PS My colleague has just informed that the explained variance can be found using the following code snippet:
load VOI_xxx.mat;
100*xY.s(1)/sum(xY.s)
Best,
Peter.
> -----Original Message-----
> From: Zeidman, Peter
> Sent: 28 March 2014 09:55
> To: 'Mason, Liam'; [log in to unmask]
> Subject: RE: VOI thresholding queries
>
> Hi Liam,
> Could you clarify whether you want to make inferences at the single
> subject or group level? And how is your ROI defined - is it based on a
> group activation? Or single subject activation? Or anatomical
> structure?
>
> Note that if you're making inferences at the group level, you would not
> necessarily expect significance at the single subject level.
>
> The variance explained gives you a sense of how much of the signal you
> have captured in your principle component (ROI) - it might be that you
> are perfectly modelling 5% of the signal, but there are more
> substantial processes going on that are not included. You'll generally
> find it varies as a function of the size of your ROI. It's shown in the
> GUI, but I'm not sure if it's stored.
>
> Best,
> Peter.
>
> > -----Original Message-----
> > From: SPM (Statistical Parametric Mapping)
> [mailto:[log in to unmask]]
> > On Behalf Of Mason, Liam
> > Sent: 27 March 2014 15:14
> > To: [log in to unmask]
> > Subject: [SPM] FW: VOI thresholding queries
> >
> > Hi there,
> >
> > Can anyone help with the following VOI queries?
> >
> > First, I am having difficulty deciding on a threshold for individual
> > subject VOI - is there a trade-off in terms of 'quantity' (number of
> > voxels included) and their 'quality'? Where poor quality might mean
> > including voxels with spurious or artifactual activations.
> > Practicaly, thresholding with P=.1 fails to find any voxels for a
> > substantial number of subjects, so I will need to relax this. P=.5
> (or
> > even P=1) is commonly used but seems too relaxed(?) For subjects
> where
> > a robust/non-artifactual peak is identified at e.g. P=.1, won't
> > dropping this threshold further increase the chance of finding an
> > artifactual peak?
> >
> > Second, is the amount of variance explained by the VOI a good
> measure
> > of 'quality'? In which variable would this value be stored for each
> > subject (can't find it in any of the fields of xY returned by
> > spm_regions function)
> >
> > Any help would be much appreciated!
> >
> > Many thanks,
> > Liam
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