hi all,
we have an event-related design fMRI study in which the conditions
of the spm analysis have been constructed retroactively based
upon the nature of the subjects' recognition responses tested after
scanning.
when i inspect the design matrix for several subjects, spm appears to be
deciding in advance not to sample certain time points on a session by
session basis. that is, if i explore the design matrix and pick a
particular
condition, the lower left hand plot, which shows the synthetic hrf and
time derivative, indicates that different peristimulus times will be
sampled
across the different sessions for that condition. for example, there
may
be red asterisks along the x-axis out to 20 seconds in scan one, whereas
it only goes out to 15 seconds in scan two.
am i interpreting this plot correctly? if so, is it that the program
is omitting time points because they are somehow inefficient or
statistically
biased and does this mean that some of the data do not make into the
Y.mad
file regardless of the overall F threshold?
if this is so, can someone please indicate in layman's terms
how and why the program selectively chooses certain time points a
priori,
and whether or not one can override this during design specification
and collect the same peristimulus time points for all conditions and
scans?
thanks in advance for your help.
ian.
--
Ian G. Dobbins
MGH-NMR Center
Bldg. 149, 13th Street
Charlestown, MA 02129
(ph) 617-724-9989 (fax) 617-726-7422
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www.nmr.mgh.harvard.edu/people_final.htm
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