Investigators at University of Heidelberg and other centers in Germany employed the N100 component of the EEG response to transcranial magnetic stimulation (TMS) as a novel marker for direct assessment of cortical inhibition in children with ADHD. Amplitude and latency of the N100 component were compared at rest, during response preparation in a motor reaction time task and during movement execution. In 20 children with ADHD compared to 19 healthy controls (8-14 years), the amplitude of N100 at rest was significantly lower and its latency tended to be shorter. During movement, N100 amplitude decreased while motor evoked potential amplitudes showed facilitation. Children with ADHD showed a smaller N100 amplitude reduction during movement execution compared with control children. The N100 amplitude evoked by TMS decreased with increasing age in both groups. The findings suggest that top-down control of motor cortical inhibition is reduced in ADHD, and the deficit in cortical control is an intrinsic qualitative deficit and not due to delayed development. 
COMMENT. The authors conclude that reduced cognitive control of the motor system may contribute to diminished motor cortex inhibition and is responsible for the deficient TMS-evoked N100 amplitude reduction during motor response preparation and motor execution in ADHD. Effects of age on TMS-evoked N100 are not compatible with a delayed development of cortical inhibition but point to intrinsic inhibition deficits in the motor system of ADHD. The TMS-evoked EEG potentials are a promising new marker of cortical inhibition in children with ADHD.