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!
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