Problem with data from BESA

Robert Oostenveld r.oostenveld at FCDONDERS.RU.NL
Wed Nov 16 18:00:52 CET 2005

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:
   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).


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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