In my experience the longer the inter-stimulus interval - the better your ability to detect the response. The instantaneous stimulus is modeled in SPM as waves with ascending slope of 5 seconds and descending slope of the same duration (approximately). So if you have two conditions spaced with only 500 ms the will be 95 % overlap between those target and BLANK hemodinamic responses. Maybe you can increase the duration of BANCKs.
Good luck,
Vladimir
--- On Fri, 3/11/11, SUBSCRIBE SPM Yihui Hung <[log in to unmask]> wrote:
> From: SUBSCRIBE SPM Yihui Hung <[log in to unmask]>
> Subject: [SPM] about timing parameters in first level analysis in spm8
> To: [log in to unmask]
> Date: Friday, March 11, 2011, 2:45 AM
> Hello,
>
> I have a question regarding defining the onset time in the
> first level analysis in SPM8.
> My experimental procedure is as follows: FIXATION (650ms),
> MASK (100ms), PRIME (35ms), MASK (20ms),TARGET (500ms) and
> then a BLANK (500ms). All of the mentioned events are
> included within one scan of 2 seconds (i.e., TR = 2 sec; we
> left about 200ms for the software of stimulus presentation
> to wait for the trigger of the
> scanner). Finally, a blank of 2 second
> serving as a baseline appears (i.e., TR = 2 sec).
> Hence, there are two scans in one trial. It’s of research
> interest to look at the BOLD signal changes of the targets
> of 500ms contrasted with the blank of 2 seconds. One
> can use the “scans”as timing parameter. In my case, the
> interscan interval is 2 seconds and then I specify which
> scans corresponding to one given condition. However, using
> “scans” as the timing parameter will include the BOLD
> signal changes across one scan (i.e., beginning from
> FIXATION to the BLANK of 500ms).
> Because we would like to look only the duration of the
> TARGET of 500ms rather than the entire BOLD signal change
> across the scan of 2 seconds , is it correct to use
> “seconds” as timing parameter and set “0.8”
> (nearly the onset of the TARGET) rather than “0” for the
> target in the first trial? Specifically, I should enter
> 0.8(target onset in 1st trial), 4.8(target onset in 2nd
> trial), 8.8(target onset in 3rd trial) and so on so forth in
> “scan” (the sublevel of Data & Design in first level
> analysis) for the targets? And then, enter 2(baseline BLANK
> of 2 sconds onset in 1st trial), 6(baseline BLANK onset in
> 2nd trial), 10(baseline BLANK onset in 3rd trial) and so on
> so forth in “scan” for the baseline. Is it
> correct? Moreover, is it still necessary to set the
> interscan interval (2 seconds) if I use “seconds” as my
> timing parameter?
> Thanks for any suggestions in advance!
>
> Sincerely,
> Yihui
>
