[Mne_analysis] source localizing frequency bands

[Pavan Ramkumar]

Hi Gus and Elli,

Thanks for the discussion. I have been doing something similar.

If your bandpass filter is a FIR filter then I think bandpass/MNE are
commutative processes. This is because FIR filtering is a convolution
which can be implemented by the right-multiplication of a 'time x time'
BPF matrix constructed from the filter coefficients, with a 'channels x
time' raw data matrix [step 1]. The linear MNE inverse operator on the
other hand (literally) is implemented by the left-multiplication of a
'source pts x channels' matrix [step 2].

It is easy to see that steps 1 and 2 are interchangeable. However, I am
not completely sure about whether step 1 holds for FIR filtering, since
presumably Matlab does it row by row for each channel...

Pavan

> Just a few thoughts...
>
> I'm not familiar enough to comment on the DICS method, but the MNE
method
> returns a distribution based on the provided sensor waveforms. It would
seem intuitive to me that performing a restrictive (say, only alpha
activity) bandpass on the sensor waveform prior to performing MNE would
significantly change the topology of the waveform, and thus
significantly
> affect how the sources are localized.
>
> The real question being asked is whether the steps are commutative;
i.e.,
> do the following two analysis streams produce identical results: 1)
bandpass, MNE
> 2) MNE, bandpass
> My gut intuition is that they are NOT, since the first method performs
the
> bandpass on the raw MEG data, so to speak, while the second one performs
the bandpass on reconstructed data. However, I can't prove this, and it
bears further investigation. Anyone else have any thoughts?
>
> Elli Kanal
>
>
>
> On Nov 30, 2009, at 5:20 PM, Gustavo Sudre wrote:
>
>> Hello,
>> I have a generic question about source localization. From what I've
seen so far, when the goal is to localize the activity in certain
frequency bands, it's common to use methods such as DICS or the variant
of MCE for frequency bands. I was wondering if it is also correct to do
source localization using a different method (e.g. MNE) with the whole
signal (i.e. prior to band-pass filtering to specific bands) and then
convert the signal in the localized sources to
>> frequency domain. Another alternative would be to band-pass the signal
to a certain frequency band in source space, and then run a different
source localization method on it (e.g. MNE).
>> Could anyone elaborate on the advantages / disadvantages ( / validity)
of these two methods?
>> Thank you,
>> Gus
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-- 
Pavan Ramkumar

Brain Research Unit
Low Temperature Laboratory
Helsinki University of Technology
Espoo, Finland