Motor imagery enhances corticospinal transmission mediated by cervical premotoneurons in humans.

Motor imagery is known to affect the reacquisition of motor functioning after damage to the central nervous system. However, it remains unclear whether motor imagery influences corticospinal (CST) excitation mediated via cervical premotoneurons, which may be important for functional motor recovery in animals and humans. To investigate this, we examined the spatial facilitation of motor-evoked potentials (MEPs) induced by combined stimulation (CS) of CST and peripheral nerves. Thirty-two healthy volunteers were included and electromyograms from the biceps brachii (BB) were recorded. Transcranial magnetic stimulation (TMS) to motor cortex and electrical stimulation of ulnar nerve at wrist (NERVE) were delivered separately or in combination with 6-15 ms of interstimulus intervals (ISIs). Subjects were instructed to imagine performing an elbow flexion at rest and during tonic BB contraction. During both motor imagery and control tasks, CS (7.5-12 ms of ISIs) facilitated MEPs, compared with the mathematical summation of responses obtained with either only TMS or NERVE (