Within-subject variability during spatial working memory in children with ADHD: an event-related potentials study.

Working memory (WM) dysfunction and increased within-subject variability are known issues in attention deficit/hyperactivity disorder (ADHD) patients. Little is known about the electrophysiological characteristics of this variability. We evaluated behavioral and electrophysiological within-subject variability taking developmental aspects into account in a group of ADHD patients. Multichannel (n = 31) event-related potentials (ERP) were measured during a visuo-spatial backmatching task; 44 children (8-16 years old) were tested: 22 children with ADHD, combined (n = 17) and inattentive (n = 5) type, and 22 age- and intelligence-matched control children. One-backmatching (BM1) and two-backmatching (BM2) tasks were performed. Classical behavioral parameters and target and nontarget ERP were compared between groups. In addition, motor response variability and ERP amplitude variability were studied. Age-related changes in both motor response and ERP amplitude variability were analyzed in each group. Attention deficit/hyperactivity disorder children made more commission errors, which was more pronounced in the difficult (BM2) task. No difference between groups was found in ERP amplitude and in motor response variability. However, ADHD patients had higher ERP amplitude variability, which was again more pronounced in the difficult WM task. A delayed maturation of amplitude variability was seen in ADHD patients with a slower than in controls decrease in variability with age. This amplitude variability was correlated with the number of commissions, but in an opposite way for ADHD and control children. Our findings indicate an impaired visuo-spatial WM processing in ADHD children with greater ERP amplitude variability compared to controls. Our results also support the view of a delayed cortical development of visuo-spatial WM circuits in this disorder.