Transcranial alternating current stimulation entrains single-neuron activity in the primate brain.

Krause, M. R., Vieira, P. G., Csorba, B. A., Pilly, P. K. & Pack, C. C. Transcranial alternating current stimulation entrains single-neuron activity in the primate brain. Proceedings of the National Academy of Sciences 116, 5747-5755, doi:10.1073/pnas.1815958116 (2019).
 
Summary:

• Krause et al. applied tACS (4mA peak to peak) in two macaque monkeys and measured the effect on single-unit neural entrainment in the hippocampus and basal ganglia (HC/BG) via depth electrodes. tACS consistently influences the timing, but not the rate, of spiking activity. Delivered 2-mA tACS (4 mA peak-to-peak) through 2 scalp electrodes and measured average electric field strengths in the hippocampus and basal ganglia of 0.28 V/m in one monkey and 0.35 V/m in another monkey.

 
Pros:

• Built Individualized head models from high-resolution MRI and intraoperative records of implant locations
• Frequency (1-100Hz, ±1Hz bin) and location-specific tACS effect (increase PLV in HC/BG but not in TEO)
• Sham condition (ramp up and down) controls for nonspecific sensory effects of stimulation
• Another control where they mirrored the tACS montage and found reduced neural entrainment. – entrainment is unlikely to be caused by peripheral or retinal stimulation

Open question:

• 2-mA tACS in a monkey will generate fields that are on average 3 times stronger than 2-mA tACS in humans. Would a weaker electric field have similar effect? – Their follow-up article: the entrainment persisted at weaker amplitudes at 1mA and 0.5mA.
• Is this effect possibly be induced by neuronal entrainment in cortex, and indirectly entrain deep structures (hippocampus)?

 
Contributed by: Wei Huang