> 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
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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
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