Hi,
I asked this question on the spm mailing list last week. But I haven't got
any answer yet. Maybe somebody could help with this question.
I specified a model for my fmri data set in SPM by using 3 conditions A, B
and C, besides there is a neutral condition (which was not specifically
modelled). So the paradigm is: neutral(45 scans) -- A(60 scans) --
neutral(48 scans) -- B(56 scans) -- neutral(51 scans) -- C(55 scans) --
neutral(50scans). The TR is 6 seconds and since each condition appeared
only
once, the SOA for each condition was specified as 353 scans (or any large
number >=353 scans should do it).
Then, I selected "no" for stochastic design, "variable" for SOA (but it
really doesn't matter even if the SOA is fixed), input the
time scans and durations; took "epochs" and "box-car" for the design; set
hrf "no". The idea was not to use any hemodynamic response function, since
I wanted to define a pure box-car function for each condition. When the
design matrix was plotted, the functions for A, B, and C
had a triangular appearance instead of a boxcar appearance. With triangular
appearance, I mean that the condition for A as a function of time was
linearly increasing up to a maximum and then decreasing to zero for the
stimulus duration. This behavior was not expected, since a boxcar i.e.
square wave-like shape was desired for the stimulus duration. For the other
regressors (B,C) similar shapes were obtained. As an example, you will find
a graph in .gif format attached.
So, my question is why are the regessors not boxcar-like since no hrf
function was used?
Thanks for your help!
Yunyu Cao
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