Questioning inhibitory control as the specific deficit of ADHD - evidence from brain electrical activity

Objective: To investigate motor response control during a cued continuous performance test (CPT-A-X) by performance and ERP parameters in children with hyperkinetic disorder (HD), hyperkinetic conduct disorder (HCD) or oppositional deviant/conduct disorder (ODD/CD), to examine the evidence for an inhibition-specific deficit as indicated by these parameters, and to analyze whether possible deviations are specific for HD/HCD. Method: Behavioral parameters and event-related potentials (ERPs) were recorded during a CPT-A-X-task in children (aged 8 to 14 years) with either HD (n=15), HCD (n=16), or ODD/CD (n=15) and normal children (n=18) and analysed. ICD-10 diagnoses of HD/HCD diagnoses in all children were fully concordant with the DSM-IV diagnosis of ADHD-combined type. Results: Children with HCD committed more dyscontrol errors and differed most from normal children on ERP measures of motor response control, while children with HD-only were more impaired during processing of the warning stimuli for motor preparation. ERP measures specific for response inhibition were not different between the groups. Conclusions: The results show that ADHD cannot be fully explained by an inhibition-specific deficit and implicate impaired response execution processes as well. This indicates that comorbid children suffer from a reduced ability to control their prepared motor responses. Further, they seem to have difficulties in timely switching attention from monitoring the sensory input stream to the monitoring of own responses and actions.