Low level of intracortical inhibition in children shown by transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) is an established neurophysiological tool to evaluate the integrity and maturation of the corticospinal tract. TMS was used in this study to compare intracortical inhibition (ICI) in children, adolescents, and adults. The paired-pulse technique of TMS with interstimulus intervals of 2 ms was used to determine the ratio of conditioned (cMEP) and unconditioned amplitudes (ucMEP) that measures ICI. In experiment 1 (Exp 1) stimulus intensity was adapted to motor threshold (50 healthy subjects; 24 male, 26 female, median age 13.5 years, range 6.3 - 34 years) and in experiment 2 (Exp 2) stimulus intensity was adapted to the ucMEP (200 - 400 microV). Children (quotient of cMEP and ucMEP: Exp. 1: 0.71 +/- 0.41, Exp. 2: 0.82 +/- 0.25) had significantly less ICI compared to adults (Exp. 1: 0.21 +/- 0.19, mean +/- STD, Exp. 2: 0.35 +/- 0.22, in both experiments p < 0.001). Recently, ICI has been linked to the regulating function of GABAergic cortical interneurons on practice-dependent neuronal plasticity. Therefore, the lower ICI in children points to maturation processes that may have implications for the greater capacity of practice-dependent neuronal plasticity in children.