Hi Laura,
On 5/19/05, Laura Mancini <[log in to unmask]> wrote:
>
> Dear SPMers,
>
> I am using SPM2 to analyse some fMRI data, and I have got a doubt on the
> choice of a couple of parameters when doing the
>
> fMRI/specify data
> SPM asks me:
> "remove global effect", and the choice is 'none' or 'scale'.
Global effects scaling is a remnant from the PET days. As Cyril
mentioned, you can read up on the different issues surrounding it in
the archives; however, my understanding of the current thinking is
that for most fMRI designs you should not scale globally.
> Also, when SPM asks for the highpass filter, and I choose 'specify' the
> default value is 128, but is there a way of choosing an optimum parameter
> for the experiment considered?
That is calculated as a default (i.e. the number 128 will change
depending on the nature of your experiment), but it is not a "hard
coded" number. It is a bit complicated the way that number is
calculated (and a lot of fun to figure out looking at the matlab
code!) so unless you have specific quantitative reasons for using
another number, I would stick with that one.
Good luck!
Max Gunther
Clinical Psychology
University of Georgia
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