Phase and frequency-dependent effects of transcranial alternating current stimulation on motor cortical excitability.

Nakazono, Hisato, Katsuya Ogata, Tsuyoshi Kuroda, & Shozo Tobimatsu. 2016. Phase and frequency-dependent effects of transcranial alternating current stimulation on motor cortical excitability. PLoS ONE. 11(9): e0162521.
 
Summary: Nakazona and colleagues (2016) sought to evaluate the phase dependency of oscillatory brain stimulation effects in the human brain. In a series of experiments, the researchers measured TMS-evoked MEPs at multiple phases of 10 and 20 Hz TACS, then used the preferred phase identified to test the same effects at multiple frequencies (5, 10, 20, & 40 Hz). At 90 degrees tACS phase, 10 Hz and 20 Hz tACS demonstrated differentiated effects on MEP amplitudes whereby 10 Hz tACS attenuated MEPs and 20 Hz tACS facilitated MEPs. In a follow-up experiment, TMS delivered at the 90 degree phase of ongoing 5, 10, 20, and 40 Hz tACS demonstrated the frequency specificity of these effects by replicating the 10/20 Hz effects while showing no modulation by 5 or 40 Hz stimulation. The facilitation by 20 Hz was found to remain significant even compared to sham, but not the 10 Hz attenuation. 
 
Pros:

• Multiple experimental stages, optimized follow-up experiments
• Online stimulation probe of excitability
• Phase specific stimulation

 
Open Questions: 

• Would tACS applied at individualized mu or beta rhythms enhance the phase specific effects?
• Do these phase-excitability relationships change during longer stimulation protocols (i.e., > 5 min)?

 
Contributed by: Christopher Walker, PhD