randomise image pairs

I need to present an event comprising 2 images simultaneously side by side, for 5 events i.e. 10 different images in all. They need to be randomly chosen without replacement and also randomly positioned left or right. I can’t see a way of doing this in Superlab so would appreciate any advice.

Positioning an image randomly on the left or right should be fairly easily done using a trial variable.

If I understand correctly, you have 10 images total, and you want to present a total of 5 trials where each trial presents two of these images. Am I interpreting the question correctly?

Yes. 5 pairs of images will be presented in sequence, no 2 images being the same. This will be repeated for each participant but the image pairs must be randomly different both in content and relative position each time the sequence is run. The same 10 images are used for each participant. (This will be the basic building block for a bigger experiment).
One problem is: If SLab chooses to put the 1st image of a pair randomly on the left, say, how will it know to put the next one on the right? As far as I can see they might both end up on the left.

randomise image pairs

Any update available for my previous question please?


I programmed that in one of my experiment. I f I understood correctly, the events are your 10 pictures and you have 5 trials of pairs ? If it’s the case, you can do that:

First, you have to go in : trial variables / screen location
(and edit)
In “level” you tell Superlab the position of your picture on the left and on the right of the screen. I didn’t change what it was already programmed:
LHS X: -360 Y:0
RHS X: 360 Y:0

  • In level Selection you have to choose:
    “Random with No replacement”. With this choice, you are sure that Superlab will not put the two pictures of your pair on the same side.

  • And choose also “Randomize levels before start of : each trial”

You’re done with the trial variable.

Now, you have to go in the “event editor / Stimulus / settings” of all your events of picture and say :

  • The position of the picture is variable
  • Select the variable : Screen Location

If you didn’t do it, you need also to program an other thing in order to present simultaneously two pictures.
In event editor / stimulus / Settings / Presentation option
You have to chek off for your first picture : “erase screen” and “keep stimulus invisible”
and for your second picture just " Use offscreen buffer "

I hope this helps


Thanks for the info. However, I cannot see how to tell the program to make alternate images invisible because this has to set at the event level, not the trial level. The random selection requirement precludes me from setting the ‘invisible’ flag on the image events. This is driving me nuts. Can anybody help?

One way to make alternate images invisible is to create a “Location” trial variable that contains two levels, with the 2nd level being off screen.