Outlier rejection in the process of pooling

Ensemble perception allows our visual system to process large amounts of information efficiently by summarizing its statistical properties. A key aspect of ensemble perception is the devaluation of outlying elements, which leads to more informative summary statistics with reduced variance and a more representative mean. However, the mechanisms underlying this outlier rejection process are not well understood. One possibility is that outliers are selectively excluded before summarization. To test this, we investigated whether only weaker items were excluded from averaging. We manipulated the encoding strength of items in a display by changing the emotional intensities of faces, the spatial location of emotional outliers, and the spatial distribution of emotional faces. We found that the response to outliers varied depending on their location. Specifically, outliers were more likely to be excluded from averaging when presented in more peripheral regions, while their exclusion was partial in parafoveal regions. In other words, outlier rejection in ensemble processing is more flexible than the supposed rigid designation of weighting against outliers. Alternatively, the results fit well with hierarchically structured pooling, during which outliers are discounted more dynamically without positing any separate selective mechanism before summarization. We propose an explanation for outlier rejection in light of a recently proposed population response model of ensemble processing.