Motor learning enhanced by combined motor imagery and noninvasive brain stimulation is associated with reduced short-interval intracortical inhibition

Background Motor imagery (MI) improves motor skill learning, which is further enhanced when MI is paired with primary motor cortex transcranial brain stimulation or with electrical stimulation of the peripheral median nerve. Applying both stimulation types (here with 25 ms intervals) is called paired associative stimulation (PAS25). The final primary motor cortex output is determined by combined excitatory and intracortical inhibitory circuits, and reducing the latter is associated with enhanced synaptic transmission and efficacy. Indeed, short‐interval intracortical inhibition (SICI) inhibits motor evoked potentials (MEPs), and motor learning has been associated with decreased SICI and increased cortical excitability. Here, we investigated whether cortical excitability and SICI are altered by PAS25 applied after MI‐induced modulation of motor learning. Methods Peak acceleration of a hand‐grasping movement and MEPs and SICI were measured before and after MI alone, PAS25 alone, and MI followed by PAS25 in 16 healthy participants to evaluate changes in motor learning, corticospinal excitability, and intracortical inhibition. Results After PAS25 alone, MEP amplitude increased while peak acceleration was unchanged. However, PAS25 applied following MI not only significantly enhanced both peak acceleration (p = 0.011) and MEP amplitude (p = 0.004) but also decreased SICI (p = 0.011). Moreover, we found that this decrease in SICI was significantly correlated with both the peak acceleration (r = 0.49, p = 0.029) and the MEP amplitude (r = 0.56, p = 0.013). Conclusions These results indicate that brain function altered by PAS25 of the motor cortex enhances MI‐induced motor learning and corticospinal excitability and decreases SICI, suggesting that SICI underlies, at least in part, PAS25 modulation of motor learning.