The following matlab code demonstrates the function resulting from
convolution of neural events of various length with a canonical hemodynamic
response (at least I THINK it does - hopefully someone more knowledgeable
will correct me if I've gotten this wrong):

hrf=spm_hrf(1);
bf1=zeros(size(hrf));
bf1(1:2)=1;
bf2=bf1;
bf2(3:4)=1;
hold off
subplot(1,2,1);
plot(bf1,'g');
hold on
plot(bf2,'b');
plot(conv(bf1,hrf)*10,'g');
plot(conv(bf2,hrf)*10,'b');
subplot(1,2,2);
bf3=zeros(size(hrf));
bf4=bf3;
bf3(5:6)=1;
bf4(5:8)=1;
hold off
plot((conv(bf1,hrf)*10)+(conv(bf3,hrf)*10),'g');
hold on
plot((conv(bf2,hrf)*10)+(conv(bf4,hrf)*10),'b');

the plot on the left  shows the single hrf for an event lasting either two
seconds (green) or four seconds (blue).  as you see, there is a large
difference in the height of the resulting response (almost twice as high for
the 4 second event), and a relatively small difference (roughly 1 sec) in the
time to return to zero.

the plot on the right shows the combined (added) response to two events,
either two 2-second events with a two second ISI (green), or two 4-second
events with no ISI (blue).  Again, the main difference is in the amplitude of
the response, with the timing being almost identical.

I hope this helps in understanding what's going on under the hood.  It might
be worth extending this demonstration to see how it works with a delta rather
than a mini-boxcar as I've used here.  My understanding from discussion a
while back with Rik Henson was that only the amplitude, and not the the
shape, of the response should differ for neural events lasting less than 5
seconds or so, and thus the delta/hrf model should be reasonable for neural
events within that window.  Hopefully Rik will correct me if I have misquoted
him.

cheers,
rp


Anat Maril wrote:

>  Hi,
>
> My question refers to event-related design with a trial duration of 4
> seconds, and a TR of 2 secs.
>
> Given that the onsets files include only the first TR of each trial, the
> second TR is never listed, like all the fixation trials. It implies that
> the following two hypothetical trial sequences:
>
>  a. trialA (2secs) -> fixation (2secs) -> trialB
>  b. trialA (4secs) -> trialB
>
> are treated by the model in the same way.
>
> treating them as identical would mean that, once a hemodynamic response
> begins, it has one possible pre-determined course.  Regardless of how long
> the actual trial lasts before the next trial/rest, the model would treat
> trial B identically.
>
> Is there a way to differentiate these two scenarios in SPM?
> To the extent that I have a reason to believe that a real cognitive effort
> does continue during the whole duration of the trial (i.e. that a 4secs
> trial does behave differently from a 2secs trial) - am I correct to assume
> that I would have to incorporate this belief to the model via one of the
> user-specified options?
>
> thank you,
> - Anat
> Anat Maril
> Harvard University
> Department of Psychology
> William James Hall
> 33 Kirkland St., Cambridge, MA 01238
> (617) 496-5909
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