I don't have any answers, but here is a small tidbit of information.
We acquired 1536 frames of fixation-only data in
two subjects. One of these subjects (our favorite) is blessed with the
ability to remain utterly motionless (as far as our realignments programs
can tell). The other squirmed more than my 2-year old nephew, yielding data
that we considered unusable because of motion artifact. I tried to
test the hypothesis that motion artifact is a contributor to the 1/f
portion of the spectrum. To this end, I computed a noise spectrum for each
of the 12 runs (128 frames per run) for each subject. I then averaged the
spectrum across runs, and then averaged the spectra across voxels
in visual cortex activated by a flickering checkerboard (activated voxels
passed a Bonferroni correction). This should give an average spectrum
across voxels that are primarily gray matter, and excludes ventricles and
large white-matter tracts. I expected the squirmy subject to be noisier.
In fact, the spectra lie on top of each other with an uncanny degree of
precision. Both the lowest and higest frequencies matched very well, with
the squirmy subject having a few more squiggles in between.
Apparently noise due to motion largely cancels when averaging across
voxels, and the residual 1/f noise is due to some other effect.
John
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John Ollinger
Washington University
Neuro-imaging Laboratory
Campus Box 8225
St. Louis, MO 63110
http://imaging.wustl.edu/Ollinger
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