Neural Representation Characteristics Of Ensemble Size Perception

When multiple stimuli are organized into an ensemble,the human visual system can extract the statistical summary information of the ensemble(such as central tendency and variability)to establish an ensemble representation,allowing people perceive the gist of environment more quickly and efficiently.The ensemble size of a set as a basic visual feature has been thoroughly studied in behavioral science and collected various achievements,but few studies have focused on its neural representation.Here,we combined with electroencephalogram(EEG)recordings and multivariate pattern analysis(MVPA),to explore the neural representation and temporal dynamic characteristics of the ensemble size.Since the previous behavioral experiments have commonly used single representation as a measure of the ensemble average representation,which implicitly assume that the ensemble average representation is equal to the representation of a single stimulus that physically averages the same stimulus set.Therefore,this study compared the neural representation of a single stimulus to explore the differences and temporal dynamic characteristics between it and the ensemble neural representation.For the behavioral results,the study confirmed that the participants could accurately estimate the average size of the circle set.For the neural results,Experiment1 showed that ensemble size and single size could be decoded rapidly from EEG activities with similar time courses,and the processing times of ensemble size and single size were similar,indicating that ensemble size could be extracted at an early stage of visual processing.As for the temporal dynamics,while the ensemble size representation was stable across time,the single size representation was dynamic.Second,the ensemble representations could not generalize to single size representations despite physical equivalence in size.These results showed that the ensemble size representation might have more stable and abstract attributes than the pictorial single size representation.Experiment 2 further explored the abstraction,and found that the ensemble representations could generalize across stimulus arrays varied in the number of items,indicating that the overall representation was not affected by low-level spatial features,and had the characteristics of high-level and abstraction.Interestingly,we found that finer ensemble size representation and better cross-stimulus generalization occurred when larger numbers of items were contained in the stimulus set,indicating that the brain adopted different strategies while facing different sets of conditions: when the number of items is small,the individual processing strategy is adopted,and when the number of items is large,the ensemble processing strategy is adopted.Finally,since the ensemble stimulus and the single stimulus in the above experiment were presented in the peripheral field and the central field respectively,Experiment 3 had confirmed that the distinction between ensemble size and single size representations was not affected by whether they overlapped spatially.Together,our results suggest that the neural representation of ensemble size is distinct from that of single size;instead,ensemble size is represented in an abstract,scene-adapted,and temporally stable form that is not a pictorial representation of the physically matched single stimulus.