Hi Philipp,
I think I got you wrong (or you got me wrong): I was not talking about any F-testing, just about the multi-session/subject tensor-ICA on FMRI time series. Here, if you have one subject with an S-mode of 1 and another with -1 you may in fact assume that their time-courses OR activation/deactivation maps behave the opposite way (i.e. are anticorrelated, as you say). However, for resting state data you cannot assume that the FMRI time-series are temporally consistent across subjects whereas the powertransformed time-series may be consistent. Thus, either you run melodic in the full tensor-ICA mode on powertransformed data or in the multi-session temporal concatenation mode on the original time-series. The latter is recommended on the web, at least if you primarily want to identify the networks.
Does that help? If not, I guess Christian will jump in...
Cheers-
Andreas
________________________________
Von: FSL - FMRIB's Software Library im Auftrag von Philipp G. Saemann
Gesendet: Fr 21.11.2008 10:42
An: [log in to unmask]
Betreff: [FSL] Meaning of S-modes in resting network group melodica
Hello Andreas,
thanks a lot for your answer on the F-test issue.
If I understand it correctly, very low negative values would indicate
strong "similarity" between the individual time course
and the first eigenvariate time course, but in an anticorrelated manner. This
would somewhat imply that the negative S-modes need to be
flipped for testing against zero AND for comparison between subjects to come
back to a parametric scale from "low strength" to "high strength" for resting
analyses.
(Take for e. g. the default mode network that is robustly detected in
practically any group)
My question now is: is the first eigenvariate (in time concatenated group
analysis) constructed from somewhat time locked signal courses or from the
unchanged mix of raw courses as they are in the group data?
If not, then it seems at first sight that generation of the subject modes and
their parametric value are somewhat not meaningful for resting data at all.
So, e. g. correlation between network "strength" (from the mode values as
they come from FSL) and a parametric behavioural measure seems obsolete to
us. However, as we intended to do so, we are stuck here...
Does anybody have an explanation of what the S-mode (across subjects of
ONE component) as coming from FSL in the time-concatenated group mode
really indicate? In published (users') work it says something like "strength of
BOLD fluctutations".
Again thank you very much in advance for any hints on this,
Philipp
Max Planck Institute of Psychiatry
NMR Research Group
Munich
|