Hi,
On Sat, Sep 12, 2015 at 8:27 PM, Yann Quidé <[log in to unmask]> wrote:
> Dear all,
>
> We are conducting a VBM analysis (using the VBM8 toolbox) and decided to use a ROI approach. We used Marsbar scripts to extract individual mean grey matter values for our ROIs.
>
> But what exactly represent this value? I initially thought it would extract the mean grey matter volume, but the numbers I get do not seem to confirm that (seems to be in a 0-1 range). Does it rather represent the grey matter density or concentration within the ROI mask? What would be the unit, % of grey matter?
Are you extracting data from the output of an SPM classification? In
that case the images will either be (at each voxel) the probability of
the given tissue type, or that probability modulated by the Jacobian.
So, if you are extracting from a GM volume the voxel values will be
probabilities that that voxel is in GM.
Best,
Matthew
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