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  • stimulus exposure time

    To check the accuracy of a 29ms stimulus exposure time, I ran a macro that repeated the block a 1000 times. It ran for 50 sec. instead of the expected 29 sec.

    The stimulus also blinked once in a while. I'm using the 4.5 version of Superlab.

    Am I in trouble with my stimulus duration assumptions?

  • #2
    forgot to mention...

    Forgot to mention: I am using a 60 Hz refresh rate Samsung SyncMaster 712n monitor.

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    • #3
      In SuperLab, there is “housekeeping” work that needs to be done such as loading files from disk, allocating and freeing memory, decompressing images, saving participant data to disk, and so forth.

      By design, SuperLab presents events within a trial as quickly as possible and shifts these housekeeping tasks to the trial and block levels.

      Presenting a stimulus for 29ms and expecting 1000 presentations to last 29 seconds is an incorrect assumption for a few reasons:
      • One is the housekeeping duties that SuperLab must still accomplish.

      • Another problem is that a stimulus is always presented for a multiple of refresh cycles. Your monitor has a refresh rate of 16Hz, which translates into 16.67 milliseconds per refresh cycle. The 29ms stimulus will be presented for a multiple of that, which is 2 refresh cycles, or 33.34 milliseconds.

      • And third, by default, SuperLab erases the screen unless you explicitely turn off that option. This erasure adds one refresh cycle every time.

      At three refresh cycles, that’s 50.01 milliseconds, which sounds about in-line with your findings.

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      • #4
        Originally posted by Hisham View Post
        In SuperLab, there is “housekeeping” work that needs to be done such as loading files from disk, allocating and freeing memory, decompressing images, saving participant data to disk, and so forth.

        By design, SuperLab presents events within a trial as quickly as possible and shifts these housekeeping tasks to the trial and block levels.

        Presenting a stimulus for 29ms and expecting 1000 presentations to last 29 seconds is an incorrect assumption for a few reasons:
        • One is the housekeeping duties that SuperLab must still accomplish.

        • Another problem is that a stimulus is always presented for a multiple of refresh cycles. Your monitor has a refresh rate of 16Hz, which translates into 16.67 milliseconds per refresh cycle. The 29ms stimulus will be presented for a multiple of that, which is 2 refresh cycles, or 33.34 milliseconds.

        • And third, by default, SuperLab erases the screen unless you explicitely turn off that option. This erasure adds one refresh cycle every time.

        At three refresh cycles, that’s 50.01 milliseconds, which sounds about in-line with your findings.
        Thanks Hisham. Two questions though:

        How do I turn off the option that you mentioned of erasing the screen?
        If the screen is not erased, what will make the stimulus disappear after the specified duration (e.g. 33.34 ms) ?

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        • #5
          For pictures: in the Event Editor’s Stimulus tab, click on the Settings... button. A “Picture Settings” dialog appears. Click on the Presentation Options tab and turn off the checkbox Erase screen before presenting stimulus.

          If you do turn off that option, then you need to simulate erasure of the screen yourself. For pictures, this is easily done if all the pictures have the same size and a solid background instead of a transparent background. In this situation, each picture overwrites the preceding one.

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