Hi, experts,

 I am trying to do a PPI analysis for a working memory task dataset. The working memory paradigm was a block-design letter n-back task, which contained a resting (fixation) condition and two stimuli conditions (0-back and 2-back). Stimulus blocks and resting periods alternated within the experiment with a total of four 2-back and four 0-back blocks.

I figure out two ways to extract VOI data for PPI analysis based on the SPM8 PPI manual, but I am not sure which one is right. Could do you help me to check them?

Method 1：

For the GLM analysis (Design setup and estimation), I am considering to define three conditions: fixation, 0-back and 2-back in the design matrix, and define two T contrasts as follows:

For the first T-contrast, Name is Working memory, and the T contrast vector is: 0 -1 1 (Note the order of the conditions in the design matrix is: fixation, 0-back and 2-back).

For the second T-contrast Name is Cognition, and the T contrast vector is: -2 1 1.

Then, I will find a PFC region (PFC1) that is activated during task and shows greater activation during 2-back condition that during 0-back condition when I check the result using Cognition contrast with the Working memory contrast as an inclusive mask.

Next, while extracting VOI data, I will

(1) Select Cognition contrast, but do not use mask.

(2) Go to this PFC1 point

(3) Press eigenvariate

(4) Name of region [PFC1]

(5) Adjust data for [effects of interest]

(6) VOI definition [sphere]

(7) VOI radius(mm) [6]

In this way, when I create the PPI variable, the Input variables and contrast weights should be:  [2 1 -1; 3 1 1]

 

Method 2:

For the GLM analysis, I think that another way is to define two conditions: 0-back and 2-back in the design matrix, and define only one T contrast as follows:

Name is Working memory, and the T contrast vector is: -1 1 (Note the order of the conditions in the design matrix is: 0-back and 2-back).

Then, I will find a PFC region (PFC2) showing greater activation during 2-back condition that during 0-back condition when I check the result using the Working memory contrast.

Next, while extracting VOI data, I will

(1) Select Working memory contrast.

(2) Go to this PFC2 point

(3) Press eigenvariate

(4) Name of region [PFC2]

(5) Adjust data for [effects of interest]

(6) VOI definition [sphere]

(7) VOI radius(mm) [6]

In this way, when I create the PPI variable, the Input variables and contrast weights should be:  [1 1 -1; 2 1 1]

 

I have highlighted the differences between the two ways.

Any help is greatly appreciated.

 

Many thanks,

Yuan