[Mne_analysis] visualizing differences between conditions in source space

[Hari Bharadwaj]

Yes.. mne_average_estimates with weights of 1 and -1 is indeed subtraction
is source space.

Hari

On Fri, July 13, 2012 12:03 pm, Matthew Panichello wrote:
> 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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>>> Mne_analysis at nmr.mgh.harvard.edu
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>>>
>>>
>>>
>>
>
>
>
>


-- 
Hari Bharadwaj
PhD Candidate, Biomedical Engineering,
Boston University
677 Beacon St.,
Boston, MA 02215

Martinos Center for Biomedical Imaging,
Massachusetts General Hospital
149 Thirteenth Street,
Charlestown, MA 02129

hari at nmr.mgh.harvard.edu
Ph: 734-883-5954