Problem with data from BESA

Michael Wibral wibral at MPIH-FRANKFURT.MPG.DE
Wed Nov 16 18:56:20 CET 2005

Hi Robert,

thanks for your help. There seems to be indeed a problem with the
ordering of the electrodes in the mul-files themselves and the
corresponding sfp files, that contain some additional fiducials - so if
electrodes and their positions are not matched by name but by order
during imports that will of course go wrong. Both files also have a
different order from the 10-10 layout used in Fieldtrip, but I guess
layout files match electrodes per name, don't they. I will try to figure
out a workaround.


Robert Oostenveld schrieb:

> Hi Michael
>> I actually switched on the electrode labels in the display and the
>> peaks sit at the wrong electrodes.
> That seams to indicate that there is a mismatch between the channel
> names and the electrode names. If you see a peak at a specific
> electrode in the topoplot, you should be albe to confirm its value by
> looking in the data. Could it be that  the ordering of the channels
> is different in the two conditions that you are reading in (compare AM
> {1}.label and vAM{1}.label)?
>> I therefore assume it is not a problem of the layout file (alone).  I
>> actually took into account that the data look heavily distorted  and
>> tried to check wether it is just a projection problem by  playing
>> around with different scalings of elec.pnt (albeit this  didn't seem
>> to affect the plot??).
> The scaling of the radius of the electrodes does not affect the
> location towards which it is projected in the 2D plane. What would
> matter however w.r.t. the 2D projection is if you would shift them.
> The interpolation algorithm that is used in topoplotER is certainly
> different from the one that is used in BESA. But I would not expect
> that to make such a big difference that peaks start shifting around.
> Maybe Ole can comment on the interpolation, since he supplied the
> topoplotER function based upon some code from EEGLAB (Ole should read
> along on the mailing list, but I also CCed to him).
>> I should have also mentioned that I'm using version 20051113.
>> However I imported the electrode positions with read_fcdc_elec from
>> the version 0.9.6 (there doesn't seem to be a read_fcdc_elec  version
>> supplied with 2005113..) - I hope this doesn't cause the  trouble.
> It indeed was missing. I have tagged the read_fcdc_elec file to be
> included in the upcoming daily release versions (which are updated
> every evening on the ftp server). You can pick it up tomorrow at
>> Meanwhile I also tried to use the elec1010.lay layout file which
>> works fine.However, in fieldtrip I find a negative peak between
>> electrodes P5 P3 PO7 PO3 and a positive central one at CPz Cz  (which
>> has no counterpart in BESA, so contourlines don't match  exactly),
>> whereas in BESA a positive peak is found on CP4. This  looks like an
>> inversion of signs, an inversion of left/right and a  difference in
>> the interpolation algorithm.
> Does the peak ly on top of an electrode or in between the electrodes?
> If it is at the electrode, you should be able to verify it's actual
> value. I am concerned that there might be an ordering/naming problem
> with your EEG channels. Please try the two low-level functions that
> you find attached. They work like this:
>   topoplot(cfg,X,Y,datavector,Labels)
> and
>   triplot([X Y zeros(Nchan,1)], [], Labels, datavector)
> You can get the X and Y value from the layout file. With the triplot,
> you can also plot 3D (just use elec.pnt, i.e. [x y z] as the first
> argument). The triplot does linear interpolation over the triangles
> that connect the electrodes. It might look coarse, but with it you
> are guaranteed not to overinterpret the data (i.e. there cannot be
> any spurious peaks between the electrodes).
> best,
> Robert
> PS if you still cannot figure it out, send me a private mail with
> your plotdata1 structure and, if not too large, the AM and vAM data.

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