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hi Satrajit

it's true, sorry,  I meant that the relative contribution to the signal of the aliased frequencies is higher at the higher frequencies, in the sense that a smaller physiological oscillation is more likely to be lost at these frequencies  (this is clearly seen in the paper John sent for spectrograms at  grey matter and a location in the jugular vein).

On Mon, Mar 19, 2012 at 8:09 PM, Satrajit Ghosh <[log in to unmask]> wrote:
dear enzo,

it is customary to look at that range because higher frequencies may be contaminated due to aliased cardiorespiratory artifacts (as well as with high frequency motion artifacts in the borders of the brain).

i would expect any aliasing to affect all frequencies, especially if some of the really slow TRs are being used. from a signal processing perspective, this would be true at any TR that's not capable of sampling twice ( and practically speaking 2.5 times) the fast changing artifactual signal.

cheers,

satra