Combining Event and Cumulative Timing in one Trial

[kyleheidtman2012 at u.northwestern.edu]

Hello,

I'm designing a task for an fMRI experiment, so I've been using cumulative 
timing in order to ensure the experiment kept pace with the scanner.

However, two of the stimuli are displayed for very brief periods (~70msec 
and ~30msec), and I found that cumulative timing altered the duration of 
the displays by a rather larger amount.

I've tried using event timing for just those two stimuli, and I found that 
they're duration is now much closer to what it should be.

Now, I'm worried if this will cause unacceptable delays in the task, and I 
have a few questions.


   1. Is it ever acceptable to combine Event and Cumulative timing in one 
   trial?  
   2. Is the ".OnsetDelay" variable equal to the *total* amount that a 
   stimulus is delayed?  Or is it just the amount of *additional* delay?
   3. Does an average OnsetDelay of ~50 msec seem acceptable to you all?
   4. Does EPrime automatically correct for OnsetDelays or do I need to add 
   a script that sums the OnsetDelays for a given trial and then subtracts 
   that time from the ITI (or something similar)?


Thank you,

Kyle

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