EEG markers of reduced visual short-term memory capacity in adult attention deficit/hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) persists frequently into adulthood. The disease is associated with difficulties in many cognitive tasks, which are assumed to be caused by neurobiologically-based basal dysfunctions. A reduction in visual working memory storage capacity has recently been claimed a testable endophenotype of ADHD. This study aimed at identifying brain abnormalities underlying this deficit by combining parameter-based assessment with electrophysiology. We compared unmedicated adult ADHD patients and demographically matched, healthy controls. We found reduced storage capacity in the patient group and delineated neural correlates of the deficit by analyzing ERP amplitudes according to (1) differences between patients and controls and (2) individual’s performance level of storage capacity K: First, the contralateral delay activity (CDA) was higher for individuals with high compared to individuals with lower storage capacity. The component differed between patients and controls only in an early time window (eCDA), in which activity correlated with patients’ symptom ratings of hyperactivity/impulsivity. Second, a broadly distributed central positivity (CP) was higher in individuals with higher compared to lower storage capacity. A later section of the CP was further overall increased in the group of ADHD patients relative to controls. Together, the findings indicate that ADHD patients show disease-specific changes in brain mechanisms underlying visual storage capacity, characterized by deficient encoding and maintenance, and increased recruitment of control processes.