What is it that you are trying to show? That subjects with a
stronger/weaker pathway have some difference in behavioral scores?
Honestly, I might correlate the waytotal values themselves (if you are using
at least one waypoint mask) with the behavioral score. I am not clear that
a voxelwise analysis is really appropriate on tractography data, because you
are inferring on the most probable location of a single connection that you
suspect exists from a priori knowledge. There are some further difficulties
with using the waytotal, in that it might be effected by things like subject
motion that create differences across subjects but affect all pathways in
the subject, so it might actually be more correct to also have a control
pathway that you do not expect to vary meaningfully with the behavioral
measure across subjects and take the ratio of that to the pathway of
interest as your variable.
I would be interested in hearing people's thoughts on the above, as it is
something I am considering doing myself.
Peace,
Matt.
-----Original Message-----
From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On Behalf
Of Massieh Moayedi
Sent: Saturday, July 31, 2010 8:03 PM
To: [log in to unmask]
Subject: [FSL] Correlating behavioural findings to Tractography results
Hi Everyone,
We've been struggling for some time to come up with an appropriate method to
correlate a behavioural measure to probabilistic tractography. Sorry if the
post is a bit long...
There have been several options offered on the FSL listserv...
In so far, I've found that we can:
1) take each subject's average tractogram probability and correlate to the
behavioural measure
2) take the seed masks # voxels (x 5000) as a denominator, and calculate the
ratio that reaches each target
3) Use the waytotals as a denominator for each subject, and calculate the
ratio that reaches each target
(2 and 3 are basically the same)...
I did a search in the literature to see how other groups are dealing with
the issue, and found a method published in Neuroimage back in 2007 (Cohen et
al), and was hoping to get your views on it...
To correlate individual differences in amygdala-seeded tractography with
behavioral performance, we correlated across subjects, at each voxel,
tractography results with the behavioral results (average trials until
learning criteria, probability of loss/switch). Due to inter-subject
variability in the spatial distribution of tracts, we included in the
analyses only voxels for which at least eight subjects had non-zero tract
estimates. This resulted in 7511 voxels being tested (.0285% of 263,340
brain voxels). Because this may have resulted in non-normal distributions
over voxels, we used the Statistical non-Parametric Mapping (SnPM) toolbox
in SPM5 ([Holmes et al., 1996] and [Nichols and Holmes, 2002]). SnPM uses a
non-parametric comparison procedure, in which pseudo-t-values are calculated
based on randomization/permutation testing; it is therefore more robust than
the standard general linear model in cases of few degrees of freedom or
non-normally distributed data. To identify significant regions, we used an
uncorrected threshold of p < .01 and a cluster threshold of at least 6
contiguous voxels. We believe this to be an appropriate statistical
threshold because (1) although it is one order of magnitude more liberal
than the standard whole-brain threshold of .001, there are two orders of
magnitude fewer voxels; and (2) all voxels are already defined as being a
priori of interest by the output of an orthogonal analysis (that is, all
voxels are identified by amygdala-seeded tractography, the results of which
could not have been influenced by functional connectivity or behavioral
performance). This is also the default setting for uncorrected tests in
SnPM. One subject had corrupted diffusion data, and was not included in
these analyses.
Specifically, I was under the impression that values in individual voxels
were meaningless- and that correlating them to a behavioural measure would
not provide a physiologically relevant finding. What does it mean if a
focal probability of a tract fluctuates with behavioural finding? Doesn't
the target of the tract- and the probability of reaching it make a better
measure?
Your insight in this dilemma is greatly appreciated.
Regards,
Massih
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