Motor system excitability was measured in 16 children with ADHD, 16 with chronic tic disorder or Tourette’s disorder (TD), 16 with comorbid ADHD and TD, and 16 healthy control children, in a study at the University of Gottingen, Germany. The technique of focal transcranial magnetic stimulation (TMS) was used in a single and paired-stimulus paradigm, to assess inhibitory mechanisms within the motor system of these 4 groups. Children with ADHD alone had a reduced intracortical inhibition compared to those without ADHD, confirming deficits in inhibitory cortical motor mechanisms in ADHD. Children with TD had a shorter cortical silent period on TMS than those without TD, correlating with a deficient motor inhibition in the sensorimotor circuit. Children with combined ADHD and TD had both a significantly reduced intracortical inhibition and a shortened cortical silent period, suggesting an additive deficit in motor system inhibition at the level of motor system excitability. 
COMMENT. Children with comorbid ADHD and TD have additive deficits in motor system inhibition, involving both cortical motor mechanisms and the sensorimotor circuit. This finding is thought to be in line with neuropsychological data suggesting most severe impairments of motor inhibition in ADHD combined with TD.
Motivational effects on inhibitory control in children with ADHD.
Lowered inhibitory control in children with ADHD is attributed to low motivational incentive in a study of 33 children using a stop-signal task, at the University of Essen, Germany . Under conditions of low incentives, children with ADHD were less able to inhibit reactions and had longer stop-signal reaction times. With high incentives, they performed as well as controls. From a practical standpoint, the implementation of motivational incentives in ADHD individuals is often a problem, however.