[Simnibs-discuss] [tACS] multi electrodes montage

[Axel Thielscher]

Dear Laurent:

> So the question is : if I am happy with the result of a 2 + 1 montage 
> by using CP3 and P5 (at -1 mA) and P1 (at +2mA) with PI electrodes in 
> a tDCS perspective...how to translate this in terms of tACS parameters ?

Opposite signs in TDCS translate to a 180° phase shift in TACS. So here 
you will have currents with the opposite phase in P1 versus CP3 and P5. 
I suggest that you look at the normal component of E on the cortex (see 
also https://simnibs.github.io/simnibs/build/html/faq.html for an 
explanation fo the normal component): All positions that have opposite 
signs have a 180° phase shift to each other. You can tell Simnibs to 
calculate the normal component in the simulation options.

For more details, please see also 
https://www.biorxiv.org/content/10.1101/500314v1 and 
https://pubmed.ncbi.nlm.nih.gov/28919407/

Best regards,

Axel


On 9/17/2020 3:36 PM, LAURENT TORLAY wrote:
> Hi,
> Thank you fo developping SimNibs software !
> I have a newbie question about tACS (I already did tDCS simulations 
> and experiments).
> First, I use Neuroelectrics (NE) device and, with their software, I 
> have the possibility to have several "input" electrodes with different 
> phases and only one return electrode if needed.
> In Simnibs, the GUI is designed fo tDCS but I understood that the 
> results can be extrapolated to tACS for low frequencies (that's our 
> case : around  10 Hz).
> So the question is : if I am happy with the result of a 2 + 1 montage 
> by using CP3 and P5 (at -1 mA) and P1 (at +2mA) with PI electrodes in 
> a tDCS perspective...how to translate this in terms of tACS parameters ?
> For example, can I manage this with phase difference but with same 
> amplitude or half ? What is exactly the role of return electrodes in 
> tACS ?
> Thanks in advance,
> Laurent
>
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-- 
Dr. Axel Thielscher
Associate Professor
Danish Research Center for Magnetic Resonance
Copenhagen University Hospital Hvidovre
DK-2650 Hvidovre, Denmark
www.drcmr.dk
&
Department of Health Technology
Technical University of Denmark
DK-2800 Kgs. Lyngby
http://www.healthtech.dtu.dk/

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