[Mne_analysis] visualizing differences between conditions in source space

[Matthew Panichello]

Hari, Sheraz,

Thanks for your help.

Manipulating the weights fed into mne_average_estimates sounds like a 
simple solution.

Sheraz, can you explain more what you mean by subtracting in source 
space? Simply subtracting the dSPM matrix for one condition from the 
other? Does manipulating the weights in average_estimates essentially 
accomplish the same thing?

Thanks,

Matt


On 7/13/12 11:50 AM, sheraz at nmr.mgh.harvard.edu wrote:
> Hi Matt,
>
> You can not perform any nonlinear operation before multiplying with your
> inverse operator, abs is not linear. Why dont you subtract in source
> space.
>
> Sheraz
>
>> Hi Everyone,
>>
>> I would like to create a signed dSPM map visualizing the difference
>> between two trial types (within subjects).
>>
>> The steps seem pretty straightforward:
>> 1) create difference waves for each subject using xplotter.
>> 2) compute the forward solution, inverse operator, and signed dSPM stc
>> file for each subject based on these difference waves.
>> 3) average the resulting stc files across subjects and load in mne_analyze
>>
>> However, I'm not sure about the proper computation to perform to create
>> the difference waves in xplotter. I believe the proper calculation is
>> either
>> Condition1 - Condition2
>> or
>> abs(Condition1) - abs(Condition2)
>>
>> I'm leaning towards the latter. I reasoned that if I just took the
>> simple difference, the sign of the difference wave wouldn't consistently
>> correspond to a greater Condition1 or Condition2 deflection. A large,
>> negative Condition1 deflection and a smaller negative Condition2
>> deflection at the same time points would produce a positive difference
>> wave, for example. By taking absolute value, positive values should
>> consistently reflect greater Cond1 deflections and negative values
>> should reflect greater Cond2 deflection.
>>
>> This method rests on the assumption, though, that greater deflection
>> from baseline always equals greater underlying neural response. I'm not
>> sure if this is the case (for example, what to make of a
>> positive-directed deflection that occurs completely below baseline?).
>>
>> Could anyone please clarify if creating difference waves and then
>> projecting these into source space is indeed a reasonable method for
>> comparing conditions, and, if so, which calculation is more appropriate?
>>
>> Thank you,
>>
>> Matt
>>
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>>
>>
>