Hi,
Our experimental task involves an ABAC pattern that is repeated four times.
Each condition is 32s long so the total time for ABAC pattern is 128s. As
recommended by the FSL software we chose a temporal filter is 128s i.e., .007
Hz. The FSL documentation states that a high pass temporal filter is effective
in filtering low frequency scanner drift and physiological noise like cardiac
and respiratory noise. Although cardiac and respiratory effects may occur at
higher frequencies than the signal of interest, the temporal sampling
frequency (the scanner’s TR setting) can interact with these high frequencies
resulting in an aliasing of the signal to a much lower frequency. It is
expected that this aliased lower frequency will be filtered out by the high
pass filter along with the low frequency scanner drifts. I calculated that
hypothetical cardiac rates of 1 or 2 Hz, with a sampling rate of 2 sec
(TR=2sec) will alias down to 0 Hz resulting in nothing really to filter out.
However, if the cardiac rate is around 1.2 Hz, with a TR of 2, it aliases down
to a frequency of .2Hz. In such a case our high pass temporal filter of .007
Hz will not be effective in filtering out cardiac noise. If this is the case,
should we be concerned about cardiac noise aliasing into our signal of
interest?
Thanks,
Appu
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