[FieldTrip] problem with leadfield

[philip Joadavi]

Dear Jan-Mathijs,

Thanks for your quick reply.
so if I sum over the channels on the selected dipole, I get these results :
so
for leadfield computed with 64 channels:

1.0e-14 *
   -0.1818   -0.6828    0.5815

for leadfield computed with 32 channels:

1.0e-13
    0.0046   -0.1538    0.0872

and for the subset of 32 extracted from 64 channels leadfield :

-0.3738   -0.4471   -0.2351

is that what you are referring to?

Best,
Philip

On Sun, Apr 11, 2021 at 6:59 PM Schoffelen, J.M. (Jan Mathijs) <
jan.schoffelen at donders.ru.nl> wrote:

> Hi Philip,
>
> If I understand you well, the 32 channel array is a subset of the 64
> channel array?
>
> Perhaps it may be useful to you know that FieldTrip computes EEG
> leadfields as if the channel array is average referenced. This means that -
> for a ‘leadfield’ topography for a given dipole - the average along the
> rows is 0, by construction. If you construct a 64-channel leadfield, which
> will have an average of 0 along its rows, a selection of 32 rows (i.e. your
> 32 channel subset) will not by default have a zero mean across the rows.
> However, had you pre-selected those 32 channels and computed the leadfield,
> the average would be 0.
>
> Could this explain the difference you describe?
>
> Best wishes,
> Jan-Mathijs
>
>
> On 11 Apr 2021, at 16:14, philip Joadavi <p.joadavi at gmail.com> wrote:
>
> Dear all,
>
> I'm trying to compute the leadfield for my EEG data.
> I've done all the steps in the tutorial, here
> <https://www.fieldtriptoolbox.org/workshop/oslo2019/forward_modeling/>.
> I run the *bemcp*, for computing the headmodel. Since in my data I have 2
> electrode sets (64 channels and 32 channels), I tried to compute the
> leadfield for 64 and then extract the leadfield from that file for 32
> channels. However, when I compute the leadfield for 32 channels
> *separately*, the results are different from when I *extract *the 32
> channels from the 64 channel leadfield.
>
> I have checked everything, the patterns that I get are exactly the same,
> but the values from the leadfield computed separately from 32 channels are
> higher when I plot the potentials (using the simulation in the tutorial).
> Can someone explain why is this the case and where it happens in the code
> for computing the leadfield? why the number of channels is important for
> leadfield computation?
>
> Unfortunately, I can not run the 'simbio' to compare.
>
> Thank you very much!
> Best,
> Philip
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