Dear Joe, Morgan,
I had prepared an answer along the same lines but I've little to add
to Joe's excellent reply. Yes, the dispersion derivative allows modelling
slight variations in response width for the same reasons. Note that if
regional variations in response delay (>1.5-2s) or width are large,
unconstrained basis functions such as a Fourier set are needed (see e.g.
http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind9911&L=spm&P=R9090). The
F-contrast
-1 0 -1 0 -1 0 1 0 1 0 1 0
0 -1 0 -1 0 -1 0 1 0 1 0 1 (two lines)
is correct and tests whether the canonical hrf or its temporal
derivative across the last three conditions is significantly different from
the first three (it is two-sided, as opposed to the t-contrast), and is
therefore equivalent to
1 0 1 0 1 0 -1 0 -1 0 -1 0
0 1 0 1 0 1 0 -1 0 -1 0 -1.
Good luck
Dick Veltman
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