Fundamental units of numerosity estimation

Humans can approximately enumerate a large number of objects at a single glance. While several mechanisms have been proposed to account for this ability, the fundamental units over which they operate remain unclear. Previous studies have argued that estimation mechanisms act only on topologically distinct units or on units formed by spatial grouping cues such as proximity and connectivity, but not on units grouped by similarity. Over four experiments, we tested this claim by systematically assessing and demonstrating that similarity grouping leads to underestimation, just as spatial grouping does. Ungrouped objects with the same low-level properties as grouped objects did not cause underestimation. Further, the underestimation caused by spatial and similarity grouping was additive, suggesting that these grouping processes operate independently. These findings argue against the proposal that estimation mechanisms operate solely on topological units. Instead, we conclude that estimation processes act on representations constructed after Gestalt grouping principles, whether similarity based or spatial, have organised incoming visual input.

Automated summary

Humans have the ability to quickly estimate the number of objects at a glance, but the fundamental units over which this estimation occurs are still unclear. Previous studies suggested that estimation mechanisms only operate on distinct topological units or units formed by spatial grouping cues, not on units grouped by similarity. However, this study demonstrates through four experiments that both spatial and similarity grouping lead to underestimation, indicating that estimation processes operate on representations constructed after Gestalt grouping principles.