Sources

[Michael Wibral]

Hi Marco,

I think there is no fundamental reason why you shouldn't be able to spot a power decrease in source space. What could be is that the source of the power decrease varies more over subjects (in anatomical location) than the source of the power increase - maybe you could run your analysis with a higher lambda to check this. Another physiological reason could be that the source of the power decrease actually consists of two sources that are at some distance, but highly synchronous - in which case they will cancel each other using beamforming.

Then there are three silly mistakes I made previously, so I give you a list to check - not necessarily assuming you did the same mistakes:
1. Do your functional limits and your opacity limits allow to plot negative values?
2. Do you do a two-sided test in sourcestatistics ?
3. Do you possibly look at a relative (% or z-score) measure at the electrode level but at absolute (non-basline corrected) values in source space - or vice versa? This can sometimes give seemingly opposing effects, especially when you are comparing two groups of subjects that might have systematic differences in baseline power already.

'hope this helps,
Michael


> -----Ursprüngliche Nachricht-----
> Von: "Marco SPERDUTI" <marco.sperduti at UPMC.FR>
> Gesendet: 05.02.09 10:18:08
> An: FIELDTRIP at NIC.SURFNET.NL
> Betreff: [FIELDTRIP] Sources


> Dear users,
> 
> i'm trying to localize the sources of time-frequency data.
> At the scalp level i have, in the gamma range, two effects: a power's  
> increase for some sensors on the right hemisphere and a power's  
> decrease on the left.
> When i run the source reconstruction i find the sources for the power  
> increase in some areas that are compatible with the power's  
> distribution at the scalp, but i don't find anything for the power's  
> decrease.
> 
> Is that normal? Is it a problem to reconstruct the sources for the  
> power's decreas?
> 
> Thank you,
> 
> Marco
> 
> ----------------------------------
> The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/fcdonders/fieldtrip.
> 



----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/fcdonders/fieldtrip.
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