Problem with data from BESA

Robert Oostenveld r.oostenveld at FCDONDERS.RU.NL
Wed Nov 16 14:16:46 CET 2005

Hi Micahel

> I see in BESA (more like something differentiated / inverted from
> the BESA maps - the foci are clearly shifted).

The projection of the 3D electrode locations towards the 2D plane (in
which the color-coded data has to be represented on screen or paper)
might be quite different. Fieldtrip uses layout files in which you
can specify the location of each sensor in the 2D plane (have a look
at one of the *.lay files). If you do not specify a layout file, the
2D layout is constructed on the fly from the 3D electrode locations
that are represented as NelecX3 matrix in data.elec.pnt.

I suggest that you turn on the electrodes in topoplotER
(cfg.showlabels option) and see whether that makes sense.

If you are using standard labels of the extended 10-20 system in your
EEG data, you can also try topoplotting with a predefined 2D layout,

cfg = ...
cfg.layout = 'elec1020.lay'  % or elec1010.lay
topoplotER(cfg, avg)

> Do I have to tell Fieldtrip somewhere that this is EEG data, so
> that it doesn't do the things it would when dealing with MEG
> gradiometer data?

No, the topoplotting of EEG data and MEG data is done just the same.

> Or is there something I have to do to let fieldtrip know that the
> data are average reference data. I can't find anything in the
> tutorials on this matter.

No, referencing of EEG data does not influence the spatial
topographical distribution. It might change the global color
(depending on the coloraxis), but not the pattern. Re-referencing
your data at one timepoint just subtract a constant value (the
potential at the reference electrode) from all electrodes. A
geographical map of the Himalayas would also look the same if you
would express the height with respect to the foot of the mountain
range instead of with respect to the sea level.

best regards,

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