Dear SPMers,
We have a query regrading group comparisons of PPIs.
We have performed a PPI analysis in a fMRI dataset acquired while subjects
were performing a cognitive paradigm which consisted of two tasks (say Task
A and and a baseline Task B). There were two groups of subjects (say Group
1, n=23; and Group 2, n=23). We have extracted VOI-timeseries from a chosen
seed region for each of the subject's 1st level SPM, derived the regressors
for the [Task A -Task B] contrast, set up the PPI GLM for individual
subjects and obtained con images for
positive and negative interactions. We then entered these 1st level con
images from subjects of the two groups into two separate second level
one-sample t-test models. This model included four confounding covariates.
One of the groups showed extensive areas of significant negative
interactions after FDR correction at p<0.05. The other group did not show
any such regions of negative interaction. However, when we entered these
same 1st level con images into a two-sample t-test model for a between-group
comparison with the covariates of no-interest, we fail to see any
significant differences in the areas of negative interaction (See attached
file for images).
Our quetions are:
1. Have we followed the correct steps for this between-group comparison of PPIs?
2. Why does the two-sample t-test comparison fail to show any significant
group differences when the one-sample tests suggest such obvious differences
in the pattern of negative interactions in the two groups?
Any comments / suggestions would be greatly appreciated.
Best Regards
Harsha
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Harsha Halahalli
PhD Scholar
MBIAL, Department of Psychiatry
National Institute of Mental Health and Neurosciences (NIMHANS)
Bangalore - 560029, INDIA
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