Dear Geraint and others,
I have a comment about a small part of one of Geraint's recent
replies which regarded low frequency fMRI noise:
> change is very low. Unfortunately many confounding signals of little
> interest (physiologically aliased signals, drifts in scanner gain and so
> on) also change at low frequencies.
Though I certainly agree that there is more noise at lower frequencies,
I have not seen any data to support their explanation in terms of either
a) drifts in scanner gain
or
b) physiologically aliased signals.
Though both of these seem reasonable a priori as explanations
for low frequency noise, I have not found any evidence to support either
in my own data or in the literature. In fact there is even some evidence
to refute them.
In terms of evidence
against model a), a drift in scanner gain would predict a correlation between
the whole brain signal and variability in signal across space. I looked
at this and saw no such correlation (has anyone else looked at this?).
Model b) would predict no (or at least a decrease in) low frequency
noise in non-physiological scanned objects. But, low frequency noise
has been observed in water phantoms that looks very similar to that seen in
human subjects. Furthermore, cardiac and respiratory rhythms have a strong
periodic component that would therefore not alias into a 1/f noise
spectrum.
I raise these issues because it would be wonderful if an engineer
or MRI physicist could do some research into understanding
the low frequency components of fMRI noise better. Or perhaps, some has
already been done, and it just has not filtered its way into the mainstream
neuroimaging literature?
Sincerely,
Eric
Eric Zarahn
University of Pennsylvania
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