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Thread: trial delay and error in timing

  1. #1

    trial delay and error in timing

    I am using DRT for stimulus presentation during an fMRI study. The input file is written to display a fixation cross at the start of the trial (500ms), followed by a movie (for the duration of the movie), and lastly by a picture (rt:1,2, full, clear-1). The style for each trial is set with a min-max! to control when it advances to the next trial.

    Timing is extremely important for us, because specific trials being presented by DRT are paired with specific images being acquired by the scanner (TR).

    I ran a test subject and found that DRT was still going when the scan had completed (the total scan time is set to match the sum of all the trial max settings). Upon examining the log output file from this subject, I see that things are actually taking slightly longer than they're supposed to.

    The fixation cross was up for slightly over 500 msec, the movies lasted a bit longer than their set duration, and the end of the trial also exceeded the max setting. At times, the fixation cross at the start of the trial didn't come up for a second or more. These small delays are adding up to major differences by the end of the experiment - and the images can no longer be paired to their appropriate stimulus.

    What's causing these delays in advancing through events/trials? Is is due to delays caused by opening/loading the stimulus? Can I get the level of precision out of DRT to match trials with images (TRs) on the scanner?

  2. #2
    Join Date
    Nov 2005
    What trial interval are you using--i.e., as set in the options of your style file?

  3. #3
    I was using 1000ms. I tried reducing the interval (to 750), in case this was throwing off the total times. It hasn't fixed the problem on my personal computer, but I need to try it still at the imaging center (since stimulus loading times would really be specific to each machine - depending on the specs).

  4. #4
    Join Date
    Nov 2005
    Can you attach a copy of your input file and a sample log data file--I'll take a look and see if I can tell what's up. It would be useful if you could indicate how long you observed that the total execution of input file took (i.e., for the session corresponding to the log data file).

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