Dear SPM-users,
in a recent reply by Karl Friston (Thank You !),
he stated:
> This approach
> eschews any problem with asynchrony between acquisition and trials and
> automatically ensures approoriate convolution with the HRF in
> 'microtime' (i.e. time-bins of TR/16).
However, whenever we difine events, SPM asks for 17 covariates,
when we define a period of exactly 1 TR. If we define a period of 2 TR SPM
asks for 33 covariates, the system seems to be 16*TR + 1. Where does this
one extra covariate come in ?
Best regards,
Peter Erhard
Florian Castrop
On Fri, 21 Jul 2000, Karl Friston wrote:
> > From [log in to unmask] Fri Jul 21 13:36:14 2000
> > Date: Fri, 21 Jul 2000 14:35:38 +0200 (MET DST)
> > Subject: covariate-timing
> > From: Peter Erhard <[log in to unmask]>
> Dear Peter,
>
> > we just stumbeled over a question that might have been discussed
> > earlier. Our fMRI-paradigm is slightly asynchronous to the image
> > acquisition. From the performance of execution of the paradigm we
> > derive a covariate. As during the images at the beginning and end of
> > the block the execution starts or ends, we do not know what to choose
> > as a covariate for these images (for all the other images, we use
> > either the average of the performance data acquired during the image or
> > zero (during rest). In this context we also would like to know,
> > whether the covariate data is convolved with the hemodynamic delay
> > function, and if so, how ?
>
> If the covariate is presumed to have a neuronal correlate then it
> should be convolved with the HRF (because this is how that effect will
> be expressed in the data). One way to model this is to treat your
> block design as an event-related design, where each block is a train of
> trials or events. Using the parametric option simply modulate each
> trial with the perfomance measure for that trial. This approach
> eschews any problem with asynchrony between acquisition and trials and
> automatically ensures approoriate convolution with the HRF in
> 'microtime' (i.e. time-bins of TR/16).
>
> I hope this helps - Karl
>
-----------------------------------------
Peter Erhard, Ph.D.
Nuklearmedizinische Klinik und Poliklinik
Klinikum rechts der Isar
Technische Universitaet Muenchen
Ismaninger Strasse 22
81675 Muenchen
Germany
Tel: 49-89-4140 4570
Fax: 49-89-4140 4938
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