Perceptual hysteresis effect.

Perception (ordinate) changes as a sigmoidal function of changing luminance S of the cube’s back-layer as stimulus morphing parameter. The hysteresis paradigm consists of two conditions (red and blue traces) with sequential stimulus presentation, with a stepwise increase (from S1 to S5) and thereafter a decrease (S5 to S9) of stimulus ambiguity. The resulting sigmoid functions change as a function of stimulus presentation order (blue trace: starting with S1; red trace: starting with S9 as indicated by the blue and red arrows). The horizontal distance between the inflection points of the two (blue and red) curves is the hysteresis distance and reflects the influence of immediately past perceptual history, stored in STM. Presenting the stimuli S1 to S9 in random sequence and averaging across repetitions eliminates the influence of STM. The resulting inflection point should thus be located at Sa related to the physically most ambiguous reference cube (Necker cube, Sa = S5). (a) Short-term memory with adaptation impact. The ordered stimulus presentation shifts the sigmoid function toward the direction opposite to stimulus presentation order, with respect to the random condition (black trace). In this case perception already alternates even though the observed stimulus still contains cues favoring the initial percept (S4 for the blue trace and S6 for the red trace). According to a predefined calculation rule (inflection point value from the blue minus inflection point value from the red trace), the hysteresis distance has a negative sign; therefore the effect is called negative hysteresis. (b) Short-term memory with priming impact. The ordered stimulus presentation shifts the sigmoid function toward the direction of the stimulus presentation order, with respect to the random condition (black trace). In this case the percept only alternates when the observed stimulus already contains cues favoring the alternative percept (S6 for the blue trace and S4 for the red trace). The hysteresis distance between the locations of the inflection points for blue minus red traces is positive; therefore the effect is called positive hysteresis.