Children with comorbid attention-deficit-hyperactivity disorder and tic disorder: Evidence for additive inhibitory deficits within the motor system

For children with attention-deficit-hyperactivity disorder (ADHD) or tic di sorder (TD), we recently reported deficient inhibitory mechanisms within th e motor system by using transcranial magnetic stimulation. These deficits-s tated as reduced intracortical inhibition in ADHD and shortened cortical si lent period in TD-could be seen as neurophysiological correlates of motor h yperactivity and ties, respectively. To investigate neurophysiological aspe cts of comorbidity, we measured motor system excitability for the first tim e also in children with combined ADHD and TD. The findings of a reduced int racortical inhibition as well as a shortened cortical silent period in thes e comorbid children provide evidence for additive effects at the level of m otor system excitability.