Transitional Confounds in SDT Masking Paradigm

Builder
1

New to the site so hope it fits here.

Hey everyone would really appricate your help regarding an issue with the experiment im currently building

The experiment is using Psychophysics (Signal Detection Theory) and there are 2 conditions: target trials - 2 Gabour stiumli in 45° and 135° orientation, and catch trials with a blank screen (matched to background colour) instead of a stimulus. There are 4 thresholds 16,32,64,128 ms (catch trials also match those intervals)

The overall structure is as follows: fixation (500 ms) → target/catch trial (16,32,64,128 ms) → mask (200 ms) → visabilty grading response (unlimited)

There is a sense of transition from the stimuli to the mask (idk the terminolgy for this phenomenon) in the target trials, whereas the catch trials have no such transition (smooth transition between blank screen and mask).

I tried adding noise to the blank screen, to create this sense of transition. Now, those noisy catch trials can also cause a sense of something that appeared, which can confound subjects responses. On the other hand, keeping these transitional differences makes the whole paradigm pointless: its easy to spot the difference in transition between target and catch trials.

So far i was unable to minimize this tranistional difference in other ways aswell:

  1. In target trials, reducing contrast between mask and stiumli, doesnt make the transition smooth, it makes the target really hard to spot at a certain point.
  2. Combining the same noise within stiumli and blank screen both keeps the transitional difference and the target harder to spot.
  3. 0% gabour contrast instead of blank screen

In the short reading ive been doing to address this issue, i saw that its known problem for target present & absent visual masking paradigms. Some researches addressed it by double masking [Peters & Lau (2015), Human observers have optimal introspective access to perceptual processes even for visually masked stimuli] but in my SDT case, it would make the stimuli very hard to spot. Unfortuenatly, its quite a rabbit hole, so any advice or suggestions would be appricated.

Currently used matrials: https://imgur.com/a/k6c4lsv

2

Maybe I am not fulling understanding the nuance behind the term “transition” and “[participants recognize] that something has happened” but there is a few things that I can comment on.

  1. Make sure the order of your stimuli in builder is optimized. Psychopy loads objects/stimuli top → down, so when you have a very short duration, it can be awkward and you might not be properly loading stimuli/objects depending on what’s all involved.
  2. What is your monitor refresh rate? Depending on that, it might be better to use frames instead. So try using a frame duration, or frame onset start time (will get into this in the next point). There is the consideration of frame slippage, but if it is run on a local machine, and it has enough processing power, that shouldn’t be an issue.
  3. If the issue is transitions between routines, put the entire trial in a single routine, or at least everything after fixation. With variable target/catch trial durations, the onset would be the same (0 if fixation is not included), then set the duration to be a variable. Then the mask start time is also a variable based on the the trial type, and then the grading response happens after that.

When building subliminal priming experiments these are things I had to mess around with in during piloting to ensure the “subliminal” part does not become “supraliminal” if you will haha.

Issac