[VBMEG-users:00146] Unit of cortical current

[Chang Cai]

Dear VBMEG users,



We have estimated cortical currents in a simple button press task from brain
activities measured by MEG and fMRI with VBEMG (ver 1.0-0.a.0).



The default parameter (bayes_parm.patch_norm = 0) was applied during the
currents estimation. According to the VBMEG manual (
<http://vbmeg.atr.jp/docs/manual/manual_e.html#toc44>
http://vbmeg.atr.jp/docs/manual/manual_e.html#toc44), the unit of cortical
currents should be the international unit, Ampere. 　



The amplitudes in some vertices in our experiment are about 20x10^12 Am
(20pAm).

We want to check whether these amplitudes are reasonable or not.



We noticed that in most papers using VBMEG (such as Yoshioka et al. 2008,
Neuroimage), the unit of cortical currents was labelled as current density
(pAm/mm^2).



We want to know whether there is any simple method that can convert the
current (Am) to current density (Am/mm^2), or vice versa.



Best Regards,

Chang Cai

CiNET



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[Yamashita Okito]

Dear　Dr. Cai

Here is my breif answer to your first question.

> The amplitudes in some vertices in our experiment are about 20x10^12 Am
> (20pAm).
>
> We want to check whether these amplitudes are reasonable or not.

I think 20pAm seems to be too smal.

According to the Hamalainen's excellent review paper (see pp.424,425 of
http://www.martinos.org/meg/pdfs/1993-Hamalainen-RMP.pdf),
the amplitude of current density measureable with MEG ranges 100~250
nA/mm^2.
If we assume the tickness of cortex to be 1mm, the dipole moment density
becomes 100~250 [nA/mm^2] * 10^-3 [m] = 100~250 pAm/mm^2.
If you further multiply patch area (say 10mm^2) to this desisty, we can
estimate a range of the dipole moment 1nAm~2.5nAm.

I hope this makes sense to you.

best regard
Okito Yamashita, ATR

[Chang Cai]

Dear Dr. Yamashita

Thank you very much for kind and detailed explanation and the excellent
review paper.
They are really very helpful.

The amplitude 20pAm was from only a very small number of vertex before
movement onset.
I checked the maximum amplitude of cortical currents before and after
movement onset from all the vertices in that subject. The amplitude is about
1.5nAm.
So, perhaps the amplitude 20pAm from a very small number of vertex is
acceptable.

Best regards,
Chang Cai