Joe's absolutely right - this is what you expect with a linear model, and
what you are seeing in the data at these two different stim lengths is a
nonlinear scaling. Your fix sounds fine.
Cheers.
On Fri, 7 May 2004, Joseph Devlin wrote:
> It's not surprising that the amplitude of the model differs for 200ms vs.
> 1200ms events. This is the basis for the difference in sensitivity between
> event-related designs (ie short events) vs. blocked-designs (which are
> equivalent to long events). If you don't expect your BOLD response to
> exhibit this effect over this particular range, then by all means rescale
> the amplitude. On the other hand, it is important to note that this is the
> expected behaviour of the model.
>
> Joe
>
> --------------------
> Joseph T. Devlin, Ph. D.
> FMRIB Centre, Dept. of Clinical Neurology
> University of Oxford
> John Radcliffe Hospital
> Headley Way, Headington
> Oxford OX3 9DU
> Phone: 01865 222 738
> Email: [log in to unmask]
>
Stephen M. Smith DPhil
Associate Director, FMRIB and Analysis Research Coordinator
Oxford University Centre for Functional MRI of the Brain
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
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