State-dependent effects of transcranial oscillatory currents on the motor system: What you think matters.

Fuerra, Matteo, Patrizio Pasqualetti, Giovanni Bianco, Emiliano Santarnecchi, Alessandro Rossi, & Simone Rossi. 2013. State-dependent effects of transcranial oscillatory currents on the motor system: What you think matters. J Neurosci. 33(44): 17483-9.
 
Summary: Tested MEPs under conditions of rest and motor imagery. The researchers applied tACS at 5, 10, 20, & 40 Hz to the left motor cortex, and to the parietal cortex as an active control. TACS delivered at 5 and 20 Hz demonstrated a double dissociation in effect based on motor imagery. 20 Hz tACS increased MEP amplitudes greater at rest, whereas 5 Hz tACS increased MEP amplitudes most during motor imagery. 
 
Pros: 

• Active control
• Frequency specificity of effects

 
Open Questions:

• TACS can bias field potentials, but it cannot induce action potentials, so what is the origin of the 5 Hz signal modulated by tACS? 

 
Contributed by: Christopher Walker, PhD