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

[Mccalley, Daniel]

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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