[caret-users] CARET: MFM ?

[David Van Essen]

Brian,

Your question gets at the heart of the differences between AFM  
(average fiducial mapping) and MFM (multi-fiducial mapping), which is  
discussed in my 2005 Neuroimage paper.

In essence, the MFM value at a given node is the average of the value  
for the corresponding nodes in the 12 fiducial subjects, and the  
positions of these nodes is inherently variable.  So the overall peak  
will typically be lower with MFM than AFM.   (The exception would be  
if the average fiducial surface happened to miss the overall maximum  
in the volume, which is possible but seems not to occur often.)  But  
it's not any simple linear transform.

The main advantage of MFM is that it gives you a better estimate of  
the location of the peak(s), and we recommend it if that is your main  
objective.  If you need to hang onto the values of individual voxels,  
then AFM is better.

If you're in a position to do surface-based analysis of individual  
subjects, then spatial resolution improves substantially.

Hope this helps.

David

On Oct 9, 2007, at 6:54 PM, bpiper at nsm.umass.edu wrote:

>
>
> Caret folks-- I'm just starting to get acquainted with Caret and am  
> enjoying the
> program.
>
> I've been mapping my zstat_thresh file from FSL onto: 1) the PALS  
> brain, 2) the
> AVG fiducial map, and 3) the multifiducial maps.  The images I get  
> from 1 & 2
> are
> identical with the same Zmax (3.6) but #3 has a Zmax that is much  
> lower (2.8).
> Is this supposed to happen?  By playing with the Zmin on the  
> display, it looks
> like there's some linear transformation going on for #2 vs. #3.
>
> thanks for any suggestions,
> Brian
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