[FieldTrip] Unit of simulated EEG using openMEEG

[Alexandre Gramfort]

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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