[Mne_analysis] SSPs and avg ref

Candida Jane Maria Ustine candida at nmr.mgh.harvard.edu
Wed Apr 3 06:40:50 EDT 2013
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Hello Hari, 

Instead of deleting the average references from the projon ave.fif data structure, I tried disabling them(setting the active tag to 0) so as to keep the dimensionality the same. Do you think this could help? (In this case the resulting averages looked a little different from an alternate option of viewing the ave.fif originally set to --projoff but manually checking all but the average ref projectors On in mne_browse_raw.) 
 
Best,
Candida




-- 
Candida Jane Maria Ustine M.Eng. 
NeuroCognition Lab
Department of Psychiatry - Neurosciences
Athinoula A. Martinos Center of Biomedical Imaging
Massachusetts General Hospital
candida at nmr.mgh.harvard.edu 


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On Apr 2, 2013, at 11:06 PM, Hari Bharadwaj wrote:

> Hi Ellen,
>   The EOG SSPs and the average reference SSP shouldn't really interact in
> anyway, except that you may get rid of some of the EOG artificat in
> your average reference (which is not a problem, I guess). You could
> apply them in any order and applying them multiple times shouldn't
> change anything.
> 
>   The average reference operator in MATLAB code would be as below if you
> want play with some toy data in MATLAB to understand this better:
> A = eye(Nchan) - ones(Nchan)/Nchan;
> 
> If X is your NChan x NTime EEG data, then A*X should do the average
> referencing. A has rank Nchan-1 where Nchan is the number of EEG channels
> that you have. This means that unless you have your reference channel as
> one of your EEG channels (as opposed to a mastoid or nose or something
> that's separate from the Nchan EEG channels), applying this operator would
> reduce the dimensionality of your data by 1 and if you save only the
> result with --projon for this projector, deleting the operator from the
> fif structure would not help you gain back that lost dimension. If you
> really want to undo the average reference at a later point, I'd suggest
> making an extra derivative channel that is the average of all your
> channels and adding that channel's time series to the each of the
> individual channel when you want to undo the average referencing.
> 
> Hope this helps..
> 
> Regards,
> Hari
> 
> 
> 
> 
> 
> 
> On Tue, April 2, 2013 9:21 pm, Ellen Lau wrote:
>> Hi,
>> 
>> Our group is applying SSPs to get rid of EOG artifact with EEG data. As I
>> posted recently, we are trying to display our EEG sensor data without the
>> average reference applied.
>> 
>> My question is about the interaction between the EOG SSPs and the average
>> reference. One method we were considering was to compute the EOG SSPs on
>> the raw data, then use the --projon tag to apply them plus the average ref
>> in computing the evoked data, and then manually remove the average ref SSP
>> from the evoked data structure. Will this procedure result in an evoked
>> file that does not include any effects of the average reference, or does
>> the average reference interact in some way with the application of the
>> other SSPs? Sorry if this is obvious, I just have trouble conceptualizing
>> the average reference as an SSP.
>> 
>> thanks,
>> Ellen
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>> Mne_analysis at nmr.mgh.harvard.edu
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>> 
>> 
>> 
> 
> 
> -- 
> Hari Bharadwaj
> PhD Candidate, Biomedical Engineering,
> Boston University
> 677 Beacon St.,
> Boston, MA 02215
> 
> Martinos Center for Biomedical Imaging,
> Massachusetts General Hospital
> 149 Thirteenth Street,
> Charlestown, MA 02129
> 
> hari at nmr.mgh.harvard.edu
> Ph: 734-883-5954
> 
> 
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