Hi Doug and Freesurfers,
I am using fcseedcor to compute the correlation between two time courses for each subject in my group. The command that I run is as follows:
fcseedcor -s $subject -fsd resting -seed seed1.dat -seed seed2.dat -xreg global.waveform.dat 1 -xreg vcsf.dat 5 -xreg wm.dat 5 -xreg mcprextreg 6 -hpf .01 -lpf .08 -nskip 4 -o cor_s1s2.dat
My first question is about the size of the vcsf.dat and wm.dat files. They seem to be (N+1)xN matrices, where N is the number of timepoints in the signal. Does this mean there are N+1 components for each time point (i.e., N+1 potential regressors to add)? What do they exactly correspond to and how do they relate to the average signal from the ventricles, CSF, and white matter?
My other question is about the number of components to include for vcsf.dat and wm.dat. I have seen 5 recommended in examples, however, five components seem to explain very different amount of variance in different subjects and if I change this number for a given subject, I see substantial changes in the resulting correlation value. I would appreciate any suggestion on how to select an unbiased value for the number of components to include. Could including up to the component at which fixed percentage of cumulative variance is explained be a solution?
Many thanks for your help!
Best, Hamdi