Object groups mutually bias each other’s perceived average size, as evidenced by neural and behavioural data

Abstract The human visual system simultaneously computes summary statistics of object features, such as average size, across groups of objects. Evidence suggests that summary statistics across groups of objects (ensembles) are not independent but influence one another. We investigated whether an ensemble’s average size affects the perceived average size of another ensemble, and whether such changes are reflected in altered activation patterns in the visual cortex. Distractor ensembles’ average sizes altered the perceived average size of target ensembles in a contrast-like manner: Large distractor ensembles decreased the target ensembles’ perceived average sizes; conversely, smaller distractor ensembles increased target ensembles’ perceived average sizes. These contrast effects were reflected in more activated voxels in retinotopic regions when target ensembles were perceived as larger. Distractor ensembles’ voxel counts were similarly modulated by target ensembles’ average size, suggesting their perceived average size was also altered. Additionally, individual differences in the magnitude of the ensemble-size contrast effect covaried with activation in higher-level regions, including the left intraparietal sulcus (IPS) and middle temporal gyrus (MTG). Together, these findings suggest that ensemble summary statistics are contrasted, resulting in changes in perceived size and altered retinotopic activation pattern, with involvement of a network including left IPS and left MTG.