Dear nonparametric experts,
I am aware that the assumption of exchangeability doesn't hold in a 1st level
fMRI analysis because of the autocorrelation of the scans. On 2nd level the
assumption surely holds for intersubject paradigms. But what happens to
intra-subject paradigms on 2nd level? The documentation I found left out this
part.
As we compare conimages which for the T-test equivalent most often are
differences of beta weights I would see no problem for
a) Block designs in which the blocks of interest are reasonable far apart (in
time)
b) sparse imaging designs with high TR
as in both cases the beta weights themselves should not be correlated. Is this
assumption correct? Is there some literature on this?
What about "normal" event-related designs in which the event types are
separated in time by about the length of an hrf?
And what about sparse imaging designs on a first level? Is there some rule of
thumb how long TR must be for autocorr. being low enough for using SnPM here?
Or am I just plain wrong and one cannot assume exchangeability in these cases.
Thanks for any hints,
Roland
--
Dr. Roland Marcus Rutschmann <[log in to unmask]>
Institute for Experimental Psychology, University of Regensburg
Universitätsstraße 31, 93053 Regensburg, Germany
Tel: +49 941 943 2533, Fax: +49 941 943 3233
http://www.psychologie.uni-regensburg.de/Rutschmann
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