[Mne_analysis] Comparing conditions

Mon Sep 29 12:52:00 EDT 2008

Well you are right - that's what we see all the time. Even if you do  
subtraction of incorrect and correct response to get the ERN - which  
works well for a subset of EEG sensors in fronto-central locations -  
in other regions not involved  in the error processing you get  
patterns difficult to interpret.

But condition comparisons or subtractions in source space must not  
necessarily be more accurate - in case of minimum norm you "smear"  
the currents following a mathematical minimization criterion  - "the  
minimum norm".   Who says that the brain does work that way in a  
particular task - minimizing the current. So it is difficult to say  
what works best in terms of "best physiological model".

At that point - I usually don't care that much about the "most  
accurate source localization"  any more and look what is the best  
measure to differentiate (e.g. conditions, groups)  - once you've  
ruled out all the trivial effects you've got a good chance to see  
correlates of physiological differences. And if you can replicate  
that I would call it "a reliable, valuable correlate", which does not  
necessarily mean "true generator". It is rather a reasonable solution  
from the infinite amount of inverse solutions, which is probably all  
what we can expect in psychophysiology anyway.

Christian

On Sep 29, 2008, at 6:09 PM, Padraig Kitterick wrote:

> If one cannot assume similar source configurations in the two  
> conditions to be compared, presuming that any noise is constant and  
> equivalent, is it safe to say that subtraction would result in  
> distorted data ('distorted' not the best choice of words but my  
> vocabulary has failed me today!), or at least data which would be  
> difficult to interpret?
>
> Thanks,
>
> P.
>
> Christian Wienbruch wrote:
>> 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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>>>>> 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  
>>> <mailto:p.kitterick at psych.york.ac.uk>
>>>
>>>
>>> _______________________________________________
>>> Mne_analysis mailing list
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>>
>> Christian Wienbruch
>>
>> University of Konstanz
>> Clinical Psychology
>> Fach D27
>> 78457 Konstanz
>>
>> Christian.Wienbruch at uni-konstanz.de  
>> <mailto:Christian.Wienbruch at uni-konstanz.de>
>>
>>
>>
>
>
> -- 
> 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
>
>

Christian Wienbruch

University of Konstanz
Clinical Psychology
Fach D27
78457 Konstanz

Christian.Wienbruch at uni-konstanz.de

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