[Mne_analysis] Get back a reference channel from acticap

[Dr. Sylvain Madec]

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Ok I will try to find a space for holding a file if I do not manage to solve 
the issues before, but here is some observations:


- the error "RuntimeWarning: The locations of multiple reference channels 
are ignored (set to zero)" appearing when I use 
mne.io.add_reference_channels is there only if previously I import the 
brainvision data by specifying a montage. For some reasons specifying a 
montage is creating a list called "dig" (I do not exactly know what it is), 
and the presence of this list is causing issue in the code 
there https://github.com/mne-tools/mne-python/blob/master/mne/io/reference.py 
line 219;

- so what I did is not importing the data by not specifying a montage. Then 
concerning the fact that the Info was not updated I am sorry I did a mistake 
in using mne.add_reference_channels instead of mne.io.add_reference_channels 
. Without io well it does not update anything.

Now still if anyone had an idea if the rationale of my original question is 
ok I would be interested.

Thank you!

Sylvain

On Tue, 16 Oct 2018 15:18:46 +0200
  Sik <mailsik at gmail.com> wrote:
>        External Email - Use Caution        
> 
> I'm not familiar with the processing pipeline, but if you drop us an
> example file I'll try to see why FCz is dropped.
> 
> On Tue, Oct 16, 2018 at 3:06 PM Sylvain <madec at uni-potsdam.de> 
>wrote:
> 
>>         External Email - Use Caution
>>
>> Dear mne users,
>>
>> I used an acticap system to collect data. This system has a common
>> reference electrode localized on the FCz location. I would like to 
>>retrieve
>> this electrode for further analysis,
>>
>> What I would like to do is as follow:
>> - adding back the reference channel by using
>> mne.add_reference_channels(raw, 'FCz'), which I guess should add 
>>this
>> channel with values equal to zeros;
>> - then computing the average reference of all channel by using
>> raw.set_eeg_reference('average') So I guess the values on the FCz 
>>channels
>> should just be 0 minus the mean of all other channels;;
>> - then well maybe I will use the linked mastoid average or I will 
>>stay
>> with the average one.
>>
>> OK so the issues I have right now:
>> - could anyone confirm my rationale makes sense?
>> - second I have troubles with  mne.add_reference_channels. For some
>> reasons when I apply it I get an error "RuntimeWarning: the 
>>locations of
>> multiple reference channels are ignored (set to zeros)". And 
>>apparently it
>> is not updating the info structure.
>>
>> Thank you for any help!
>>
>> Sylvain
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