With a randomized stimulus presentation sequence (black trace) the influence of perceptual STM should disappear after averaging across repetitions.
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Accordingly the inflection point from the random sigmoid function should be located at the physically most ambiguous cube (the Necker cube, Sa = S5), serving as a reference stimulus. However, perception of the Necker cube is known to be biased towards the front-side down perspective (FDP), reflecting the contribution of LTM during perceptual disambiguation. This perceptual bias will be indicated by larger probabilities for cube percepts favoring the FDP interpretation. As a consequence, the inflection point of the related sigmoid function, indicating the perceptually most ambiguous cube variant (between S3 and S4) will already contain 3D cues favoring the non-preferred FUP interpretation. This results in a horizontal shift of the sigmoid function from the random condition (S4 in a and S3 in b, black trace). (a) Assuming an additive impact of FSTM and FLTM during the perceptual process, the sigmoid functions from the two ordered conditions should be shifted by the same amount and in the same direction as the sigmoid function from the random condition. (b) However, there must be a threshold for memory contribution to the perceptual process: If this threshold Sthres is approached by FLTM (i.e. if the bias is strong enough), the effective influence of STM (FSTM pointing in the same direction as FLTM) on the sigmoid function will attenuate, and FLTM and FSTM influences become subadditive. The related sigmoid function will then be closer to the one from random presentation order, as indicated by the red shaded traces. In the extreme case of very strong bias, the two sigmoid functions may coincide.