Hi Harsha,
there seems to be a problem with your Group 2 negative interactions
one-sample t-test. The threshold was T = Inf. You can try to look at
this contrast with "uncorrected 3.12" an I'll expect a figure like Group
1. This may be an explanation for seeing no difference in the two-sample
t-test.
Michael.
Harsha Halahalli schrieb:
> 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
>
> -------------------------------------------------------------
> Harsha Halahalli
> PhD Scholar
> MBIAL, Department of Psychiatry
> National Institute of Mental Health and Neurosciences (NIMHANS)
> Bangalore - 560029, INDIA
>
>
> ------------------------------------------------------------------------
>
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Dr. Michael Erb
Sektion f. experimentelle Kernspinresonanz des ZNS
Abteilung Neuroradiologie, Universitaetsklinikum
Hoppe-Seyler_Str. 3, D-72076 Tuebingen
Tel.: +49(0)7071/2987753 priv. +49(0)7071/61559
Fax.: +49(0)7071/294371
e-mail: <[log in to unmask]>
www: http://www.medizin.uni-tuebingen.de/nrad/sektion/
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