADHD and healthy people without the condition.

Both groups of participants were given either a dose of Ritalin or a placebo. The study was double-blinded, which means neither the participants nor the clinicians who administered the medication knew whether they were using Ritalin or the placebo.

Before and after taking their given dose, participants completed tasks that tested their ability to concentrate and pay attention over a period of time.