When is it necessary to use gPPI instead of PPI?

I conducted a traditional PPI analysis in SPM. However, the group analysis results of this PPI appear odd. The whole-brain activation pattern of the PPI interaction regressor closely resembles the activation pattern of the total of all conditions in my experiment (contrast: condition 1+ condition 2+ condition 3+condition 4+ condition 5 + condition 6). I’m uncertain what went wrong. Could this be due to the necessity of using gPPI in this case? Any insights would be appreciated.

Experiment setup:
• Conditions: 6 (condition 1, 2, 3, 4, 5, 6)
• Contrast for testing experimental manipulation:
(condition 1+ condition 2+ condition 3) – (condition 4+ condition 5 + condition 6)

Used standard PPI with a weight matrix: [1 1 1 -1 -1 -1]
The code is:

	 abatch{1}.spm.stats.ppi.type.ppi.u =       [1 1 1
	                                           2 1 1
	                                           3 1 1
	                                           4 1 -1
	                                           5 1 -1
	                                           6 1 -1];

Thank you !

Hi @sata,

From the gPPI paper (Mclaren et al., 2012):

Second, the standard implementation described by Friston et al.(1997) and Gitelman et al. (2003), using a psychological vector of A or a psychological vector of A–B, does not span the space of all conditions and as such is potentially limited to simple experiments with only one or two conditions, respectively, or experiments that can be collapsed into two conditions for analysis.

Based on the conclusions of the paper, it seems like gPPI would always be preferred to traditional PPI, and especially when the experiment does not conform to the limitations described above.


Thanks a lot! It’s so kind of you.