# [FieldTrip] Unit of simulated EEG using openMEEG

HINDRIKS, RIKKERT rikkert.hindriks at upf.edu
Wed Apr 8 11:20:46 CEST 2015

```Hi Alex,

I see, thanks. I have specified the meshes in mm's so a unit dipole
moment is expressed in Amm right?
Thus a homogeneous dipole field with moment x Amm leads to scalp potentials
of 10^(-2)*L*x (where L
is the computed leadfield matrix).

The thing is that if I place a single dipole with moment Q = 10 nAm =
10^(-6) Amm (which is about the
required value to pick up a measurable MEG signal), this gives scalp
voltages LQ in the order of picoV
(at most something of the order 10^(-11) for appropriately placed dipoles)
while you would expect it to
be in the order of microV no?

Could it be that in the specification of the headmodel, I should use S/mm
instead of S/m for the conductivities?
(I use the "standard" values of 0.33 S/m , 0.0041 S/m, and 0.33 S/m for
brain, skull, and scalp, resp.)

Kind regards and thanks,
Rikkert

On Wed, Apr 8, 2015 at 10:08 AM, Alexandre Gramfort <
alexandre.gramfort at telecom-paristech.fr> wrote:

> hi,
>
> if your meshes are in meters then moments in Am produce volts.
>
> hope this helps
> Alex
>
> On Wed, Apr 8, 2015 at 9:32 AM, HINDRIKS, RIKKERT
> <rikkert.hindriks at upf.edu> wrote:
> >
> > Dear all,
> >
> > Could anyone tell me what are the units of the entries in BEM leadfield
> > matrices generated using openMEEG?
> >
> > For example, if I multiply the matrix with a vector of dipole moments in
> Am,
> > do I get microVolts? (after taking the
> > average reference)
> >
> > Thanks a lot,
> > Rikkert
> >
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