[Mne_analysis] Comparing conditions

[Christian Wienbruch]

Not necessarily - the sensors pick up signals from multiple  
generators  following the  superposition principle. This basically is  
the sum of the products leadfield L_ij times dipole moment p_j plus  
noise forming the magnetic induction B_i at sensor i

B_i = L_ij *p_j + n

If you subtract two field B1 and B2 and they share a common generator  
- this one is excluded in sensor space. In theory this works  
perfectly well in absence of noise.
Practically speaking  this concept works pretty good  for certain EEG  
components like the ERN which is the difference between incorrect and  
correct responses.

"Distortions" usually come from the noise term and generators not  
being active in both conditions, or differently active in both  
conditions. Additionally you have source model related distortions  
e.g. regularisation.
So it depends mostly on your signal to noise ratio if it is wise to  
subtract fields in sensor space or not.

It also depends on how accurate you can position the MEG helmet in  
headframe coordinates across conditions, subjects, ...
If that can not be done reliably don't even think about subtraction  
in sensor space. You just add additional variance to your data.

-Christian


On Sep 29, 2008, at 5:19 PM, Padraig Kitterick wrote:

> Because you are distorting the dipolar topographies when you do a  
> subtraction at the sensor level. The resulting data is likely  
> contain field patterns which do not relate to the leadfields of the  
> actual sources that gave rise to the data. Thus, any source  
> reconstruction which relies on lead field models, i.e. minimum  
> norm, will give spurious results.
>
> -Padraig
>
> Yury Petrov wrote:
>> Why not first subtract one average response from the other and  
>> then  localize?
>>
>> On Sep 29, 2008, at Sep 29, 2008 | 6:28 AM, Alex Clarke wrote:
>>
>>
>>> Hi there,
>>>
>>> I have a question regarding how best to statistically compare  
>>> two  conditions. So far I have only being comparing between 2  
>>> conditions  using ROIs and comparing current estimates over time.  
>>> However, I'd  also like to see the difference between two  
>>> conditions across the  whole brain. I was wondering what the best  
>>> approach to this was  (Ideally ending up with a dSPM map of  
>>> condition1 - conditon2).
>>>
>>> Any help on this would be appreciated
>>>
>>>
>>> Thanks,
>>>
>>> Alex Clarke
>>> _______________________________________________
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>>> Mne_analysis at nmr.mgh.harvard.edu
>>> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis
>>>
>>
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>>
>>
>
>
> -- 
> Pádraig Kitterick
> Graduate Student
> Department of Psychology
> University of York
> Heslington
> York YO10 5DD
> UK
>
> Tel: +44 (0) 1904 43 2883
> Email: p.kitterick at psych.york.ac.uk
>
>
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Christian Wienbruch

University of Konstanz
Clinical Psychology
Fach D27
78457 Konstanz

Christian.Wienbruch at uni-konstanz.de



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