Abstract
Visual perception is serially dependent: features of a current stimulus are judged more similar to previously shown stimuli than they really are, demonstrating an attractive bias towards the past. However, certain studies have also revealed repulsive biases in perception. They are considered a consequence of short-term adaptation to a stable sequence of stimuli, while the attractive bias may correspond to a post-perceptual bias associated with decision stability (resulting in a two-process model). However, this issue is highly debated, in part because it is typically difficult to disentangle stimulus effects from decision effects. In two experiments, we managed to dissociate the stability of stimuli within-trial (by showing a sequence of 4 to 12 Gabors presenting either a stable or volatile orientation across the sequence) and the stability of decisions across trials (by making the orientation to report on each trial stable from one trial to another or random). In line with the two-process model, opposite serial dependence was found for within-trial and between-trial sequences. Within a single trial, serial dependence was repulsive, but only when the sequence of stimuli was stable. Between trials, serial dependence was attractive, with a stronger effect when the orientation reported was random, compared to stable. Interestingly, the number of stimuli presented within the same sequence affected the strength of serial dependence: when the sequence within the trial was stable, the repulsive effect became larger as the number of stimuli increased, which was not the case for the volatile sequence. Overall, the results reinforce the idea of a two-process model of perceptual decisions, with a repulsive bias strongly dependent on the stability of current perception and an attractive bias towards previous decisions, whose strength is modulated by the stability of decision history.