Dear Ruth and PPI experts,

 

       I am a PPI user, not an expert. There are two points I concentrated on. One is that you convolved the psychophysiological interaction with HRF. I don’t think it’s necessary. Maybe you can read the SPM manual and there is an example which treat the psycholphysiological interaction as a regressor instead of a condition.

       The other is the method you used to extract VOIs. I am always puzzled by the most appropriate approach to define VOIs when I use DCM and PPI. You used an extremely low threshold and an extremely big sphere. Maybe that won’t miss any information, but redundant information, even wrong information, will be included into your ROIs. It’s a dilemma. If I use a low threshold or a long distance between individual peak loci and group-level peak loci, I won’t be confident in the reliability of information.  However, if I use a high threshold, I won’t find so many subjects with activated VOIs (maybe 20 in 30).

       On more question, what is the relationship between the individual peak voxel and the group-level peak loci?

 

Best Wishes!

 

   Lu Feng

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Dear PPI experts,

 

I am currently working with Psychophysiological interaction (PPI) analyses and have some question about my applied method so far.

 

I did the following: 

To identify the left IFG activation for each participant we examined the F contrast of all conditions at p<0.999 uncorrected, to obtain similar amount of voxels in or VOI across all subjects.  The deconvolved time series from a 16 mm radius sphere around the individually defined peak activated voxel within the left IFG was extracted (30 participants). The PPI was calculated as the element by element product of the left IFG time series (the first eigenvariate from all voxels' time series) and a vector coding for the effect of task. These products were subsequently re-convolved with the hemodynamic response function (HRF). The interaction term was then entered as a regressor in a first level model together with the time series of the left IFG and the vector coding for the task effect. The models were estimated and contrasts generated to test the effects of positive PPI. These contrast images for the PPI effects were then entered in a second level analysis.

 

These choices were based on he SPM forum list, were I understood the following:

“you should think of the contrast you use to display the activations as simply a window to which voxels will be extracted. SPM goes to the xyz locations of the active voxels in your VOI, gets the actual data from your processed images and then adjusts the data for any effects of interest you specify. The contrast you use to display activations merely identifies the voxels to extract data from it does not affect the data that are extracted.” By Darren Gitelman

 

Therefore I used a absurdly low threshold of p<0.999 to capture all activity in the VOI and to make sure that the below mentioned advise could be assured.

 

 “Always use ALL the voxels in the ROI. You don't want to compare a subset of voxels in one subject to a subset in other subjects, then you might be getting apples and oranges. This is of course a bigger issue if you were to use a larger ROI.” By Donald McLaren

 

I do think that a sphere of 16 radius is quite large, but when using a smaller radius I could not detect activity in this region for each participant. This would be a problem because excluding participant form this analysis would be a waste as well.

 

The results from this analyses look sensible, but I am unsure if my method is decent enough to pursue writing it down.

 

Best wishes

Ruth