[Simnibs-discuss] Scalp to cortex distance and eField thresholding

Guilherme B Saturnino guilhermebs at drcmr.dk
Mon Jul 27 10:33:04 CEST 2020


1) As of now you would need to write a small python or matlab script 
which loads the mesh file and calculates the distances from your coil to 
the cortex. But I've implemented this calculation as a part of the 
standard pipeline for the next version. Beginning in version 3.2 this 
value will be written out in the log.

2) I'm by no means an electrophysiology specialist (my background is 
physics and computer science) but the time dynamics of tDCS and TMS are 
very different. While tDCS uses a direct current applied over a long 
time (typically minutes), TMS happens at a quite short time scale 
(typically microseconds) and have strong variations in time, even 
monophasic pulses. So I guess that, while it is valid to speculate if 
small fields can have an effect in TMS, direct comparison with tDCS 
using only field strength is not very appropriate.

Best Regards,


On 24/07/2020 18.50, Mccalley, Daniel wrote:
> Hi all,
> I have two questions about some eField models that I've created:
> 1) Is there a simple way to extract the measured scalp-to-cortex 
> distance between the defined TMS position and the cortex? I imagine 
> SIMNIBs must calculate this in creating the eField. I would be very 
> interested in extracting that information for other analyses.
> 2) In interpolating my electric field models to .nii files, I'm 
> attempting to discern how best to threshold the eFields such that I 
> might create a Region of Interest of mask based on the precise site of 
> stimulation. I know that there is much debate surrounding 1 V/m as a 
> critical value in tDCS modeling. In TMS, I've noticed that even in my 
> modeling of stimulation intensities that are particularly low (40% 
> MSO, 58.4 dI/dT on Magventure), the resulting eField has a rough value 
> of about 45 V/m at the site of stimulation (crosshairs at the first 
> image), but even areas near the occipital cortex have a rough value of 
> 3-4 V/m (crosshairs at second image). While proportionally this V/m is 
> much smaller than the actual site of stimulation, I could imagine one 
> might argue that a value of 3-4V/m may influence cortical neurons.
> Is this a typical result or is it possible I've made some critical 
> mistake in my modeling?
> Thank you so much for your help!
> Best,
> Dan McCalley
> Medical University of South Carolina
> College of Graduate Studies, Neuroscience
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