source localization given induced spectra

[jan-mathijs schoffelen]

Dear John,

No, it is not possible to perform source localization on the
spectrograms as you define them. You quite rightly point out that a
spatial topography of power is always positive, so cannot account for
a proper dipolar pattern. However, source localization of induced
changes in oscillatory activity is possible. There's actually a nice
tutorial on the fieldtrip website: "localizing oscillator sources,
using beamformer techniques".
Alternatively, you can actually call dipolefitting using frequency
data as an input, but this requires either fourier-data, or cross-
spectral densities between all channel combinations. Usually the
fourier-data is more memory efficient. In this case I would propose a
two-step strategy: compute spectrograms to identify your time-
frequency region(s) of interest. Then call freqanalysis again, with
cfg.output = 'fourier'. Then I would guess that dipolefitting runs
through... At least it's worth a try.

Yours,

Jan-Mathijs


On Nov 3, 2008, at 5:26 PM, John Iversen wrote:

> Hello,
>
> Is there a way to do source localization on induced spectrograms?
> (Induced spectra being the mean of individual trials' power
> spectra.) Conceptually I am not sure how this would work, given
> that one starts with topographies of real, positive-valued power,
> with no phase information, so any dipole fit could be at best sign-
> indeterminate.There is no facility within fieldtrip to do such a
> thing as far as I can tell (induced spectra were calculated
> freqanalysis on multi-trial data and are within the .powspctrm
> field of the result, which is not handled by freq2timelock, and
> thus cannot feed any of the localization routines).
>
> What is of actual interest are task-related fluctuations of the
> power around a (much larger, and topographically varied) baseline.
> Is there a way to say where in the brain are the (presumed) subset
> of neural sources that vary in power with time?
>
> Thanks,
>
> John
>
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> 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.