Hello everyone,
I want to disgard first four scans during fmri preprocessing. I use the presentation generated by neurobehavioural systems (neurobs) and my question is when I disgard first four scans, am I supposed to change onset times generated by this system by subtracting (4 scans* TR)= 8 seconds (TR=2 seconds) or onset times should stay the same?
Well, I have not used neurobs, but I think that you most likely need to to change your onset times. It depends on how your stimulus program deals with the triggers from the scanner. In most cases that I have seen the scanner sends a trigger at the beginning of each volume. If your onset times are relative to the trigger for the first volume, you will need to adjust the onset times. This is most likely the case.
You should be able to tell for yourself, if you don't change the onset times, is the last onset time to close to the end of the run (or after)? Then obviously you need to adjust the onset times.
You could also analyses your data twice, with both reduced and original onset times. The correct onset times should give much better results.
Hello,
as the timing that is logged by Presentation is relative to the start of the scenario, yes, you have to correct the onsets, if you want to discard the first few scans. Question is, do you think it's necessary to remove the first few scans? Several saturation scans should occur automatically prior to the first "real" scan, but of course it depends on the sequence and the scanner.
Best,
Martin
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