Dear Patrick,
> The file spm_hrf contains the modelling-section of the HRF function by
> means of a sum of 2 gamma-functions, defined by a p-vector. Can someone
> tell me the theory/experiments behind the values of this p-vector.
> E.g. the first-value represents the hemodynamic delay, which is set by
> default to 6. But how are the other values defined/related to the
> hemodynamic delay or other ?
These parameters where chosen as approximations to those that reproduce the
first principal component of HRFs over voxels in the data described in:
Friston KJ Josephs O Rees G Turner R Non-linear event-related
responses in fMRI Magnetic Resonance in Medicine 1998;39:41-52
In relation to their physical interpretation:
>> 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
_______________________________________________________________________
@(#)spm_hrf.m 2.7 Karl Friston 99/05/17
Each Gamma variate has 3 free parameters. One of these can be cast as
overall amplitude [fixed such that sum(hrf) = 1]. This gives 2 x 3 - 1
= 5. The last two simply specify the begining and end of the kernel in
relation to peri-stimulus time.
I hope this helps - Karl
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|