Dear Patrick,
> I've taken a look at the file spm_hrf.m (and spm_Gpdf.m). The hrf is
> function of the TR and a vector p, containing 7 parameters. The
> haemodynamic delay (normally about 8 sec) is however not explicitly
> mentioned. Is it in some curious way part of the p vector, or is it
> "hard coded" in the construction of the hrf, based on the Gamma-PDFs ?
The default is 6 seconds but can be changes be explcitly specifying
the parameters of the HRF (p)
>> help spm_hrf
returns a hemodynamic response function
FORMAT [hrf,p] = spm_hrf(RT,[p]);
RT - scan repeat time
p - parameters of the response function (two gamma functions)
defaults
(seconds)
p(1) - delay of response (relative to onset) 6
p(2) - delay of undershoot (relative to onset) 16
p(3) - dispersion of response 1
p(4) - dispersion of undershoot 1
p(5) - ratio of response to undershoot 6
p(6) - onset (seconds) 0
p(7) - length of kernel (seconds) 32
hrf - hemodynamic response function
p - parameters of the response function
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@(#)spm_hrf.m 2.7 Karl Friston 99/05/17
> My second question is related to that vector p: the seventh parameter
> (kernel-length), defines -if I'm correct- some kind of temporal
> smoothing kernel (although it's a convolution of course). On what
> parameters does this kernel-length depend ? (by default it's set to 32
> sec.)
This parameter simply allows you to make longer kernels if the delay
(or dispersion) was greater than the default values. The reason it
defaults to 32 seconds is that the HRF is generally zero by that time.
I hope this helps - Karl
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