Paired associative transcranial alternating current stimulation increases the excitability of corticospinal projections in humans.

McNickle, Emmet, & Richard G. Carson. 2015. Paired associative transcranial alternating current stimulation increases the excitability of corticospinal projections in humans. J Physiol. 593.7: 1649-66.
 
Summary: These authors induced spike-timing dependent plasticity (STDP)-like effects with a modified paired associative stimulation (PAS) protocol that uses tACS instead of TMS. A wide range of frequencies (80, 140, & 250 Hz) and stimulation amplitudes (1, 2, & 3, mA) were found to induce increases in corticospinal excitability measured with TMS-induced MEPs over a 30 minute period. TACS presented at 80 and 140 Hz generated the greatest increases (140 Hz evaluated as low as 1 mA). The paired effect is most robust when the peripheral afferent stimulation is long in duration (~1 sec) and this effect was observed even when applying vibrational (non-electric) stimulation to the peripheral nerve. This novel paradigm introduced new avenues for plasticity induction in the motor system that do not rely on precise timing of classic STDP protocols or more expensive equipment.  
 
Pros:

• Wide parameter space
• Active controls
• Multiple experimental stages
• High overall sample size

 
Open Questions; 

• Can we leverage these protocols to facilitate stroke rehabilitation with lower seizure risk?
• Can oscillatory mechanisms explain plasticity induction? 

 
Contributed by: Christopher Walker, PhD