Dear SPMmers

 

I am attempting to run a PPI analysis across a number of subjects, although I am struggling with setting this up at even the single-subject level and would greatly appreciate some assistance.

 

In section 36.4 of the SPM12 Manual, on page 334, I follow steps 1 – 7 (although not correcting for MC and keeping threshold to .001). I identified the coordinates for my VOI (angular gyrus) based on the group-level results for that contrast, and these are [-32, -74, 42]. Unfortunately, when I select these coordinates and then hit ‘eigenvariate’, it resets the coordinates to [-8, -90, 8] and doesn’t allow me to extract. SO, I used this source to make the following code:

 

xY.xyz  = spm_mip_ui('SetCoords',[-32 -74 42]); % Input the coords for the VOI

xY.name = 'AG'; % Angular Gyrus

xY.Ic   = 3; % 1: don’t adjust, 2: adjust everything, 3: Eff.of.Interest.

xY.Sess = 1; % for each run, iterate in loop

xY.def  = 'sphere';

xY.spec = 6; % 6mm

[~,xY]  = spm_regions(xSPM,SPM,hReg,xY);

 

This works, but after the execution of the final line it says Warning: Empty region. > In spm_regions (line 155). The glass brain contains no suprathreshold voxels there, but is this not expected behaviour? We are interested in extracting the data from this region and doing the PPI analysis at group level, so is it not expected that some subjects won’t have suprathreshold voxels? Moreover, I have run this in a loop across all subjects and found that most subjects have no suprathreshold voxels here for at least a couple of their runs (there are 4 runs in this case), despite there being an overall group-level effect with the contrast of interest.

 

Have I misunderstood something?

 

Thanks in advance

Lewis

 

 

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