On Tue, Aug 3, 2010 at 9:08 AM, Fred Sanders
<[log in to unmask]> wrote:
Dear SPM Experts,
I am trying to run a PPI analysis and want to make sure I am interpreting it the correct way. Using the GLM, I compared two groups (Group A and Group B) using a simple task. I found that with the contrast Interest - Control Condition, Group A had greater activation in my ROI than Group B. I then wanted to run a PPI based on this ROI. However, Group B did not demonstrate any significant activation in the ROI in the contrast Interest - Control Condition. So I ran a contrast with Interest - Fixation. With this contrast, both groups showed activation in my ROI. Here are my questions:
Question 1) To me, it does not back sense to run the PPI with the Interest - Control Condition b/c with this contrast in my ROI Group B does not demonstrate significant activation - is that correct??
Correct in the sense that if a group does not show any activation in a region you cannot extract that VOI. As you suggest you can display activations using a different contrast, so that the VOI can be extracted. Note that you should think of the contrast you use to display the activations as simply a window to which voxels will be extracted. SPM goes to the xyz locations of the active voxels in your VOI, gets the actual data from your processed images and then adjusts the data for any effects of interest you specify. The contrast you use to display activations merely identifies the voxels to extract data from it does not affect the data that are extracted.
Question 2) If Group A has greater activation than Group B in my ROI during the Interest - Fixation condition, how does this affect the PPI when I compare across the groups? My understanding is that the ROI activation serves as a regressor in the PPI. So even if one group has greater activation in the ROI, this should not matter. In other words, a region with a stronger interaction in Group A vs Group B still means the that there is stronger connectivity between the ROI and the region identified in the PPI in Group A. Is that correct?
Two groups do not have to show equal activity in the source region in order for the PPI comparison to be valid, just like two groups would not need to have equivalent main effects in order for an interaction to be tested (indeed it is the main effects that are suspect when there is an interaction and not vice versa). The first eigenvariate from the ROI activation is included as part of the PPI design matrix, in order to discount the main effect of the ROI activity.
Regards,
Darren
Thanks!!!
Fred
