With a TR of 8s it is very unlikely that your physiological components
will be located at a single frequency. As the cardiac pulsation and
respiration are non-stationary (their frequencies vary over time), the
aliased noise will look like white noise in a spectrogram. So what you
see in your fft is most likely due to scanner drift. Scanner drift can
be removed using the SPM high-pass filter. For further information see https://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind00&L=SPM&P=R111434
or have a look at the paper bellow.
Hope this helps
Torben Ellegaard Lund
Assistant Professor, PhD
The Danish National Research Foundation's Center of Functionally
Integrative Neuroscience (CFIN)
Aarhus University Hospital
Building 10G, 5th floor
8000 Aarhus C
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Lund et al. Non-white noise in fMRI: Does modelling have an impact?.
NeuroImage (2005) pp. 13
Den 02/07/2009 kl. 08.23 skrev Woo-Suk, Tae:
> Dear SPM experts
> I have an aliasing problem in my FFT with time-course data.
> TR of EPIs was 8 sec. and it seem that there were aliasing
> components in frequency domain
> graph related with low frequency drift, respitory and cardiac cycles.
> Can these aliasing components could be controlled by bandpass filter?
> Woo-Suk, Tae
> Chuncheon, Korea