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Stimulus presentation sync to TR onset?


#1

Hi all,

I’m wondering if it is necessary to sync every stimulus presentation to TR onsets? My stimulus lasts for 3s and TR is 2.6s.

Thank you!


#2

This question is posed as a technical one, but really it is fundamentally related to your fMRI design and analysis plan. I’d gently suggest that before proceeding, you should ensure that you are thoroughly versed in fMRI design principles: it would be ideal if you can confidently express the detailed implementation details that arise from those design considerations, rather than rely on the kindness of (potentially misguided) random strangers on the Internet.

But to get you started at least, I can only speculate, as you haven’t given any details of your design. For example, with a standard block design, the issue might be (largely) unimportant, whereas your protocol would be exquisitely sensitive to this in an event-related design.

Let’s say you have a standard block design, comparing summed activation between blocks in which different classes of stimuli are presented. Then no, the stimuli don’t generally need to be synced to specific TR onsets. What is (generally) crucial is that the onset and offset of the train of stimuli corresponds to the onset of the first TR in your block and the end of the last TR in the block. If they are misaligned, then you are reducing the signal contrast between the block boundaries. But the timing of individual stimuli relative to individual TRs within the block is not so important, and may be driven by a myriad of other considerations specific to your particular domain of research (although there are still technical considerations, like maintaining a steady pace of presentation throughout the block, and so on.)


#4

Thanks for your reply. My design is event-related. Right now all of my stimuli are synchronized to TR onset, but I don’t really know why this is or is not necessary. I read in a book that this synchronization is often the practice because in event-related designs ‘it is crucial to know when the presentation of each stimulus occurred relative to TR onset’. But I suppose it’s still possible to know this info even stimuli are not synced to TR onsets? Is there any other reason to do this sync?

And I’m asking this question because syncing to TR onsets makes my task length longer than desired so I’m wondering if I could get rid of this sync and make my task shorter.


#5

^^^ This is certainly true fort event related designs.

This can actually be a bad idea for event-related designs. Often it is best to systematically jitter your stimuli relative to TR onset, for example it can have great benefits in increasing the temporal precision with which you can measure HRFs (if that is what you are after).

But this takes a lot of background reading. You really should get to grips with that (i.e. the intricacies of event-related fMRI designs) before you even consider trying to balance trade-offs with task duration. I strongly suspect you’re not yet at the stage to benefit from brief answers on a forum like this, without the background knowledge required to critically evaluate them.

fMRI is still too expensive and time-consuming to risk (or even guarantee) getting the design wrong, just in order to avoid getting immersed in some texts for a week or so.

Hopefully you also have experienced supervisors/mentors/experienced peers to consult with?