Hi Donald,

Thanks for your answer. It paints a very clear picture for me. I'm somewhat getting the gist of what regressors are and how to work with them, and I started reading parts of the SPM book by Friston et al. which is a good source.

I'll also paste the answer to my question I got from Chris Bishop, which did not make it to the list (I hope he doesn't mind):

"Because time starts at 0.  If you use time, then  time starts at zero. If you use scans, the number of scans is multiplied by your TR to figure out when the event occurs. Thus, scans must start at 0 as well."

Regrads,
Glad

On 07.05.2012 19:32, MCLAREN, Donald wrote:
[log in to unmask]" type="cite">What I think is meant by the statement is that the onset times are in relation to the beginning of the scan. The first TR is collected at time 0. 
Thus, your onset times should be in relation to time 0. You do not need a trial with the onset of 0, in fact it would be better if there was some delay before the first trial. Your onsets are just a list of onsets - not a column of 1s and 0s.

If you have a regressor, then it needs to be the number of TRs in length.

If you have a parametric modulator, then it needs to be the number of trials in length.

Hope that helps.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Mon, May 7, 2012 at 4:01 AM, Glad Mihai <[log in to unmask]> wrote:
Dear SPM fans,

can someone explain to me and if possible guide me to some literature as to why the onsets need to start at 0 (zero) when entered in SPM?

Also, if the onsets start at zero and are written into the design matrix, should regressors that I input manually (not the motion parameters rp_*.txt file) also start at 0? If they don't then I suppose they won't line up with the onsets, since both of them are columns in one matrix.

Cheers,
Glad

--
Glad MIHAI, M.Sc. Biomedical Physics
Funktionelle Bildgebung
Uni-Klinikum Greifswald
Walther-Rathenau-Straße 46
17489 Greifswald
www.baltic-imaging-center.de


-- 
Glad MIHAI, M.Sc. Biomedical Physics
Funktionelle Bildgebung
Uni-Klinikum Greifswald
Walther-Rathenau-Straße 46
17489 Greifswald 
www.baltic-imaging-center.de