Fig. 1.

Example of stimuli with different numerosity (4 vs. 8 elements), controlled for the main continuous variables (sketched in red). a Inter-distance (ID): both groups are constructed with the same mean inter-distance between all elements. b Convex hull (CH): both groups are constructed with the same convex hull. c Density (D): both groups are constructed with the same density (n/CH). d Radius (R): both groups are constructed with all equal elements (fixed radius). e Total perimeter (TP): the sum of all the elements’ perimeters is the same for both groups. f Total area (TA): the sum of all the elements’ areas is the same for both groups. Combinations of one variable from the left group of controls (ID or CH or D) with one from the right (R or TP or TA) are also possible, allowing for maximum flexibility (see also Table 2 and Fig. 6)