Hello,
I have a question regarding global normalization for VBM. I've been following John Ashburner's SPM Course 2010 VBM Tutorial, with absolute thresholding at 0.1. I've been using GM volume (from the native space segmentation volumes) calculated using get_totals.m. When using Proportional Normalization to the globals I get an error saying "no in-mask voxels". To my understanding the proportional normalization divides the voxels by the global prior to masking, so with modulated data (voxels running from 0 to 1) and GM volumes (running around 800 mm^3), the data clearly does not survive the absolute masking. As John's manual suggests both proportional normalization to either TIV or GMV, and Absolute thresholding, for modulated data, I'm wondering if I'm simply using the wrong values for the globals.
Any help would be kindly appreciated!
With thanks,
Jared
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