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[solved] very fast stimulies

edited November 2013 in OpenSesame

A couple of months ago I read an old paper about seeing fast alternating blue and yellow lights as a steady green light (I haven't been able to find it again, if anybody could help me I'll appreciate it). Trying to do the same thing in opensesame ( with varying frequencies) did not lead to the expected green light, actually every back-end gave a different result, is this a hardware problem or is there a way to reach the pseudo-green stimulus?

Thanks in advance.



  • edited 8:09PM

    Hi Ezer,

    This seems like a hardware problem: most modern monitors have refresh rates of 60Hz, which means your light will alternate 60 times a second. In general, OpenSesame should have no problem with updating the display within a single frame refresh (thus being able to display 60 stimuli per second).

    Actually, the difference you are seeing is most likely due to the software running too fast in some conditions. For example, with the legacy back-end, you might be telling the screen to update more than once every frame refresh. This means that halfway through the refreshing of the screen in the blue colour, OpenSesame is telling it to update to the yellow colour. The result would be a half-yellow, half-blue screen. When you pick the psycho back-end, OpenSesame will wait for the screen to be refreshed before telling it to refresh again. Therefore, only single-coloured screens should occur. The xpyriment back-end should do the same, but on my system it seems like it's too slow to keep up (and on a Windows 8 PC I've tested in the past, it didn't wait for the refresh). I should note, though, that the difference in xpyriment functioning seems highly system dependent, as there are people that have absolutely no problems when using it!

    Although I wouldn't be able to tell you at what rate the human visual system will start to interpret the two alternating colours as one, I think it might be a bit over 60 Hz. There are monitors around with higher refresh rates, and for most labs it's not unlikely to have one that goes up to 100 Hz. If you have one of those monitors around, testing on that would be a good idea. Otherwise, you might want to try using something else than a computer monitor, e.g. a LED light.

    Good luck!

  • edited 8:09PM

    So. correct me if I'm wrong, in order to display a faster than 16.6 ms stimulus I'll need a diffrent screen, I'll look around for one.

    Thnaks again,

  • edited 8:09PM

    Yes, exactly! Or something other than a screen, e.g. a LED light. Or you could try to establish the same effect in a different modality; hearing, for example (although I guess that would be a bad idea, as you'd actually be creating a mixture of sound waves, rather than alternating between to discrete sounds).

    Anyhow, good luck with your study!

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