[Mne_analysis] visualizing differences between conditions in source space

[sheraz at nmr.mgh.harvard.edu]

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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