dipole moment

[Robert Oostenveld]

Hi Nathan,

On 7 Sep 2006, at 23:19, Nathan Weisz wrote:
> my intention is to steal some code of the dipolefitting function to
> model source activity of raw EEG data (keep fixed locations, just
> analyse changes in dipole moment; i have a leadfield matrix using
> prepare_leadfield). to test things i used a visual ERP data set as
> outputted by timelockanalysis.

Probably you could use dipolefitting with cfg.nonlinear='no' and
cfg.gridsearch='no', then the dipole(s) stays where you initially put
it(them), and only the moment is estimated. But your approach is also
fine.

> the waveforms look fine, i just get really huge values: ~14000 as
> maximum dipole moments (the N1 is about 6000). so my question is
> what units i have? do i have to scale by 1000 to get nAm (14 & 6
> nAm would make sense) or do these values imply that something has
> gone completely wrong.

There are different physical entities with different units involved,
you have to ensure that they are all consistent: electrode positions
and volume conductor geometry represent distance (e.g. meter), tissue
resistance is in ohm per distance (ohm/m -> conductance is the other
way around), potentials are in volt (or often in uV), currents are in
A (or mA, uA, nA). If you mix them up, it is easy to get a factor of
10^n difference. E.g. I presume that your N1 is not "6000" Volts (if
so, then I would rather stay away from your subject).

Robert