As I am new to SPM the following idea may sounds odd, so please
correct me if it is wrong.
I tried to model the hemodynamic response to the onset of each event type
with two basis
functions in SPM2: a canonical HRF and a delayed HRF, shifted to onset 2
sec(1.33 TR)
later than the canonical HRF. According to Henson' paper(Confidence in
Recognition Memory
for Words: Dissociating Right Prefrontal Roles in Episodic Retrieval), the
covariates for
the late HRF need to be orthogonalized with respect to those for the early
HRF. If my
understanding is correct, SPM2 has incorporated Gram-Schmidt procedure.
The way I created the delay HRF is creating another event type with onset
values 2 sec
later in time.
Here is the specific values in my design.
SPM.Sess(1).U(1).ons = [33.33333 47.83333 62 69.16667 112 126.8333 177.8333
221.5
235.6667 257.6667 ];
SPM.Sess(1).U(1).dur = [0 0 0 0 0 0 0 0 0 0 ];
SPM.Sess(1).U(1).name = {'run1_event1_early_hrf'};
SPM.Sess(1).U(2).ons = [34.66333 49.16333 63.33 70.49667 113.33 128.1633
179.1633 222.83
236.9967 258.9967 ];
SPM.Sess(1).U(2).dur = [0 0 0 0 0 0 0 0 0 0 ];;
SPM.Sess(1).U(2).name = {'run1_event1_late_hrf'};
After I estimated the model, I checked the design orthogonality.
It showed these two event types were still correlated.
Does anyone know what's wrong with my method?
Thanks for any help.
Chih-Chen
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