The influence of short-interval intracortical facilitation when assessing developmental changes in short-interval intracortical inhibition

Objective: Measures of short-interval intracortical inhibition (SICI) can be contaminated by excitatory influences of short-interval intracortical facilitation (SICF), unless examined at individually-optimized interstimulus intervals (ISIs). We hypothesized that age-related differences in SICF would explain previously reported reduced SICI in children and adolescents compared with adults. Methods: Fifty-one participants, aged 8-29 years, underwent transcranial magnetic stimulation. SICF curves were constructed to determine the ISI at which SICF was minimal (i.e. the first trough). SICI curves were constructed at this individually-determined ISI with conditioning stimulus (S1) intensities of 60-110% of active motor threshold. Results: There was no effect of age on the ISI corresponding with the SICF trough. However, there was a main effect of age on the amplitude of the conditioned motor-evoked potential at the different ISIs, such that children aged 8-12 years demonstrated greater SICF than those aged 16-18 and 19-21 years. There was no effect of age on SICI, and no interaction between age group and S1 intensity. Conclusions: Compared with that in older adolescents and young adults, SICF is enhanced in children aged 8-12 years. Surprisingly, this enhanced SICF does not appear to reduce the degree of SICI that can be evoked at the first trough in this age group. Significance: This is the first report of enhanced SICF in young children. It remains possible that enhanced SICF may have confounded earlier reports of reduced SICI in children less than 8 years.