[Mne_analysis] source localizing frequency bands

[Gustavo Sudre]

Thank you for all the responses. Elli did get to the heart of the  
question, and my gut feeling said the same (i.e. that they would not  
result in the same thing). It's good to see that the following answers  
seem to provide explanations for it.

However, if we abstract ourselves from the details of the math behind  
it, shouldn't these processes be the same? For example, let's say we  
recorded raw MEG signal with a strong 10Hz component to it.

1) If I run MCE/DICS in this 10Hz activity, I should get some source  
activity that caused it.
2) If I band-pass the signal to 10Hz and run MNE on it, I will get  
some source activity that caused it.
3) If I run MNE on the signal prior to any filtering, band-pass the  
activity in all sources estimates to 10Hz, and focus on the sources  
with highest power in 10Hz.

In all 3 cases, I'm looking for the sources that caused the 10Hz  
activity I see in the MEG signal. Then, shouldn't the results of all 3  
methods above agree to a certain degree?

Before I end this e-mail, I realize that "abstracting from the math"  
is not a luxury we can have, since they're the heart of every source  
localization technique. But maybe one technique or the other would be  
more in accordance with the intuition above?

Thanks,

Gus

On Dec 2, 2009, at 2:36 PM, Hari Bharadwaj wrote:

> Hi Kanal,
>   You are right, they are are not the same. The reason they are not  
> the
> same practically is that the noise covariance when generated after
> bandpass  filtering the signal first can be very very different (in
> scale and possibly also in the spatial structure) and hence the  
> inverse
> operator is different.
>
> I would vouch for computing one inverse operator with the entire  
> band of
> interest (0-100Hz for example) and then looking at narrower bands  
> once the
> data is in source space. My reason for that would be to not impose too
> much temporal correlation in the noise by band-pass filtering in  
> sensor
> space. MNE as such is equivalent to a maximum aposteriori probability
> (MAP) estimate of the source space activity only under the  
> assumption that
> the noise is uncorrelated in time. It is robust to violations of the
> assumption to a large extent (correct me if I'm wrong here) but  
> filtering
> around 7Hz, for instance to look at alpha makes the noise heavily
> different from its assumed no temporal correlation behaviour.
>
> Having said that, if its an event related paradigm and you have enough
> trials and excellent SNR, doing it either way should not make a
> difference.
>
> Regards,
> Hari
>
>
>
>
>
>
>
>
> On Wed, December 2, 2009 2:11 pm, Kanal Eliezer wrote:
>> 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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>>
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>>
>
>
> -- 
> Hari Bharadwaj