triggering and timing

SuperLab SuperLab 4 Support
,
1

Hello,

I have some questions about issues brought up on this page:

http://www.cedrus.com/support/superlab/tn1504_mri_sync.htm

Our trigger sends an “=” sign when the scan is starting so that the stimulus presentation can start at the same time as the scanner. I programmed SuperLab to look for this “participant input” before starting the other events.

I’m confused by the sentences “And last but not least, you need to link this event to all your trials. You can create just one event and link it to all the trials.” Why would you be linking it to all your trials? The way my program is set up is that there are about 50 trials of about 18 seconds each. Why would the trigger event be synced to all of the trials?

I am wondering about the section “Why using time limits doesn’t work well” and its impact on data analysis for MRI paradigms. If a cue that is supposed to last 5 seconds takes a little more than 5 seconds each time, wouldn’t that build up over the course of the experiment and cause a delay by the end of the experiment? If there is a delay, fMRI analysis becomes hard because you can’t reliably identify the timepoints when a subject was seeing certain stimuli.

I am confused about whether that last section applies only to the “software only” section or if it somehow can be avoided if you are triggering via the scanner.

Thanks,
Emily

2

Hi Emily,

About linking to all the trials, this assumes that you are synchronizing the start of each trial with the scanner’s TR. When assisting other users, I have noticed that many synchronize only the start of the experiment. But the better approach is to sync the start of each trial to ensure that there is no drift.

About the second question, if a cue is supposed to last 5 seconds but takes a little more than 5 seconds, what happens depends on whether the following checkbox is enabled or not:

If it is not enabled, then you get the drift scenario that you describe. If it is enabled, then SuperLab waits until the next multiple of time before starting the next trial. In your example, it will wait until 10 seconds have elapsed before starting the next trial.