Stephen -
> Christian Buechel's post
>
> http://www.mailbase.ac.uk/lists/spm/1999-07/0154.html
>
> gave a formula for correcting an onset time, to account for SPM allowing a
> user-specified choice of which time point to sample regressors (fMRI_T0).
> Is this formula still applicable, or has spm99 now do this automatically
> with no user input?
That formula is still valid, and nothing has changed in SPM99.
We thought about linking the reference slice chosen in slice-timing
correction to the value of fMRI_T0, but decided against it because
some people do not use the slice-timing stage during preprocessing
(eg with long TRs, when the interpolation error may be large, or for
blocked designs, when model/slice timing differences are often
negligible).
> Minor related question: is there any reason that fMRI_T (the number of
> sample points per TR for e.g. the hrf) should be the same as the number of
> slices? E.g., if I have 24 slices and slice-timing correct to the middle
> slice, I think it should be OK to use other values of fMRI_T, as long as
> fMRI_T0/fMRI_T = 0.5. Is that right?
You are right. The value of fMRI_T could match the number of
slices, but doesn't have to - it is the ratio you describe
that is important. Basically, increasing the value of fMRI_T
may gave you more temporal precision, but the advantage is
likely to be negligible (unless you have a long TR, eg >4s)
and the computation of covariates will take longer.
Rik
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