tACS – transcranial or transcutaneous alternating current stimulation?
Ahmad Khatoun
Research Seminar
When?
Wednesday, 16 May 2018, 15:30-16:30
Where?
Seminar room HP2
O&N 2 - KU Leuven
Herestraat 49
B-3000 Leuven
Abstract
Transcranial alternating current stimulation or tACS is a neuromodulation method in which electrodes are placed on the scalp and used to deliver sinewave electrical stimulation. The electric field passes through the scalp, skull and CSF before a weak current reaches the brain. It is assumed that the weak alternating current in the cortex directly modulates the membrane potential causing neural entrainment. tACS has been shown to modulate speech, motor, cognitive and memory function. However, these findings are highly controversial, as recent studies have shown that the current reaching the cortex in humans is not be strong enough to directly entrain neural activity. We propose a new hypothesis that can reconcile these two sets of opposing results: the ‘t’ in tACS represents transcutaneous stimulation of peripheral nerves in the skin and not transcranial stimulation of cortical neurons. The induced rhythmic activity in peripheral nerves then entrains cortical neurons through an indirect route. To test this hypothesis we first recorded from neurons in the rat motor cortex using an experimental design that isolates the putative transcranial and transcutaneous mechanisms. Our results established that transcutaneous stimulation does entrain cortical neurons and that this is similar to entrainment caused by transcranial stimulation. Then, in healthy volunteers, we used physiological tremor as an outcome measure to test which mechanisms is dominant. In one experiment, we blocked the transcutaneous mechanism by applying topical scalp anesthesia and found that tACS effects were significantly decreased. In another experiment, we isolated the transcutaneous mechanism by placing tACS electrodes on the contralateral arm and still found tremor entrainment. We conclude that tACS effects on the motor system are mostly caused by transcutaneous, and not transcranial, stimulation. The role of a transcutaneous mechanism in mediating tACS effects on other systems should be considered.
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