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OSWeb - Collecting keyboard response while presenting multiple sketchpads

Hi all!

I am programming an experiment where I am trying to combine the AB and the PRP paradigms in OSWeb. In doing so, speeded responses are required to the first target stimulus (T1). Therefore, I need to collect a keyboard response (either the left arrow or the right arrow key) within the time period starting from the onset of T1 until the end of the trial.

I would use a coroutine item to implement this if I were going to run this experiment locally -In fact, I did to illustrate what I'm trying to achieve, see attached.- However, it seems it doesn't work in OSWeb. Though, mentioned here in this thread that coroutines are supported as of OSWeb 1.3.8. (Oops, I revealed the secret!). I also checked this thread out, but I didn't quite get it.

Could anyone help me implement this in OSWeb?

Many thanks in advance!



  • edited October 2020

    Oh, the links above don't work. So, here they are.

    This is the one where @Daniel mentioned that coroutines are supported as of OSWeb 1.3.8:

    And this is the second one:


  • Hi @bugay,

    What I can quickly see is that you have entered LEFT;RIGHT in the correct response field, which is not allowed. You can only enter one character here (for instance by using a variable), or leave it empty and specify the correct response in a correct_response column in your loop table.

    Also, Osweb is sometimes a bit more picky how you specify things. I see that you now specified all responses in uppercase characters (e.g. `LEFT;RIGHT`) while osweb may only work with lowercase (`left;right`). Maybe you can try if that works, and if it does, then it is a bug in osweb, as it should accept the same inputs as OpenSesame. If so, can you report it at osweb's issue tracker at

    Buy Me A Coffee

  • Dear @Daniel,

    Thanks for the prompt reply. I made the changes you mentioned, but it is still the same. However I noticed something very interesting.

    I use a white background, and present stimuli in black. When I run the experiment in OSWeb, as the trial_coroutine starts the screen suddenly goes black, and then it goes back to normal (white) by the end of the coroutines. When I tried switching the color of the stimuli in the trial_coroutine to white, I noticed that it actually displays the stimuli, but on a black background. And this is why it seems they are not being displayed at all. Is there a way to overcome this issue?


  • Hi @Bugay,

    I've run your experiment on both osweb and OpenSesame and I can see the discrepancy. Very strange indeed. It appears as if the coroutines item resets the background color to black somehow and doesn't honor the general settings. I have also managed to reproduce it in a simple test experiment:

    Since this happens only in osweb and not in OpenSesame it is clearly a bug in osweb.

    I currently don't have the time to dive into the code and find the cause. I have created an entry on osweb's issue tracker to keep it on our radar though: Check back there for the status on this bug and its fix.

    I can't really think of a possible workaround. The only one I could come up with was setting the background color explicitly on each sketchpad, but that is also ignored.

    Sorry for currently not being of anymore help.

    Buy Me A Coffee

  • Thank you!

    I'll be checking the link.


  • Hey guys! Bugay and I were wondering if you can provide a (rough) indication of when the co-routines issues might be solved / a new update of OSweb for Mac will be available. Keep up the great work!


  • I don't know if you meant all the issues, but this background color coroutines bug should be solved with a fix that I just pushed.

    Buy Me A Coffee

  • Hi Daniel,

    I downloaded the latest update of OpenSesame and tried the example experiment that you updated on the GitHub page. However, the problem remained unresolved for me, when the coroutines starts, the background switches to black instead of remaining gray. I also tried to set the background explicitly for each sketchpad, but it didn't work either. I'm a bit confused, should I do some inline coding for it to work? (Sorry, I'm a javascript newbie.)


  • In case there's someone else struggling with this issue, there's a quite simple solution that we've been missing out until recently.

    What you could do is to add a rectangle (in whatever color you like) behind everything else on the sketchpad, and then display stimuli on top of that.



  • Thanks @bugay! :)

    Did you like my answer? Feel free to Buy Me A Coffee :)

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