[Mne_analysis] Comparing conditions

Padraig Kitterick p.kitterick at psych.york.ac.uk
Mon Sep 29 12:09:56 EDT 2008
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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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>>>>
>>>>     
>>>>
>>>
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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
>> Mne_analysis at nmr.mgh.harvard.edu 
>> <mailto:Mne_analysis at nmr.mgh.harvard.edu>
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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


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