Dear Peter,
Thank you very much for the reply. I have two questions then:
1) Do you think the design we tried is wrong? We did it that way because we are particularly interested in comparing each polarity against sham separately, because anodal and cathodal stimulation should exert very different (and, in theory, opposite) effects
2) Either with this new design or with the one we tried, would the procedure I described in the previous email be correct if we then want to look at the direction of the interaction (as in increase or decrease of excitation or inhibition)?
I'm referring to this from my past email:
"We tried performing 6 separate PEBs for each polarity and time, and saving the updated DCMs like this: [PEB,rGCM] =spm_dcm_peb(GCM,M,fields) We then averaged the values in GCM.Ep.A across participants and plotted them for each node of our A matrix to see the direction of change (e.g. changes between anodal and sham pre and post stimulation), but it seems like these values are not exactly concordant with what we see in the interaction results…is there a better way of doing it?"
(So essentially I'm trying to understand how to interpret the final results)
Many thanks for your help!
Best regards,
Sara
On 28/10/2019, 13:00, "Zeidman, Peter" <[log in to unmask]> wrote:
Dear Sara
I think you should simplify this so that you've just got one PEB-of-PEBs (I'll call that the 3rd level). To recap, you've got three 2nd level PEBs (which I'll order anodal, cathodal, sham below), and from each one you're getting two parameters: mean of pre/post, and differences between pre/post.
The 3rd level regressors for the between-subjects design matrix will be: commonalities [1 1 1]', TMS vs sham [1/3 1/3 -2/3]' and anodal vs cathodal [1 -1 0]'. This design matrix will automatically be replicated over 2nd level parameters, giving you the following regressors in your 3rd level PEB:
1. Commonalities of all 3 groups' pre/post mean (i.e. overall mean)
2. Commonalities of all 3 groups' pre/post difference (i.e. main effect of time)
3. TMS vs sham on the pre/post mean (i.e. the main effect of TMS vs sham)
4. TMS vs sham on the pre/post difference (i.e. interaction of TMS and time)
5. Anodal vs cathodal on the pre/post mean (i.e. main effect of anodal/cathodal)
6. Anodal vs cathodal on the pre/post difference (i.e. interaction of anodal/cathodal and time)
Hopefully this will be easier to interpret. Please let me know if anything remains unclear.
Best
Peter
-----Original Message-----
From: SPM (Statistical Parametric Mapping) <[log in to unmask]> On Behalf Of Sara Calzolari
Sent: 26 October 2019 10:39
To: [log in to unmask]
Subject: [SPM] Interpreting interaction results (PEB framework)
Dear DCM experts,
I have a question regarding the interpretation of PEB results when dealing with INTERACTIONS.
Here’s the context of the analysis:
We’ve applied 3 tDCS sessions to each participant (anodal, cathodal and sham) and scanned them PRE and POST tDCS stimulation in each session.
We wanted to compute the interaction of time (pre/post) and polarity of stimulation (anodal vs sham and cathodal vs sham).
Therefore, in our analysis we’ve performed:
3 first-level PEBs (one for anodal, one for cathodal, one for sham stimulation), in which the contrast is [-1 1] for scans Pre and Post stimulation
2 second-level PEBs, one for the contrast anodal vs sham [1 -1] and one for the contrast cathodal vs sham [1 -1] that take as input the first-level PEBs
The results of these two final PEBs should therefore be the interaction of time and polarity.
Now, we would like to interpret these results, and in particular we would like to understand the direction of the resulting changes in each node (e.g. result in one node shows a decrease in connectivity - is this a decrease in excitation or a decrease in inhibition?) I know we can’t infer that information from these results only, so my question is: how can we get information regarding the direction of these interaction effects?
We tried performing 6 separate PEBs for each polarity and time, and saving the updated DCMs like this: [PEB,rGCM] =spm_dcm_peb(GCM,M,fields) We then averaged the values in GCM.Ep.A across participants and plotted them for each node of our A matrix to see the direction of change (e.g. changes between anodal and sham pre and post stimulation), but it seems like these values are not exactly concordant with what we see in the interaction results…is there a better way of doing it?
NB: it’s resting-state data, so we have results for the A matrix only.
Many thanks for your help!
Best regards,
Sara
