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my inclination would be to model the stimulus compounds as a single event with an onset at the time of the first stimulus, and then use the temporal and dispersion derivatives to model differences in onset or length of response between combinations. At such a short separation, the HRFs are going to be highly correlated if you try to model the individual stimuli as separate events. cheers rp Daniel Weissman wrote: > Dear SPMers, > > Would anyone know the best way to model 'events' in which 2 stimuli are > presented? This is a fast-paced, event-related fMRI paradigm in which each > 3-second trial contains 2 stimuli separated by 1 TR (1.5 sec). Any advice > would be greatly appreciated! > > :> Daniel > > Daniel Weissman, PhD > Center for Cognitive Neuroscience > Duke University > Durham, NC 27705 > phone: (919)-681-1029 > fax: (919)-681-0815 > e-mail: [log in to unmask] > ----- Original Message ----- > From: "John Ashburner" <[log in to unmask]> > To: <[log in to unmask]> > Sent: Tuesday, May 15, 2001 4:56 AM > Subject: Re: sgi, matlab6 and spm99 > > > I don't know if you are doing the "adjustment" or not. If you are, then > it > > is likely to slow the processing down considerably. > > > > Changing maxMem makes no difference to the realignment. I don't think > that > > increasing swap will have any effect on speed, although it can prevent the > > occasional crash through lack of memory. > > > > Best regards, > > -John > > > > > Hello all,I have set up and compiled spm99 on our sgi o2s with matlab6. > > > We can get things to run, but they are incredibly slow...it can take > 4-30 > > > hours to do realignment, for example....we are frustrated both by the > > > inconsistencies in time and the length of time it takes to do the > > > processing (the dataset is 264 reps and 19 slices-fMRI). > > > > > > We have fooled with the maxMem variable, and we have increased the swap > > > space to 1gb...given the inconsistencies, it is difficult to determine > if > > > these changes have had any effect....but it looks like our problem > > > persists. > > > > -- > > Dr John Ashburner. > > Wellcome Department of Cognitive Neurology. > > 12 Queen Square, London WC1N 3BG, UK. > > tel: +44 (0)20 78337491 or +44 (0)20 78373611 x4381 > > fax: +44 (0)20 78131420 > > http://www.fil.ion.ucl.ac.uk/~john > > mail: [log in to unmask] > > > > -- Russell A. Poldrack, Ph. D. MGH-NMR Center Building 149, 13th St. Charlestown, MA 02129 Phone: 617-726-4060 FAX: 617-726-7422 Email: [log in to unmask] Web Page: http://www.nmr.mgh.harvard.edu/~poldrack