Hello,
I'm trying to track down a problem with my experiment design where
occasionally I get a grey bar (actually, 2 grey bars adjacent to one another,
one corresponding to the hrf and one to the derivative) in the parameter
estimability bar. I know this usually results from specifying the same onset
for two conditions, but I have verified multiple times that this is not what
is happening (and moreover, I am generating the design matrices automatically
and most of these matrices have given me no grief at all).
So, I have one theory. In my experiment, bins are determined partly by a
subsequent behavioral (memory) task. This unfortunately results in bins with
very few (sometimes 0 or 1) members. The grey bars have only appeared twice,
and each time they have corresponded to a condition with only one member
(those with 0 are tossed out by the program generating the design matrix).
Could small bins be preventing the SPM package from deconvolving the
hemodynamics? And could this be why I'm seeing the grey bars?
I'm usually just going to throw out these conditions for subjects that have so
few trials anyway, but I want to make sure that I'm not letting something
slide by.
Also, there are some cases where out of 4 scans, only one has this problem
with parameter estimability, and the rest of the scans have sizable bins. So,
although scan 1 might only have 1 exemplar, the rest might have a total of 20.
Should I be averaging in that 1 exemplar from the first scan, or should I
toss it?
Thanks!
Dav Clark
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