See below.

On Tue, Oct 20, 2015 at 1:11 PM, Aser A <[log in to unmask]> wrote:

Dear Donald and all,

I have two PPI questions:

1- if I have a PM modulation (e.g. linear changes) and done +1 on the subjects levels and then did group analysis using +1 or -1 ? is this correct (i.e. do I have to return to the subjects level again and do the contrast -1 in order to perform one sample t tests group analysis ? 

>> Correct. Both directions at the group level can be tested with the single contrast from the first level. 

Now the PPI related question here is that what does it mean a negative first order linear PPI analysis between ROI (A (seed)) and ROI (B) ? 

>> Do you mean for a first level contrast of -1 over one PPI column? If so, this would mean that the connectivity amplitude is less than during baseline.

 

2- The second question is related to the group analysis. If I have two groups patients (A) and healthy (B), when performing two sample t tests to investigate PPI (A) > PPI (B) I do not get any significant even at very low threshdol. Is it always difficult to get differences between group and this need very high number of samples ?

>> It depends on the effect size. Some effects will be greater than others. Without knowing anything about the task or how the task was modeled, its hard to say if you'd need a large number of subjects or not. We are working on spatial analysis approaches that would be less dependent on the actual effect size and more dependent on the spatial distribution of the effects.

 

Thanks

Aser