[Eeglablist] Regarding : reference choice, baseline removal and ICA

[arno delorme]

Dear Amani,

> I need to analyze data recorded using BIOSEMI (reference free), and  
> I have a few questions :
> I read the eeglab tutorial page about referencing, but I still have  
> a few questions
>
> 1.  I record two earlobe channels and two eog channels.
> Which is the best reference I can choose during data import : One  
> earlobe or two?
> How does the calculation differ between the two?
> If I choose average reference, how is the calculation done when  
> there is no  former reference?

The calculation is done by averaging the potential from both  
electrodes and subtracting it from all others.

> - If the reference channel has drifts during some period of the  
> recording, how will it affect the data?

This will make all the data drift of course.

> - Do I need to reference the data before epoching or can I do so  
> after?

If you do not remove the baseline after epoching, it is equivalent.  
However, if you do, it is not. It only valid to rerefence non-epoched  
data. When you remove the baseline, you introduce some offset in each  
data epoch and for each channel. If there is an EEG/ERP source that  
project linearly to all channels, the linear relationship will be lost  
after baseline removal. We usually do not subtract baseline or use  
long baselines (1 second). Simulations have showed that ICA is  
unstable if the epochs baseline is too short (and baseline is removed  
of course). It is described in more detail in David Groppe's  
conference papers.

> 2. How is baseline removal done?
>  - Is the average of the baseline period of each epoch subtracted  
> from every sample in the activity period of that epoch?

Yes, baseline is subtracted independently for each channel.

> 3. How does re-referencing before and after  baseline removal differ?

I consider that it is not valid to do that. See above.

> 4. Can I include the EOG channels and earlobe channels during ICA  
> for artifact removal, where I reject components by visual inspection?

Yes you can especially if they are recorded with the same reference  
(or reference free in your case) as the other channels.

Best,

Arno