[Mne_analysis] Comparing conditions

[Padraig Kitterick]

You can ignore my comment. I was mistakenly thinking of non-linear 
dipole fits. The solution for the difference data should be identical to 
the difference between the solutions for the individual conditions as 
long as the same regularisation parameter and noise covariance matrices 
are used for both conditions so that the inverse operator is identical 
in all cases.

-P

Yury Petrov wrote:
> I didn't understand Padraig's arguments. Minimum norm is a linear  
> method. This means that the same as for the forward matrix the inverse  
> matrix is calculated irrespective of the actual signals measured,  
> except for the noise covariance term. So if you add the noise  
> covariance matrices for the two conditions and feed the resulting  
> "difference" covariance matrix to the inverse routine, it will only  
> change the signal to noise ratios in the trivial way. But the matrix  
> will remain essentially the same, and the topography of the solution  
> should not be "distorted".
>
> On Sep 29, 2008, at Sep 29, 2008 | 11:19 AM, 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
>>
>>
>>     
>
>
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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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