Frequency-dependent, bi-directional plasticity in motor cortex of human adults

Objective: To determine whether the plastic changes induced in human motor cortex by afferent stimulation depend on stimulus frequency. Methods: Transcranial magnetic stimulation was used to examine changes in corticospinal excitability in 20 subjects before and after combined peripheral (motor point) and central stimulation. Peripheral stimuli were given as either low frequency (3 Hz) or high frequency (30 Hz) trains. Results: Low frequency stimulation induced prolonged depression of corticospinal excitability, while high frequency stimulation induced prolonged facilitation. These effects persisted for approximately 40–50 min after stimulation ceased. Conclusions: Corticospinal plasticity induced by dual peripheral and central stimulation is bi-directionally-modifiable in the adult human, with the direction of change being frequency-dependent. Significance: Therapies using peripheral stimulation to alter human motor cortex excitability could be tailored to exploit the differential effects of stimulus frequency on the direction of the excitability change.