Dear SPM community,
I have a question related to estimability of contrasts. To simplify my
design somewhat, this design consists of two runs, with two conditions
(say C1 and C2) occurring in each run. Besides a condition-specific
regressor in each run, I also include run-specific predictors.
Hence, my design matrix (in simplified form) is
1 0 0 0 1 0
0 1 0 0 1 0
0 0 1 0 0 1
0 0 0 1 0 1
First column is for C1 in run 1, second column for C2 in run 1, third
column for C1 in run 2, fourth for C2 in run 2, and columns 5 and 6 are
run-specific regressors.
Now, if I ask SPM to compute a contrast for C1 in run 1 vs C1 in run 2 (a
learning effect, contrast vector (-1 0 1 0 0 0)), (s)he calculates and
reports the value of this contrast. However, my understanding is that this
contrast is not estimable since it is not in the row space of the design
matrix. But I thought SPM refuses to report contrasts that are not
estimable? Can anyone help with this problem?
Thanks in advance!
Tom
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