Top graph: Grand mean probability of happy face percepts (filled dots) ± SEM (ordinate) and sigmoidal fits from the sad→happy (blue trace), sad←happy (red trace) and random (black trace) conditions.

A clear negative hysteresis effect is visible: the sigmoids from the differently-ordered stimulus presentation conditions (indicated in blue and red) are horizontally shifted with respect to one another. The sigmoid from the random condition (black trace) shows a mid position in between the two sigmoids from the ordered conditions. This mid position indicates that memory contribution to the perceptual resolution of ambiguity has not yet reached a threshold. The green difference trace results from a subtraction of the blue trace minus the red trace, indicating the percentage of perceptual variances for individual stimulus variants. Bottom graph: The reaction times (ordinate on the left) related to the perception responses. Reaction times increase with perceptual instability. Furthermore a temporal preparation effect was observed: Less temporal preparation is possible for the first stimulus within an ordered presentation sequence, resulting in longer reaction times compared to the subsequent reaction times.