Many thanks Donald! 

I would like to elaborate more on this as I may missed some of your points relative to my understanding of PPI. So to start with I have only a task (c1) and rest.

- I think PPI or gPPI is done by correlating the signal of ROI A with the task but not the rest. The interaction between the two is the PPI of ROI A. So if ROI B is significantly (0.05) connected with ROI A using PPI of ROI A, my understanding is that this means the signal of B follows (and has) the same shape (i.e synchronised) with the A. So I cannot say why the PPI of B may or may not shows significant changes in A giving relative to the rest. 

- What is the best description then when performing PPI of ROI A that ROI A and B are: connected or correlated or can this statement said changes in A introduced changes in B..etc Or what is the acceptable way of explaining this? 

Many thanks

Aser

On 23 Oct 2015, at 15:03, "MCLAREN, Donald" <[log in to unmask]> wrote:

PPI doesn't measure the connection between two regions, for that you would want to use resting state data. PPI measures how the connectivity changes with a task.

If you find that for c1, the PPI model for region A has a significant change from baseline or another task in region B, then one may or may not find that for c1, the PPI model for region B has a significant change from baseline or another task in region A. The reason for this difference is that you don't know what all the sources of activity in region B. For example, region A may increase its connectivity to region B during task c1 relative to baseline; if region B is connected to many other areas as well, then the connectivity of region B to A may not change because B doesn't match the signal from A.

As to your last point - which again should be performed with resting state data since you are looking at the connectivity between regions and not changes with task. No that is not the case. If A and B are correlated at 0.5 and A and C are correlated at 0.5. This won't tell you how well B and C are correlated. 



Best Regards, 
Donald McLaren, PhD


On Fri, Oct 23, 2015 at 8:13 AM, Aser A <[log in to unmask]> wrote:
Dear Donald,


If I have two ROIs A and B and did a PPI using those two ROIs as seeds and say if I get a significant connection between those two regions as well. 

Do I have to expect that in either PPI of A or B and I should get B or A significantly connected respectively ? i.e. 

Also do I have to expect that they both should share a network (i.e. any region that is significantly connected to A should be without performing the analysis connected to B because A and B are connected ?

Thanks

Aser