In children with symptoms of attention deficit hyperactivity disorder (ADHD), the brain responds differently to unexpected sounds than in peers. The nature of this response changes with age. The study was conducted by specialists from Boston Children's Hospital and Harvard Medical School, and the results were published in the journal Developmental Cognitive Neuroscience.
The brain constantly monitors sounds in order to notice new ones. Normally, sensory systems learn to distinguish signals through repetition. According to predictive coding theory, the brain builds a model of expected sounds based on recent signals. When a new sound violates this sequence, a rapid electrical response occurs - mismatch negativity. It appears within fractions of a second without conscious effort. If the sound is completely unusual, for example a drumbeat or the chirping of a cricket, the brain generates a later signal associated with involuntary attention - the novelty response.
Another important process is habituation: when a sound is repeated, the brain's electrical response weakens. This helps avoid distraction by background noise and conserve resources. Without habituation, a person may feel constant overload.
Researchers Maria Pranjic, Jirapat Likitlersuang and Anne B. Arnett studied these processes in children with ADHD. The experiment involved two age groups. The younger group had 115 children aged two and a half to five years. In 51 children development was typical, while the rest had pronounced ADHD symptoms. A diagnosis is rarely made at this age, so the group was determined by clinical assessments. The children wore a cap with safe sensors to record an electroencephalogram (EEG) - a method of recording the brain's electrical activity. They watched a silent video and listened to a sequence of sounds: mostly a repeating tone, sometimes slightly altered, as well as unique noises. No active response was required.
It turned out that in young children with ADHD symptoms the amplitude of mismatch negativity was greater than in children with typical development. The difference persisted after adjustment for age and mental abilities. In addition, their habituation worked less well: the brain did not reduce its response to repeated sounds as quickly.
Then 71 children aged seven to eleven years underwent a similar test. Of these, 53 had a confirmed ADHD diagnosis, and 18 children were developing typically. Here the picture was the opposite: in children with ADHD, mismatch negativity was weaker than in peers. No differences in habituation were found between the groups.
The authors suggest that sensory information processing in ADHD changes as children grow: in early childhood the brain may overreact to deviations, while in middle childhood it may underreact. However, the study compared two different groups of children at a single point in time rather than following the same children. Therefore it cannot be stated exactly how the response changes in a particular child. The results also do not prove that unusual sound processing causes ADHD: only a statistical association is shown, which by itself does not indicate causality. Further long-term observations will help answer these questions.