Detecting cortical circuits resonant to high-frequency oscillations in the human primary motor cortex: A TMS-tACS study.

Guerra, Andrea, Federico Ranieri, Emma Falato, Gabriella Musumeci, Alessandro Di Santo, Francesco Asci, Giovanni Di Pino, Antonio Suppa, Alfredo Berardelli, & Vincenzo Di Lazzaro. 2020. Detecting cortical circuits resonant to high-frequency oscillations in the human primary motor cortex: A TMS-tACS study. Scientific Reports. 10: 7695.
 
Summary: The authors sought to address the question of whether the descending corticospinal volleys traditionally observed under stimulation conditions at 333 and 667 Hz shared generators or were functionally distinct. To that end, the authors applied 333 Hz and 667 Hz tACS and a sham condition and used TMS before, during, and 5 minutes after tACS to measure MEP amplitudes. TACS at 333 Hz increased the amplitude of MEPs during mild tonic motor contraction. These differences were only observable during voluntary muscle contraction (i.e., active motor threshold) suggesting 333 Hz tACS was able to enhance existing motor precepts, but not to generate them outright. 
 
Pros: 

• Multiple stimulation protocols to target distinct neural populations

 
Open Questions: 

• What does this mean for variability in TMS-EEG studies of the motor cortex? 
• Could the amplitude of tACS play a factor in the observed effects? What if the amplitude of 667 Hz tACS was increased?

 
Contributed by: Christopher Walker, PhD