Abstract: PURPOSE: The visual system forms stable visual representations from noisy surroundings by virtue of an effect called Serial dependence, which refers to a phenomenon that current perception is biased towards recently visual experience. For example, a current face is perceived as more attractive when an extreme attractive face could be previously presented. Although the phenomenon has widely been observed in visual perception, previous studies focused on single objects and little studies investigated serial dependence in the ensemble perception. Ensemble coding in visual perception reflects that the visual system can extract an average feature from a set of stimuli. Here we examined whether serial dependence could occur on the ensemble perception of facial attractiveness and facial expression.
METHODS: Experiment 1 (N = 27) aimed to investigate whether serial dependence in the ensemble perception of facial attractiveness could emerge. In each trial, a set of six face images were presented for 1000 ms, and participants were instructed to rate the average facial attractiveness of the set on a visual analogue scale, where 0 denoted “extremely unattractive” and 10 “extremely attractive”. Three types of face sets (i.e., attractive, unattractive and mixed) were included, each type including 30 sets (i.e., 90 trials in total). The attractive and unattractive faces were determined according to our pilot assessing results from 116 female face images selected from the Oslo Faces database. This assessment was conducted by additional 27 participants. Experiment 2 (N = 12) aimed to exclude that serial dependence found in Experiment 1 could originate from single face in the set of six faces. The experimental procedure was the same as Experiment 1, except that face positions of two consecutive trials did not overlap. Experiment 3 (N = 12) aimed to examine whether serial dependence could occur between a single face and a set of faces in the ensemble perception of facial attractiveness. The experimental procedure was the same as that in Experiment 1, except that face stimuli were alternately presented between a single face and a set of six faces. Experiment 4 (N = 12) aimed to further examine whether serial dependence could occur on the ensemble perception of an unstable face attribute (i.e., facial emotional expression). The experiment procedure was similar as that in Experiment 1, except that facial emotional expressions rather than facial attractiveness were manipulated.
RESULTS: Results of Experiment 1 showed that serial dependence emerged in the ensemble perception of facial attractiveness. Results of Experiment 2 demonstrated that serial dependence still existed even when face positions did not overlap in two consecutive trials in the ensemble perception of facial attractiveness. Results of Experiment 3 further showed that serial dependence in ensemble perception of facial attractiveness also occurred when a set of faces and single faces were alternatively presented. Experiment 4 extended above-mentioned results from stable facial features (i.e., facial attractiveness) to unstable facial features (i.e., facial expression), suggesting that serial dependence of ensemble perception also occur in the perception of facial expression.
CONCLUSIONS: Our results suggested that serial dependence occurred in the ensemble perception of facial attractiveness and facial expression. We conclude that the visual system may employ serial dependence mechanism to integrate the perception of ensemble faces, which can improve visual stability in complex environments.

Key words: serial dependence, ensemble perception, facial attractiveness, facial expression