The Effects of Bilateral Priming on Motor Cortex Function in Healthy Adults.

Bilateral priming is a rehabilitation adjuvant that can improve upper limb motor recovery post-stroke. It uses a table-top device to couple the upper limbs together such that active flexion and extension of one wrist leads to passive movement of the opposite wrist in a mirror symmetric pattern. Bilateral priming increases corticomotor excitability (CME) in the primary motor cortex (M1) of the passively driven wrist, however the neurophysiological mechanisms underlying this increase remain unclear. This study explored these mechanisms by using transcranial magnetic stimulation over the right M1 and recording motor evoked potentials from the passively driven left extensor carpi radialis of healthy adults. Intracortical measures were recorded before and five and 35 minutes after a single 15-minute session of priming. 1 ms short-interval intracortical inhibition, long-interval intracortical inhibition, late cortical disinhibition (LCD) and intracortical facilitation were recorded with a posterior-anterior (PA) intracortical current, while CME and short-interval intracortical facilitation (SICF) were recorded with both PA and anterior-posterior (AP) currents. CME with PA stimulation was also recorded approximately one hour post-priming. PA CME was elevated 35 minutes post-priming and remained elevated approximately one hour post-priming. LCD decreased and AP SICF increased at both 5 and 35 minutes post-priming. However, these changes in LCD and AP SICF are unlikely to be the cause of the increased PA CME due to the differing timelines of their effects and AP and PA currents activating separate interneuron circuits. These results suggest bilateral priming does not increase CME through alterations of the intracortical circuits investigated here.