source derivation

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Yueqing Li
Posts: 51
Joined: 08 May 2007, 16:41

source derivation

Post by Yueqing Li » 13 Feb 2009, 01:31

Hi:

I have several simple questions about the source derivation.

1. What is the function of the Ground and Reference electrode on the EEG cap? Are they used to get the amplitude (or potential) for the other electrodes?

2. I know they are several ways to get the amplitude, such as CAR, etc. What method is used in the P300 Speller in BCI2000?

3. Does the source derivation happen in the BCI2000 (software), or the amplifier (hardware)?

Thanks a lot.

gschalk
Posts: 615
Joined: 28 Jan 2003, 12:37

source derivation ...

Post by gschalk » 13 Feb 2009, 06:44

Yueqing,

Regarding the ground and reference electrodes, please read the corresponding section in the tutorial:

http://www.bci2000.org/wiki/index.php/U ... ment_Setup

The P300 speller does not use CAR (i.e., subtracting the average of all channels from each channel). At least when you use the P300GUI, it linearly combines the channels based on the outcome of a stepwise linear discriminant analysis. This is described in a recent article by Krusienski and Wolpaw.

This combination is done in BCI2000 in the Classifier matrix.

Gerv

Yueqing Li
Posts: 51
Joined: 08 May 2007, 16:41

reference and ground

Post by Yueqing Li » 15 Feb 2009, 01:30

Hi, Dr. Schalk:

Thank you for your reply. I have another two questions.

1. Since the measurement is unipolar, so I think the potential measured is in fact the difference between each electrode and the reference electrode. Is this the amplitude we use in the data analysis or just the amplifier input?

2. I am a little confused about the function of the ground. From my knowledge of electronics and circuits, the ground is a zero reference point. However, in the link you gave, it is said that "amplifier inputs must be kept within a small voltage range relative to the amplifier's zero (ground) voltage level", and "this is achieved by connecting yet another electrode, a "ground" (Gnd) electrode, to the subject's scalp".

Then, what is the difference between the function of reference and ground electrode? How is the function of the ground electrode described above achieved? Thanks.

gschalk
Posts: 615
Joined: 28 Jan 2003, 12:37

Amplifier questions ...

Post by gschalk » 16 Feb 2009, 20:48

Yueqing,

1.) Any analysis, offline or online, will operate on the difference between the electrode and the reference.

2.) The function of the Ground in this context is to basically improve the signal-to-noise ratio.

Gerv

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