source localization given induced spectra

John Iversen iversen at NSI.EDU
Tue Nov 4 18:02:17 CET 2008


Dear Jan-Mathijs,

Thanks for the quick reply.  I'm sorry to hear I was right :)

I may misunderstand what you've suggested, but is it not the case that
if I chose cfg.output='fourier' it will average fourier spectra across
trials (cares about phase) instead of power spectrum (phase blind, as
I had been doing). In the end wont I simply get the equivalent of the
fourier spectrum of the timelock average, which is substantially
different from the induced spectrum that interests me?

In many cases the sensor topography of the induced power looks
somewhat dipolar, with two power peaks, so I may well try to do a fit,
but optimizing not on the field but the field power (this would
require modifying the output of the forward model within
dipolefitting)--it will not be able to get the polarity of the dipole,
but should be able to get a location. Maybe? It seems possible in
principle, but I wonder if anyone has practical experience with this.

I feel there should be a way to study this kind of question!

Best,

John

On Nov 3, 2008, at 1:18 PM, jan-mathijs schoffelen wrote:

> 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
>>
>> ----------------------------------
>> 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.

----------------------------------
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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