I am in a project where we are preparing a visual decision making experiment which requires participants to detect vertical grating in a patch of dynamic noise (the noise pattern is updated on every frame; the experiment design is partly a replication of a published paper). We need to find signal contrast level that would result in 75% accuracy rate, and after multiple attempts with QUEST and the method of constants, finding this level seems impossible — the thresholds estimated by those two methods result in either close to 100% accuracy, or chance performance. The experiment is implemented in PsychoPy.
What could be the reason for these difficulties finding the "right" contrast level for the grating? The image below is an attempt to fit a psychometric function to the experimental data -- the slope is almost vertical. The horizontal orange line is the 0.75 level, the vertical axis shows the proportion of "yes" responses, and the values on the horizontal axis are contrast levels.