Dear all,

After revisiting some threads here and the Design Efficiency monograph
I am confused about the effects of duration on the HRF shape. I
thought the difference between a delta function and a boxcar should be
the length of the peak because (citing Design Efficiency, Part VII,
http://imaging.mrc-cbu.cam.ac.uk/imaging/DesignEfficiency) "for longer
than 2s duration trials, the response begins to plateau".

But in the same section is noted:
"After convolution with the IR, a difference in the duration of a
trial causes a difference in the scaling (size) of the predicted
response". This is shown in their Fig 19 with trials of 8s having huge
amplitudes compared to the others. Same effect is shown in a Matlab
example from Poldrack in this thread:
https://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind01&L=SPM&P=R30702&I=-3&K=1&X=10D5DD76B0C377C9B3

Here come my questions:
1. What is the real difference between the HRF convolved for a
duration of 0s and a duration of 5s ?

2. Can somebody suggest some commands (similar to Poldrack's) to
simulate different durations and the resulting HRF for an event?


I tried to create a simulation myself and found out that SPM.xBF.bf
has the delta function of my experiment (attached as JPG). Apparently
it is created in spm_get_bf.m and help shows this:
"spm_get_bf prompts for basis functions to model event or epoch-related
  responses.  The basis functions returned are unitary and orthonormal
  when defined as a function of peri-stimulus time in time-bins.
  It is at this point that the distinction between event and epoch-related
  responses enters."

3. But if SPM.xBF.bf is the same for the whole experiment, how are
durations 0s and durations 5s convolved in a mixed model design
(event-related + blocked)? Isn't it enough that I put different
durations in the SPM interface?


Would really appreciate your answers.

Dorian.