[Mne_analysis] SSPs and avg ref

[Hari Bharadwaj]

Hi Candida,
    In the files saved with --projon, you would have already applied the
average reference and saved only the resulting time series.. In this
case, setting the active tag to 0 would not have any effect. To make
sure that is indeed the case, you could check if the projon-ave.fif
data looks identical with or without the active tag set to 0.

Regards,
Hari


On Wed, April 3, 2013 6:40 am, Candida Jane Maria Ustine wrote:
> 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
>>> _______________________________________________
>>> Mne_analysis mailing list
>>> Mne_analysis at nmr.mgh.harvard.edu
>>> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis
>>>
>>>
>>>
>>
>>
>> --
>> 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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>>
>>
>
>


-- 
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