Hi,
My question refers to selecting VOIs at single subject level, that lie near to a set of co-ordinates already identified at group level. I also constrain this within an anatomical structure.
Any further input from the list greatly appreciated.
Thanks,
Liam
________________________________________
From: Zeidman, Peter [[log in to unmask]]
Sent: 28 March 2014 09:54
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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