 |
 |
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/