From shape to number: Shape-from-dots homogeneity boosts groupitizing enumeration.

Enumerating a large set of objects (e.g., more than four items) can be accomplished more quickly and/or accurately when the objects are grouped into clusters based on Gestalt principles such as proximity and color similarity, a phenomenon known as "groupitizing." However, whether other visuospatial features can similarly influence this mechanism remains unclear. This study investigated the impact of a novel feature: shape-from-dots homogeneity. Participants performed a simple enumeration task involving dot patterns, ranging from four to 20 items, spatially arranged in small clusters. In Experiment 1, the dots within the clusters were placed to form either homogeneous patterns of regular quadrilaterals (e.g., squares) or heterogeneous patterns of irregular, randomly shaped quadrilaterals. To test whether the effect was simply due to symmetry/canonicity, in Experiment 2, the dots within the clusters were placed to form either homogeneous patterns of regular quadrilaterals (e.g., squares) or homogeneous patterns of irregular, randomly shaped quadrilaterals. The results revealed that enumeration reaction times were significantly faster when clusters formed homogeneous shapes compared to heterogeneous ones (Experiment 1), while no difference was found when both patterns contained homogeneous arrays independently of the shapes (Experiment 2), ruling out that the effect was merely driven by spatial symmetry or canonicity. These findings indicate a close interaction between general shape processing and numerosity perception in the Groupitizing mechanism. This suggests that shape-from-dots homogeneity can facilitate numerosity processing akin to other Gestalt principles, likely promoting a multiplication mechanism.