Jerry -
See second paragraph of first reply of:
http://www.mailbase.ac.uk/lists/spm/2000-04/0188.html
Regards
Rik
> With regards to adjusting occurence of the onset of the first stimulus.
>
> If the first stimulus appears at the beginning of the first functional scan, why not use the first temporal slice of the scans as the reference in the slice timing routine? What do you gain by changing the temporal basis of the stimuli?
>
> Jerry Allison
> Medical College of Georgia
>
> >>> Rik Henson <[log in to unmask]> - 6/30/00 4:02 AM >>>
>
> > 1. In an event-related design, the first stimulus appears at the
> > beginning of the first functional scan (excluding dummy scans). If I use
> > the middle slice of the scans as the reference in the slice timing
> > routine and set the refences bin in the statistcs default to 8 (of 16),
> > is it correct that the onset of the first stimulus should be 0?
>
> Yes
>
> > 2. In a study with more than one session per subject, is it possible to
> > treat all sessions as one session and model the different sessions as
> > seperate epoch-covariates in the design matrix?
>
> You could do, but the single multisession covariate will be slightly
> inaccurate in that the influence of last few events of one session may
> continue into the first few scans of the next (via the HRF, depending
> how close in time the last events are to the end of the sessions),
> where this is not the case in reality if the scanner has been stopped
> between sessions. The high/lowpass filtering will also be less valid
> in this case, because you no longer have a continuous timeseries.
> Nonetheless, providing you still remove the low frequency (mean) session
> effects (via your epoch-covariates), the above errors are likely to be
> negligible.
>
> Rik
>
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> DR R HENSON
> Wellcome Department of Cognitive Neurology
> 12 Queen Square
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DR R HENSON
Wellcome Department of Cognitive Neurology
12 Queen Square
London, WC1N 3BG
England
EMAIL: [log in to unmask]
URL: http://www.psychol.ucl.ac.uk/rik.henson/index.html
TEL1 +44 (0)20 7833 7483
TEL2 +44 (0)20 7833 7472
FAX +44 (0)20 7813 1420
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