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The cutoff frequency is generally defined as the frequency where the signal is down by -3 dB (relative to the passband). So at the cutoff frequency some of the signal is removed (half power) but signal is still present. If you are not using a brickwall filter (very high order), then you will see the amount of signal removed gradually increasing as frequency decreases. On Mon, Sep 25, 2017 at 9:04 AM Wiktor Olszowy <[log in to unmask]> wrote: > Dear SPM community, > > Recently I have analysed the code of the high-pass filter in SPM > (spm_filter.m). The order of the filter is determined via: > > n = fix(2*(k*K(s).RT)/K(s).HParam + 1); > > where fix is the matlab's function that rounds towards zero, k is the > number of time points/volumes, whereas K(s).RT is the repetition time. My > understanding is that if e.g. number_of_volumes*TR=256 seconds and the > cutoff has not been changed (from 128s), then the signal at 1/128 Hz will > be removed (and at lower frequencies), as 256 is a multiple of 128 and > there is no rounding in the above formula. However, the SPM manual says: > "The default high-pass filter cutoff is 128 seconds. Slow signal drifts > with a period longer than this will be removed.". Do I misunderstand sth or > is there a small inaccuracy in the manual? For the default of 128 it does > not make a large difference, but when one is interested in making the > cutoff paradigm-dependent, it makes a huge difference if the cutoff > frequency is filtered out or not. > > Best wishes, > > Wiktor Olszowy >