tACS: transcranial or transcutaneous alternating current stimulation?
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1
Department of Neurosciences, KU Leuven, Belgium
Introduction
Transcranial alternating current stimulation (tACS) is a relatively novel brain stimulation method. Electrodes are placed on the head and a current is passed through them. The current passes through various tissues among which the skin until a relatively weak electric field is generated in the brain. The tacit assumption has been that this weak field entrains neurons and causes behavioural effects (this is the transcranial mechanism). Many studies have indeed shown behavioural effects of tACS. However, the field is controversial. Recently various studies have shown that the generated electric field in the brain may be too weak to underlie the behavioural effects. Thus there are conflicting results which give rise to the ‘tACS paradox’.
Aims
We set out to resolve this paradox by hypothesizing that current passing through the skin causes peripheral nerves to become active. The nerves then give rhythmic input to the brain and affect neurons in the cortex. Thus we proposed a transcutaneous mechanism underlying tACS effects.
Methods
In a series of experiments in rats, in healthy volunteers and in essential tremor patients we tested this hypothesis by separating the transcutaneous mechanism from the transcranial mechanism.
In anaesthetised rats we recorded single unit action potentials. At the same time we stimulated either directly on the skull (transcranial-only) or the skin of a limb (transcutaneous-only).
In healthy volunteers and ET patients we looked at the effect of tACS on tremor. First we applied standard tACS by passing current through scalp electrodes placed above the motor cortex. In a second experiment we blocked the transcutaneous mechanism by applying a topical anaesthetic to the stimulation area. In healthy volunteers, we also blocked the transcranial mechanism by stimulating on an arm, away from the head.
Results
In the rat neurophysiology transcranial-only stimulation entrained neurons at electric fields above 1V/m. This was as expected. However, transcutaneous-only stimulation also entrained neurons in a similar fashion.
In healthy volunteers and in ET patients tACS caused entrainment of tremor as expected. However, when the transcutaneous mechanism was blocked there was no more entrainment. When, in healthy volunteers, the transcranial mechanism was blocked tremor was entrained.
Conclusions
The rat neurophysiology shows that there are two possible mechanisms in tACS; namely transcranial and transcutaneous. The human experiments showed that, at least for the motor system, the transcutaneous mechanism is the only viable mechanism.
The role of the transcutaneous mechanism should be considered in tACS studies. Furthermore the rat neurophysiology shows that when the electric field in the brain is strong enough neurons can be entrained.
Acknowledgements
This work was supported by: KU Leuven Research Funding STG/14/024 and EGM-D2929-C24/17/091. EIT Health Innovation by Ideas, NEURO-WEAR Project. Boateng Asamoah is SB PhD fellow at FWO.
Keywords:
tACS (transcranial alternating current stimulation),
peripheral nerve stimulation,
transcutaneous mechanism,
transcranial mechanism,
Electric field strength
Conference:
13th National Congress of the Belgian Society for Neuroscience , Brussels, Belgium, 24 May - 24 May, 2019.
Presentation Type:
Poster presentation
Topic:
Behavioral/Systems Neuroscience
Citation:
Asamoah
B,
Khatoun
A and
McLaughlin
M
(2019). tACS: transcranial or transcutaneous alternating current stimulation?.
Front. Neurosci.
Conference Abstract:
13th National Congress of the Belgian Society for Neuroscience .
doi: 10.3389/conf.fnins.2019.96.00044
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Received:
24 Apr 2019;
Published Online:
27 Sep 2019.
*
Correspondence:
Prof. Myles McLaughlin, Department of Neurosciences, KU Leuven, Leuven, 3000, Belgium, myles.mclaughlin@kuleuven.be