EEG source analysis with one missing channel

Discussions
1

Hello brainstorm people,

I’ve recently started to use Brainstorm for EEG source analysis. I have data collected with EGI Geodesic with a 128(+1) channels hydro cell net. I did all the pre-processing steps with EEGLAB, but at the beginning I didn’t import the channel 129, which is used during the acquisition as the default on-line reference and is located at the vertex. I re-referred then my data to an average reference. For the ERPs analysis, the missed channel is not a big problem, since I’m not interested in the central scalp components, but I was wondering whether this can lead to problem with the source localization, maybe biasing the sources’ position. Can anyone give me some tips about this? Is there anyone that has experienced the same problem?

Many thanks in advance,

Luca

2

Hi Luca,

You shouldn’t have any problems with your files.
The inverse and forward models are coded to work only in average reference. During the source estimation process, there is an implicit conversion to the average reference that is done for the EEG recordings.

The important part is to have correct positions of the electrodes on the scalp, and make sure that the order of the electrodes is the same in the channel file and in the recordings (.F matrix). You shouldn’t worry about the second point if you are using standard EGI nets (it’s not a cap where you can easily exchange the position of two electrodes).

Cheers,
Francois

3

Hi Francois and thanks a lot for your fast reply,

Luckily, I did not have any problems in electrodes position. My concern was generally how much influence can have missing data (in my case from the Vertex electrode) on the source localization accuracy.

best,
Luca

4

I would say: none. What you’re doing is ok.

The electrode at the vertex was a reference. When you convert to an average reference, the reference does not disappear, it gets propagated everywhere.
You recorded only 128 signals with a bipolar montage with a common reference.

Keeping an extra electrode to store the average of all the electrodes is useful in one case: when you want to revert to the original values.
In your case, you don’t really need it. It would just add an artificial 129th value that is just a linear combination of the other 128 electrodes.

Francois

5

Perfect, now is clear :slight_smile:
Thanks again for your precious help

Luca