[Mne_analysis] visualizing differences between conditions in source space

[Matthew Panichello]

Great, thank you for the clarification.

On 7/13/12 12:41 PM, Hari Bharadwaj wrote:
> 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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>>>>
>>>>
>>>>
>>
>>
>>
>