[Mne_analysis] Comparing conditions

Padraig Kitterick p.kitterick at psych.york.ac.uk
Mon Sep 29 12:56:50 EDT 2008
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Thanks Christian. This is really helpful. It is so challenging to fully 
understand the limitations of what one is comparing, and how all of the 
analysis steps up to that point have contributed to any effects that one 
finds...

But it wouldn't be as much fun if it was easy ;-)

-Padraig

Christian Wienbruch wrote:
> 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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>>>>>> <mailto:Mne_analysis at nmr.mgh.harvard.edu>
>>>>>> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis
>>>>>>
>>>>>>     
>>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Mne_analysis mailing list
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>>>>> <mailto:Mne_analysis at nmr.mgh.harvard.edu>
>>>>> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis
>>>>>
>>>>>
>>>>>   
>>>>>
>>>>
>>>>
>>>> -- 
>>>> 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
>>>> Mne_analysis at nmr.mgh.harvard.edu 
>>>> <mailto:Mne_analysis at nmr.mgh.harvard.edu>
>>>> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis
>>>
>>> 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 <mailto:p.kitterick at psych.york.ac.uk>
>>
>>
>
> 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


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