[Mne_analysis] visualizing differences between conditions in source space

[Matthew Panichello]

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