[Mne_analysis] visualizing differences between conditions in source space

[Hari Bharadwaj]

Hi Matt,
   If you just want to visualize the difference between 2 conditions in
source space I think the mne_average_estimates (using weights of +1 and
-1 for the stc files of the 2 conditions of interest) method would be
very straightforward...

Based on your questions I would like to mention a couple of issues to keep
in mind:
(1) Subtraction in sensor space and then inversion is equivalent to
inversion of each condition and then subtraction is source space as long
as you use a common inverse operator and you don't do any non-linear
transformations - So taking abs() in sensor space is not appropriate

(2) If you choose to do a simple subtraction in sensor space i.e WITHOUT
abs(), then it is important to make sure that the number of trials going
into the 2 conditions are similar and to adjust the nave for scaling the
noise covariance.


Regards,
Hari


On Fri, July 13, 2012 11:36 am, Matthew Panichello wrote:
> 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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>


-- 
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