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Interleaved staircase and colour

Hi everyone,

I’m building a contrast threshold experiment. The participant sees a letter which is either normal (e.g. b) or flipped (e.g. d). He must press the keyboard which corresponds to the letter that had been shown. The contrast decreases when the responses are correct. To obtain the threshold of the participant, I’ve built an interleaved staircase : one staircase for the “normal stimuli” and one for the “flipped stimuli”.

The problem with interleaved staircases is that it uses a db stepType and I want a linear stepType because my background is [140,140,140] in rgb255 which corresponds to [-0.059, -0.059, -0.059] in the psychopy colour scale. Then, because I want my stimuli darker than the background, their colour will have a negative value (from - 1 to -0.059). You might know that we can only use a linear steptype with negative values. How to fix it in an interleaved staircase?

But, I have to say that, for unknown reasons, my experiment is not completely bugging with a db steptype… The contrast adapts with wrong and correct answers but … at one point, around the threshold, the staircase seems to bug and jump from low values (darker letters than the background) to higher values (lighter letters than the backgound), even if I fixed the max colour value to -0.059.

Could we change the colour scale to get around these problems…?

Many thanks,