Hi again,
I wondered if the two interpretations are correct.
Take into account that I’m not using the PPI to establish directionality, but I’m trying to interpret my PPI results according to the directionality reported in the previous literature.
I know that if I perform the reversed PPI (using the target region as the source, under condition B compared to condition A) is possible that I don´t obtain results in the source region (to my understanding, due to the variance in the target region and the source region may be not equal). Nevertheless, a reviewer told me that, for example, in a positive PPI, I can interpret both: first, the change in the region Y for any given change in the region X is higher under condition A. Conversely (and equivalently) the change in the region X, for any given change in the region Y is lower in condition A relative to condition B.
Following this discussion in SPM list:
I think that the reviewer comment is correct (if not, please let me kwon).
Given that I don’t have a main effect, but a correlation with personality scores I want to be sure that my interpretation is correct.
To be actually sure about my conclusions, I have some questions:
The first question is about the correlation between the PPI term and an external variable. I do not have a significant main effect between two regions in a PPI analysis, neither positive nor negative. If I do a correlation with the contrast images of the PPI term extracted from a condition A>B contrast, and I get a negative correlation with personality scores... what does it means? For example, may I conclude that the personality factor reduces the influence of the source region to target region activity under the condition A in comparison with the condition B?
In relation to the first question, I have a second question: is this interpretation correct independently of whether the main effect for PPI in the target region (tested with one sample T test) is positive, negative or nothing (as in my data)?
Finally, the third question comes from the interpretation regarding the previous literature. May I interpret my data in this way: the higher the personality score, the higher the contribution from Y to X activity in condition B relative to condition A? Or, in other words, the higher the personality score, the lower the contribution from Y to X activity in condition A relative to condition B?
Thank you very much in advance.
Victor.
Are you asking which interpretation is correct: "I interpret this positive correlation as the higher the psychological score, the higher the contribution from Y to X activity in condition B relative to condition A. Or conversely, the higher the personality factor, the lower the contribution from Y to X activity in condition A relative to condition B. "?
The only way to know this would be to test this using Y as a seed region. Remember that PPI X->Y is not equal to the negative of PPI Y->X.
While PPI can't tell you direction, it can test hypothesis about effects.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Tue, May 29, 2012 at 10:31 AM, Víctor Costumero Ramos <[log in to unmask]> wrote:Dear SPMexperts
I need some help with the interpretation of my PPI results.
In my task, I perform a PPI analysis from seed region X (source) under condition A compared with condition B.
When I perform a one sample T test with the interaction term I don´t find any significant result (testing for both positive and negative possible effects). Nevertheless, in a whole-brain voxelwise multiple regression analysis I found that a personality score display a negative correlation with the PPI term for region Y. Previous studies have interpreted similar results as the higher the personality factor, the lower the X-Y connectivity for the condition A relative to condition B (i.e. Cremers et al. 2010)
Due to previous literature I assume that is more plausible that the direction of connectivity goes from Y to X. Following previous discussion in the SPM list the PPI can be interpreted as the contribution from Y to X but with the reverse sign.
Then, if I interpret that the direction goes from Y to X the sing of my ppi term will be reversed so the correlation with the personality factor becomes positive. I interpret this positive correlation as the higher the psychological score, the higher the contribution from Y to X activity in condition B relative to condition A. Or conversely, the higher the personality factor, the lower the contribution from Y to X activity in condition A relative to condition B.
My question is: Is there any error in the rationale of the previous paragraphs? May I get to both conclusions?
I appreciate any advice.
Thank you in advance.
Victor.