Thanks Doug.
    In my experiment, each trial (event) consists of 1 sec stimulus presentation and 5 sec blank interval (TR=2). I have also added a condition with just blank presentation which will be repeated as much as other events.

    I have used optseq2 commands to generate the trial sequence. But I faced two problem/questions:

    1) Using this command, I expected to see a Null event (other than my blank condition) with variable length (irrespective of TR) between other trials. But to my surprise, length of the Null event was always a multiplication of TR. Is that correct?
    2) Also, for "psdmin psdmax" I wasn't sure if they are number of TRs or number of seconds and what is a safe window length when my stimuli are very simple visual objects (and no cognitive task).

Once again, thanks for your help.



On Fri, Dec 9, 2011 at 1:10 PM, Douglas N Greve <greve@nmr.mgh.harvard.edu> wrote:
Hi Shahin, there are several ways to answer this question. If you are going to analyze assuming a shape to the HRF, then the actual delay between stimuli is technically not important (though 4s will be good to avoid attenuation in the following stimulus). If you're using an FIR model of the HRF (no assumed  shape), then I usually advise that you have, in total, as much time spent on fixation (or whatever your null stimulus is) as you have for any other stimulus. You can use optseq to compute the actual schedule.
doug


SHAHIN NASR wrote:
Hi Surfers,

  It is my first experience with Event-related paradigms. In my experiment, trials take minimum 6s (TR=2s). I wonder what is the optimum variable delay between trials to be able to analyze data without any confound.

Regards

--
Shahin Nasr

PhD in Cognitive Neuroscience
Martinos Imaging Center, MGH
Harvard Medical School


--
Douglas N. Greve, Ph.D.
MGH-NMR Center
greve@nmr.mgh.harvard.edu
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Shahin Nasr

PhD in Cognitive Neuroscience
Martinos Imaging Center, MGH
Harvard Medical School