[Simnibs-discuss] anodal vs. cathodal tDCS but same e-field patterns

[Axel Thielscher]

Dear Fabio,

when you switch from anodal to cathodal stimulation with otherwise 
identical settings, you only change the direction of the currents. 
Everything else stays the same. For example, the electric field strength 
will not be affected by that.

To see the changes, you can look at the "E" instead of "normE" (the 
former are the field vectors; the latter is the field strength, i.e. the 
length of the vectors). E will change the direction. Another way to look 
at the changes is to visualize the component of the field that flows in 
or out of the cortex. This component is usually termed "normal 
component" (https://simnibs.github.io/simnibs/build/html/faq.html).

You can get the normal component when you enable "Interpolate to 
cortical surface" in the simulation options 
(https://simnibs.github.io/simnibs/build/html/tutorial/gui.html). It 
then looks similar to Fig 11A in this paper: 
https://www.biorxiv.org/content/10.1101/500314v1

Best regards,

Axel

On 12/19/2020 1:29 PM, Fabio Masina wrote:
> Dear colleagues,
>
> I tried to perform two tDCS simulations (both setting 2mA) using the 
> default head model (i.e., Ernie). The electrode positions were:
>
> 1st simulation_anodal tDCS (target electrode on F7, return on FP2)
> 2nd simulation_cathodal tDCS (target electrode on F7, return on FP2)
>
> Basically, I only changed the current direction, by changing the 
> polarity of the target electrode accordingly to the kind of 
> stimulation (2mA for anodal tDCS and -2mA for cathodal tDCS).
> After launching these simulations separately, I realized that the 
> outputs were exactly the same. Actually, I would have expected to view 
> two oppositive outputs. I mean, I would have expected two opposite 
> e-field patterns. Maybe, am I missing something?
>
> Thanks for any suggestion.
>
> Best.
>
> Fabio
> -- 
> *Fabio Masina, PhD
> *
>
> IRCCS, Ospedale San Camillo
> Via Alberoni 70, 30126
> Venice, Italy
>
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> Simnibs-discuss at drcmr.dk
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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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