Dear All,
I've been looking at the spm_hrf function and there are
a few things I don't understand:
1) With a TR (?RT) of 1 second and a default kernel length of
32, 33 values are created by the function, the first of
which is zero. I'm assuming that these values represent
the HRF at 1 TR (in this case 1 second) intervals?
How then does SPM obtain a time resolution of TR/fMRI_T?
2) Altering the P(1) parameter (delay of response relative
to onset) in isolation say from 6 to 12 also seems to
impacts the HRF mamima. Is this expected?
3) Altering the P(2) and P(3) parameters (dispersion & dispersion
of undershoot causes the HRF to suddenly disappear (flatten to
zero) after a certain point (say from 5 to 7 with a TR of 3).
Is this also expected?
Much appreciated,
A Haddadi
Clinical Research Fellow
Institute of Neurology
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