Stimulus in lower right quadrant most salient?

URL of experiment:

Description of the problem:
this is probably not entirely Psychojs specific, but I am trying to present four different grating stimuli at once with a relatively short presentation time (~250 ms) in an online study. Since I can’t generate these on the fly I am using a .png picture and need calculate and set its opacity and orientation in every trial (which potentially also creates some timing delays along the way… maybe generally not really the most ideal study to run online).
The four gratings are all arranged around a fixation cross and one of them will have a slightly different orientation, shifted clockwise or counterclockwise than all others (this is the target in a search task).
When running this in my trial routine, it seems that the grating in the lower right is always more salient and easily perceived than all other ones and generally attention seems always drawn towards the right half of the screen, while the other two stimuli on the left are barely seen. Somehow, perceptually it also seems that the two gratings on the right often have an orientation difference (even if they don’t) and that the oddly oriented target is in this lower right patch, triggering wrong responses.
I was wondering whether this had something to do with timing issues and the drawing order, with this stimulus always being drawn last and potentially for one more frame than the others by bad luck. I tried to randomize which of these grating stimuli is drawn first by also randomizing their location on screen (such that the grating component lowest in the builder view and drawing order is not always the one positioned in the lower right quadrant), but this also does not really seem to help.

The timing parameters and most others (except for their individual orientation and condition-dependent opacity) of these four grating components are simply copy-pasted in the builder and all look as expected when presenting them for longer timeframes. All of the grating parameters are either set within my trials-loop or in a code-component within a routine called welcome at the start of the experiment.
I am not entirely sure whether this is some code/screen issue or a weird visual effect, but it is perceptually relatively strong and was experienced by multiple people on different screens. Did anyone maybe experience something similar in their study?


I took a peek, but I couldn’t find see an obvious issue. I’d like to take a more in-depth look though; could you share the gitlab repo with me? (tpronk)

Some expectation management :slight_smile:

  • Annoyingly, timing validation studies tend to only examine how accurately a single stimulus is presented, so we don’t have data on multiple stimuli, but based on what I know about screens and presentation tech, I wouldn’t expect a bias to the right. If you got a photometer at Radboud, might be nice to take a look.
  • I’ll be off a couple of days, so it might take a while before I pick it up.

Best, Thomas

Hi Thomas,
thanks a lot! You should be able to access it now, please let me know if you have any questions or issues.
Enjoy your holidays! :slight_smile:
Best, Maëlle

Hey again!

I looked around a bit and I didn’t find any issues in your configuration. No straightforward ones at least. The best workaround I can think of is merging the four gratings into a single picture and present varieties of that picture depending on your condition. This way we’re sure drawing order isn’t an issue.

Sorry I couldn’t think of anything more elegant.

Hi Thomas,
thanks a lot for taking the time! Yes, this is also what I am settling for now and it definitely helps.

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