To what extent are task-switching deficits in children with attention-deficit/hyperactivity disorder independent of impaired inhibition?

Executive functions, higher-order cognitive functions needed for goal-directed behavior, have been studied extensively in the search for endophenotypes for ADHD, yet results have been inconclusive. We examine the performance of children with ADHD in task switching as an as yet understudied potential endophenotype. A group of 20 children with ADHD and a group of 23 children without ADHD (ages 7-12) performed a task-switching paradigm and a Go/No-Go Task. Children with ADHD displayed significantly greater specific switch costs, that is, compared to control children they were especially impaired directly after task switches. There were no group differences with respect to the general switch costs, which are estimated by comparing performance on single task blocks to the block where both tasks are intermixed. Specific switch costs and Go/No-Go error rate were significantly correlated; yet, group differences in the task-switching paradigm remained significant even when inhibition was controlled for. This pattern of results suggests that children with ADHD are neither generally impaired in executive function nor only impaired with respect to inhibition. Instead, they display a highly specific deficit with regard to the flexible suppression and amplification of different task rules according to the context. Our conclusion that task switching has the potential to be added to the list of ADHD endophenotypes is strengthened by the independence of task-switching deficits and inhibition.