Efficacy of tRNS and 140 Hz tACS on motor cortex excitability seemingly dependent on sensitivity to sham stimulation.

Kortuem, Victoria, Navah Ester Kadish, Michael Siniatchkin, & Vera Moliadze. 2019. Efficacy of tRNS and 140 Hz tACS on motor cortex excitability seemingly dependent on sensitivity to sham stimulation. Exp Brain Res. 237: 2885-95.
 
Summary: Kortuem and colleagues addressed the question of how individual difference in responsiveness to sham electrical stimulation predicted the efficacy of actual electrical stimulation. TMS-induced MEPs were collected before and after 10 minutes of 140 Hz tACS, tRNS, and an active sham stimulation condition. Participants who showed a significant increase or decrease in their MEP amplitudes after receiving sham stimulation did not respond to either 140 Hz tACS or tRNS of the motor cortex, but those who did not respond to sham demonstrated a significant positive modulation to both stimulation modalities. 
 
Pros: 

• Multiple stimulation techniques demonstrates that the effect is more generalized to the sham responsiveness and not a single stimulation protocol.
• Within-subjects, counterbalanced design
• Both subjects and the investigator collecting MEP data were blinded to condition.

 
Open Questions: 

• Does the motor cortex responsiveness to sham translate to a lack of sensitivity to stimulation in other brain regions as well?
• How might we operationalize "response to sham" in a way that is generalizable to treatment protocols? 

 
Contributed by: Christopher Walker, PhD