Absence of transcallosal inhibition following focal magnetic stimulation in preschool children

Focal transcranial magnetic stimulation of the hand-associated motor cortex was used to study normal healthy preschool children (n = 7; mean age, 4.6 years) and adults (n = 7; mean age, 29.4 years) under the conditions of standardized tonic voluntary contraction of small hand muscles. Callosally mediated inhibitory as well as corticospinally mediated inhibitory and excitatory motor effects were investigated. Although children had no detectable transcallosal inhibition, their corticospinally mediated postexcitatory silent period was present (mean, 140.8 +/- 30.2 msec). It was significantly shorter than in adults (mean, 192.5 +/- 32.0 msec). The motor thresholds of the cortically elicited muscle responses, measured as the lowest stimulus intensity, were significantly higher in children (mean, 89 +/- 5%) than in adults (mean, 46 +/- 6%). The corticomuscular latency of transcranially elicited motor responses revealed no difference between children and adults. These observations may reflect maturation processes in the motor system. Maturation of at least some direct corticospinal fibers occurs early in life and is followed by that of intracortical excitatory and inhibitory connections. The maturation of functionally competent callosal connections appears to occur after the age of 5 years.