Inverse Correlation Between Resting Motor Threshold and Corticomotor Excitability After Static Magnetic Stimulation of Human Motor Cortex

BackgroundHigh-strength static magnetic field stimulation (SMS) results in a period of reduced corticomotor excitability that may be mediated through a decrease in membrane excitability.ObjectiveAs resting motor threshold (RMT) is thought to reflect membrane excitability, we hypothesized that SMS may increase RMT and that there would be an inverse relationship between RMT and motor-evoked potential (MEP) amplitude.MethodsTen healthy subjects (aged 20–29; 4 females) participated in a double-blinded crossover design comparing MEP amplitude and RMT before and after a 15-min period of SMS or sham stimulation over primary motor cortex (M1).ResultsMEP amplitude was initially significantly reduced post-SMS (∼20%), and returned to baseline by 6 min post-intervention. MEP amplitude and RMT were inversely correlated (r2 = 0.924; P = 0.001). Sham stimulation had no effect on MEP amplitude (P = 0.969) or RMT (P = 0.549).ConclusionAfter SMS, corticomotor excitability is transiently reduced in association with a correlated modulation of RMT. SMS after effects may be mediated in part by a reduction in membrane excitability, suggesting a possible role for non-synaptic (intrinsic) plasticity mechanisms.