## Calculating/utilizing surface Laplacian coefficients

Working with OpenViBE signal processing scenarios and doing scenario/BCI design
sinnerfedeli
Posts: 2
Joined: Sat Dec 21, 2019 7:06 pm

### Calculating/utilizing surface Laplacian coefficients

Hi All,

I've been trying to find a clear example of how to calculate and use new coefficients for the surface Laplacian filter in the example motor imagery project for some time now, with no clearer explanation than that found in the documentation for the spacial filter box. Perhaps I don't understand the math necessary to create new coefficients. I want to limit the number of input channels to 6 (focused specifically on C3 and C4, but with input from Fp1, Fp2, F3, and F4 used to train the classifier), but need help assigning coefficients to do such.

I would love a walkthrough on how to use the supplied equation or create my own ASCII matrix files, if that isn't too much to ask.

Thank you!

Thibaut
Posts: 144
Joined: Wed Oct 31, 2018 9:14 am

### Re: Calculating/utilizing surface Laplacian coefficients

Hi,
The Example on the web site can help you,n we have 10 input to 2 outputs.
http://openvibe.inria.fr/documentation/ ... ilter.html

But for the coefficient you must use, I don't know (If you want CX only with FPX and FX electrodes I don't know how to make that if it's possible. It's too far.

Thibaut

sinnerfedeli
Posts: 2
Joined: Sat Dec 21, 2019 7:06 pm

### Re: Calculating/utilizing surface Laplacian coefficients

Thanks, Thibaut,

I’ve read that documentation and appreciate your insight about the electrodes being too far apart. I’m utilizing a 16-lead system laid out in a 10-20 pattern identical to the attached picture that does not have all the same electrodes used in your example. Assuming I still want to focus my Laplacian around C3/C4 for training my classifier, how should I decide the coefficients to use? Or are my electrodes so far apart that using a Laplacian at all is irrelevant?

I plan to use a smaller number of electrodes to train the classifier while using the remaining electrodes to passively collect information about other brain areas. If a Laplacian is inappropriate in this situation, do you think there is any wisdom in selecting C3/4 unfiltered to train left vs. right?

Thanks again!

P.S., if image is difficult to read, the channel listing is;
Fp1/2
F3/4
F7/8
C3/4
T7/8
P3/4
P7/8
O1/2
Attachments
My 10-20 arrangement with 16 electrodes.
48E2EF0B-4E78-48D7-97FE-0AABD969F9E0.jpeg (63.92 KiB) Viewed 727 times

Thibaut
Posts: 144
Joined: Wed Oct 31, 2018 9:14 am

### Re: Calculating/utilizing surface Laplacian coefficients

Hi,
I don't know enough about neurophysiology and Laplacian to answer this question. Naively, I would say to use only C3 and C4 if that is what you want and also that you have them, especially since the activity picked up by the electrode is diffuse (it is not the activity at the precise point but in the area).
Thibaut