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