Sebastian Moeller wrote:
Hi Doug,
On 11. Jul 2006, at 19:56 Uhr, Doug Greve wrote:
Hi Sebastian,
those are scaled to a global mean of 1000 (so you can think of them as tenths of percents) -- but it's global, not local.
So (I am a bit dense, sorry for that), I could just use thosevalues as basis to directly (multiply by 10 ;)) compute the percent change induced by the different conditions. Without having to resort to the h-offset files, and ugly edge effects.
Yes, you can multiply by 10 to get percent.
Yes, you can divide the h volume by the h-offset to give you local percent signal change.
What is the difference between local and global in this matter.
The global mean will contain everything in the brain - white matter, CSF, as well as gray matter, and so it might not be the right baseline to use as a percent signal change.
This makes me a little uncomfortable because you can get some funny business at the edge of the brain.
A, how does the global scaling avoid that, or is the division theproblem (say values close to zero)?
The effects on the edge will be averaged with the rest of the brain with the global method.
doug
ahoi & thanks for the information & the fine tool Sebastian
doug
Sebastian Moeller wrote:
Hi Doug, hi List,
I got a question regarding the h volumes inside the analysis sub directories. The content of those file are the regression coefficients for each voxel (XY planes) for each condition (Z stack). Supposedly those estimates are corrected for the noise and would make really nice descriptors of the overall activation of voxels/rois for the given conditions. Now, here is my question, what is the unit of the values in the maps? And is there a way to transform those values into something ala percent change? Would dividing a XY voxel of a slice though the matching voxel value from the h-offset volume result in percent of total?
ahoi Sebastian
-- Douglas N. Greve, Ph.D. MGH-NMR Center greve@nmr.mgh.harvard.edu Phone Number: 617-724-2358 Fax: 617-726-7422
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