[FieldTrip] Unit of simulated EEG using openMEEG

[HINDRIKS, RIKKERT]

Hi Alex,

Ok, clear, thanks.

But in any case, when using a typical dipole moment of 10 nAm, you would
like to end up with voltages in the experimental range, that is, in the
order
of microVolts (after taking the average reference)

I would appreciate it very much if someone could elucidate this for me.


Kind regards,
Rikkert

On Wed, Apr 8, 2015 at 11:29 AM, Alexandre Gramfort <
alexandre.gramfort at telecom-paristech.fr> wrote:

> hi Rikkert,
>
> the unit of conductivities have no impact as it's only their ratio
> that is used in computation.
>
> For the rest run simple simulations to get more intuition.
>
> HTH
> Alex
>
>
> On Wed, Apr 8, 2015 at 11:20 AM, HINDRIKS, RIKKERT
> <rikkert.hindriks at upf.edu> wrote:
> > 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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