You need to model the 2-12 response period as maintenance of working memory. Additionally, you might want to modulate the maintenance period by load.
You are right that the signal overlaps, but the neural activity doesn't; so, the events should be separable. Just make sure you model everything.
If you are still concerned, you could create 3 separate events for the 2 second delay periods and ignore them.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School
Office: (773) 406-2464
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On Wed, Dec 8, 2010 at 9:07 AM, Andres Roman
<[log in to unmask]> wrote:
Hi,
I have been following the discussion about jittered trial design and had a
couple of questions related to my design. I am analyzing data from 4 groups
of a delayed response task. The task consist of two runs (32 trials by run)
with 6 different loads of stimulus. The trials start with a fixed 4 sec
period were the stimulus is shown (series of letters) followed by a variable
delay of 2 to 12 seconds, following this there is a fixed period of 1.5 secs
were the subject is supposed to match and answer according to to the series
of letters that were shown at the start of the trial, finishing the trial
with an ITI of variable duration (3-5) seconds. I am able to to model both
the start of the trial quite well and the response section of 1.5 sec. I am
using a duration of 1 second and a parametric modulator that uses the load
effect for the 4 sec section. In the case of the response section I used a
duration of 1.5 (length of the whole answer period) and the same parametric
modulator used before (load). My problem is that I am afraid that since the
jittered minimum interval is 2 secs the signal in those trials overlaps
between each other. Is there a way to model this variable jitter interval in
a way that could maximize my signal? A proposed contrast to model the jitter
interval? Or exclude the jittered trials that have a value of less than 4
seconds?
Thank you very much for any ideas to deal with this.
--
Andres Roman-Urrestarazu