One step beyond! The conjunction-bias between non-spatial (Color) and spatial Grouping cues over approximate number perception extends to Symmetry and Connectedness.

Humans and animals share the cognitive ability to quickly extract approximate number information from sets. Main psychophysical models suggest that approximate numerosity relies on segmented units, which can be affected by Gestalt rules. Indeed, arrays with spatial grouping cues, such as connectedness, closure, and even symmetry, are underestimated compared to ungrouped arrays with the same low-level features. Recent evidence suggests that also non-spatial cues, such as color-similarity, trigger numerosity underestimation. However, in natural vision, several grouping cues may coexist in the same stimulus. Notably, conjunction of grouping cues (e.g., color + closure) reduces perceived numerosity following an additive rule. Here we investigated the effect of two Gestalt cues, connectedness and symmetry, over numerosity perception both in isolation and, critically, in conjunction with color. Participants performed a comparison-task between a reference and a test stimulus varying in numerosity. In Experiment 1, test stimuli contained two isolated groupings (connectedness or color-similarity), a conjunction (connectedness and color-similarity), and a neutral condition (no groupings). We found that the point of subjective equality (PSE) was higher in both isolated grouping conditions compared to the neutral condition. Importantly, in the conjunction condition, the biases from single grouping cues added linearly, resulting in a numerosity underestimation equal to the sum of the isolated biases. In Experiment 2 we found that conjunction of symmetry & color followed the same rule. These findings strongly suggest that both spatial and non-spatial isolated cues affect numerosity perception. Crucially, we show that their conjunction-effect extends to Symmetry and Connectedness.