Phase of beta-frequency tACS over primary motor cortex modulates corticospinal excitability.

Schilberg, Lukas, Tahnée Engelen , Sanne ten Oever, Teresa Schuhmann, Beatrice de Gelder, Tom A. de Graaf, & Alexander T. Sack. 2018. Phase of beta-frequency tACS over primary motor cortex modulates corticospinal excitability. Cortex, 103: 142-152.
 
Summary: The authors sought to address the phase specificity of alpha and beta frequency tACS on motor cortex excitability. Using a classic montage (M1-Pz), the authors used resting eyes closed alpha and beta recorded during finger tapping to define their frequencies of interest. TMS pulses were applied during and after tACS to measure the impact of both online tACS effects and aftereffects on the TMS-induced MEP. MEPs did not change after stimulation, but beta phase significantly modulated MEP amplitudes online with the strongest effect in subjects with lower individualized beta frequencies.  
 
Pros: 

• Individualized alpha (eyes closed) and beta (finger tapping) targeting, though a non-active sham
• Phase-specificity of effects. Demonstrates a combination of transient excitatory and inhibitory influences. 

 
Open Questions: 

• The authors used eyes closed alpha to define their individual alpha frequency, but something closer to a mu rhythm generated by the motor cortex may be a more appropriate comparison. 
• Why is a lower individualized beta more affected by tACS? 
• How could we design a better active sham for this?

 
Contributed by: Christopher Walker, PhD