tACS motor system effects can be caused by transcutaneous stimulation of peripheral nerves.

Asamoah, B., Khatoun, A. & Mc Laughlin, M. tACS motor system effects can be caused by transcutaneous stimulation of peripheral nerves. Nature communications 10, 266, doi:10.1038/s41467-018-08183-w (2019).
 
Summary:
Asamoah et al. proposed tACS effects are caused by transcutaneous stimulation of peripheral nerves in the skin and not transcranial stimulation of cortical neurons. They conducted 4 experiments: Single neuron activity recording in the rat motor cortex. They found both transcranial and transcutaneous can entrain neuronal oscillations (~1Hz). (spike-stimPLV). Anesthetizing the scalp significantly decreases the effect of tACS on tremor in humans. When the cortex was not being directly stimulated (upper arm), tACS still causes significant tremor entrainment. Rhythmic stimulation of peripheral nerves in healthy volunteers entrains EEG beta activity.
 
Pro:

• Anesthetize the scalp as a control for peripheral nerve input
• Conducted experiments across species: from rodent to human
• Combined date across scope: from single unit to EEG

 
Open question:

• How would this finding generalize to other cortical areas?
• Could this exclude the effect of neuron entrainment?

 
Contributed by: Wei Huang