Dear Christopher,
both time scales are treated in the same manner in SPM. Which one you want
to use depends mostly on your stimulus presentation software: some
software will log events and scanner triggers in seconds, other software
may synchronise to your scanner trigger and log in units of your triggers.
Some things to remember:
* Time _always_ starts at zero - even if units of design is scans.
* You may specify onsets in fractions of your units (if your units are
scans, then 0.5 would specify an onset at 0.5*TR seconds).
I don't think negative onsets will work, but I may be wrong here.
Volkmar
On Tue, 28 Aug 2007, Christopher Benjamin wrote:
> Dear SPM experts,
>
> I am struggling to understand (or even find discussion of) the impact of
> using seconds or scans when specifying an efMRI model (spm5). Can anyone
> point me to find a clear discussion of this, and the effect on SOTs? With my
> limited knowledge the spm5 efMRI example seems counterintuitive - 'Units for
> design' is 'scans', but the SOTS are in seconds.
>
> Also, is possible to specify the first onset time in a SOT train as a
> negative value?
>
> Any assistance much appreciated.
>
> Christopher Benjamin
>
--
Volkmar Glauche
-
Department of Neurology [log in to unmask]
Universitaetsklinikum Freiburg Phone 49(0)761-270-5331
Breisacher Str. 64 Fax 49(0)761-270-5416
79106 Freiburg http://fbi.uniklinik-freiburg.de/
|