Driving human motor cortical oscillations leads to behaviorally relevant changes in local GABAa inhibition: A tACS-TMS study.

Nowak, Magdalena, Emily Hinson, Freek van Ede, Alek Pogosyan, Andrea Guerra, Andrew Quinn, Peter Brown, & Charlotte J. Stagg. (2017) Driving human motor cortical oscillations leads to behaviorally relevant changes in local GABAa inhibition: A tACS-TMS study. J Neurosci. 37(17): 4481-4492.
 
Summary: Nowak and colleagues conducted a study to investigate the effects of GABA-A receptor mediated inhibition on beta- and gamma-band tACS effects in the motor cortex. Using ppTMS before, during, and after 20 minutes of individualized beta and 75 Hz (gamma) tACS and a sham condition, the researchers identified a transient decrease in SICI (less inhibition) during the first 5 minutes of 75 Hz tACS, which shifted towards increased inhibition by the end of the stimulation period (15 min). The early transient reduction in SICI strongly correlated with motor learning tested during a serial reaction time task completed prior to the stimulation sessions. These findings suggest gamma-band tACS engages GABAergic systems in the motor cortex with duration dependent effects, and that the responsiveness of the GABAergic system is predictive of individual motor learning capabilities.  
 
Pros: 

• Individualized stimulation frequency protocols
• Task-driven localizer for target identification
• Identifies clear relationship between response to stimulation and behavioral capabilities.

 
Open Questions: 

• Can we leverage behavioral task information to identify patients who may be more responsive to different stimulation modalities?
• Why did individualized beta stimulation not induce an increase in MEP amplitude as shown in prior literature? Can stimulation amplitude or intersession variability help to explain these findings?

 
Contributed by: Christopher Walker, PhD