> > Also, I don't believe that it is the case that using the middle slice
> > affords any greater "accuracy" in making the correction. The slice
> > correction routine makes the assumption that no meaningful power is present
> > in the data above the Nyquist frequency. (For which there is some empirical
> > support). Given this assumption, all shifts in time using
> sinc-interpolation
> > are equally valid.
>
>I don't think that there are any theoretical reasons why there should not
>be any power above the Nyquist frequency. A typical HRF to a delta function
>will contain frequencies higher than the frequency of the acquisition of
>sucessive scans.
Dear Geoff & John,
Just another comment/question on sinc-interpolation. When you say
the routine assumes no meaningful power is present above the Nyquist
frequency, does this mean you are not doing anything, or do you pre or
post-filter the data to force it to fit this assumption? When we apply a
sinc correction to our data, we generally will window the spectrum. If we
do nothing, a high frequency ringing (sawtooth) will generally be
introduced into the time series which are shifted due to the small bit of
high-frequency power. And it will be dependent upon how much it is shifted
in time. The SPM algorithm (only the one file) I saw doesn't do any
filtering...but this is probably handled by low-pass filtering downstream I
assume...eh?
Comments?
Vince
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Vince D. Calhoun, MA, MS, PhD Student (UMBC), Research Engineer
WORK: Johns Hopkins Medical Institutions, Psychiatric Neuro-Imaging
600 N. Wolfe St., Meyer 3-166, Baltimore, MD 21287-7362
VOICE: (410) 955-7861, FAX: (410) 614-3676
WWW: http://pni.med.jhu.edu/vcalhoun
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"An author [or researcher] should never conceive himself as bringing into
existence beauty or wisdom which did not exist before, but simply and
solely as trying to embody in terms of his own art some reflection of
eternal Beauty and Wisdom...And always, of every idea and of every
method the Christian will ask not, 'Is it mine?' but 'Is it good?", CS Lewis
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