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

• This cross-sectional study examined the developmental trajectory of SICF.
• We report that children aged 8–12 years exhibit enhanced SICF.
• Reduced SICI in children and adolescents is unlikely due to SICF.

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.