On 04/28/2012 09:45 PM, Mahinda Yogarajah wrote:
> Dear Experts,
>
> I had 2 questions with regard to vertex wise analysis using qdec.
>
> 1) How should one interpret the dependent variables area/pial area and 
> volume when comparing 2 groups with qdec and a vertex wise (as opposed 
> to ROI based) analysis - or are they uninterpretable in this context ? 
> I read briefly about areal as an alternative measure - could someone 
> explain the difference, and how thickness, volume and area related to 
> one another ?
volume = area * thickness. The "area" of a vertex is the average of the 
triangles that surround it. For a group analysis, you can think of it as 
drawing a small circle around a vertex in fsaverage space and asking how 
big that circle is in each individual subject.
>
> 2) When running monte-carlo cluster wise multiple correction in qdec - 
> if one finds a significant cluster is it ok to view them within qdec 
> by highlighting the "mc-z ... " file in the analysis result box, and 
> then changing the minimum threshold to whatever value one selected for 
> the monte carlo simulation ?
No, the mc-z output has values equal to the cluster-wise p-value, so it 
is not appropriate to apply the vertex-wise threshold.
> The reason I ask is that the text ouput given by the monte carlo 
> cluster analysis gives different coorinates in the cluster table to 
> that output given when one clicks on the "find clusters and go to max" 
> button ...
Are you using the "abs" when you do the clustering? Is is possible that 
it found a negative peak in the table but qdec goes to the positive peak?
>
> Thanks.
>
> Mahinda