[Eeglablist] relation between EEG data and other signal, average ref question

[Pål Gunnar Larsson]

1. The relation between an event and EEG may be very complex. Breathing must be very complex. First there will be factors influencing (starting/doing) the activity of breathing. It may sometimes have a clear cortical relation, but usually more subcortical/spinal. Then of course the breathing produces muscle and movement artefacts. Then the slight changes in pO2 will change the EEG. So you have coherent activity before, at the same time and after the breathing. Any of these may have a variable response in the EEG both in time and between patients.

2. Differential amplifiers have both GND and REF. REF is directly part of your EEG-signal and is the one to care about. In a REF-montage a channel shows the voltage difference between the active electrode and REF. The average is as the name says the average of all (most) channels: avg=sum(chani-ref)/(number of channels). This may be written: sum(chani)/(number of channels)-ref. So rereference to avg would be: (chan-ref)- (avg-ref) -> chan -avg. The ref is not in the montage any more.


Pål


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Fra: eeglablist-bounces at sccn.ucsd.edu [mailto:eeglablist-bounces at sccn.ucsd.edu] På vegne av ida miokovic
Sendt: 17. juni 2013 13:50
Til: EEGLAB List
Emne: [Eeglablist] relation between EEG data and other signal, average ref question

Dear list,

I have 2 questions which I will try to present as clear as I can and to thank you in advance for any help:

1. I am trying to find relation (causal connection) between EEG dataset and certain breathing signal (let's say it is B signal) recorded at the same time as EEG.

 I performed EEG data preprocessing (filtering, ICA and removal of components recognized as artifacts).

 So on one side I have continuous signal of EEG channels and on the other side continuous signal B (of the same size as EEG channels recording). What are the best ways to perform this in EEGLAB? I suppose everyone is always trying to find certain relation between something and EEG signals so I suppose it is not unusual request.

2. This question is about re-referencing to the average reference - EEG dataset I am working with has been recorded with AFz as GND and FCz as REF (128 chennels Acti Cap is used). Nothing from these channels isn't present in data I have (I did not participate in recording procedure) and I would like to re-reference data to the average reference. Is it usual that activity from REF and GND channels are not given/measured? In tutorial  of EEGLAB, I found suggestion to include reference channels in the average reference if they are on the scalp. Mine are on the scalp, but I don't have any recordings of them. Also, everyone is mentioning REF electrode, and I have GND too...

Kind regards and thank you for your time and help

Ida

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