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Thread: DirectRT exposure presentation duration

  1. #1
    Join Date
    Oct 2007
    Posts
    1

    DirectRT exposure presentation duration

    Hi,

    I am new to DirectRT but I have browsed through the discussion boards so hopefully what I’m about to ask hasn’t already been asked before.

    I am doing a study for my PhD that involves subliminal exposure durations (ideally around 8-10ms – depending on the computer’s capabilities). I understand that the exposure duration of a stimulus is limited by the computer refresh rate. I have a laptop computer with an LCD screen that has a refresh rate of 60 Hz, but when I connected it to an external CRT monitor it was capable of a refresh rate of 100 Hz. I’m not too familiar with the concept of refresh rates but I assume that this means it takes 10 ms to write an image to this screen.

    If I want to display images one after another (separated by a white background with black fixation cross) for 10 ms each, does this mean that the person will actually see the image for 10ms in its entire form – plus 10 ms either side in which the image is written on and off the screen? So in effect they are seeing the image in some form for about 30 ms? If this is the case would you recommend using some external device such as shutter goggles to act like a tachistoscope and ensure that the images are presented for the desired duration?

    Thanks in advance for your help!
    Last edited by jarvis24; 10-22-2007 at 03:15 PM.

  2. #2
    Join Date
    Nov 2005
    Posts
    3,318
    Say you were to display three images, back to back on a single trial, each for 10ms. They would in fact be shown for 10ms each with no interval--resulting in the total sequence taking 30ms. Does that answer your question?

    Note that it does get a bit trickier when you are dealing with full screen rather than partial screen images or text because, on a CRT, the top of the screen is drawn before the bottom. Consequently, with a full screen image you see the top of the image earlier than the bottom--but the bottom stays on longer because the top is also erased first. Every pixel is displayed for the same amount of time but this needs to be considered in terms of how it could affect perception. When using a relatively small stimulus on the center of the screen, this issue becomes close to moot.

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