Statistical regularities compress numerical representations.

Numerical information can be perceived at multiple levels of abstraction (e.g., one bird, or a flock of birds). The unit of input for numerosity perception can therefore involve a discrete object, or a set of objects grouped by shared features (e.g., color). Here we examine how the mere co-occurrence of objects shapes numerosity perception. Across three between-subjects experiments, observers viewed arrays of colored circles and estimated the number of circles in the array. In Experiment 1, unbeknownst to the observers, each array was constructed from either color pairs (i.e., regularities) in the structured condition, or the same circles in a random arrangement in the random condition. Aside from the regularities, the two conditions were identical in terms of numerosity, color, and density. We found that the number estimates were reliably lower in the structured condition than in the random condition, although observers were not explicitly aware of the regularities. This underestimation could be driven by either the presence of color pairs, or attention implicitly drawn to individual circles. To tease these two ideas apart, in Experiment 2 we examined the effect of grouping on numerosity perception, by introducing color duplicates (e.g., two red circles) in the structured condition. Number estimates were again reliably lower in the structured condition, suggesting that the underestimation could be explained by grouping. Finally, in Experiment 3 we examined the effect of local attention on numerosity perception, by presenting either two distinct colors in the array in the pop-out condition, or circles of the same color in the uniform condition. We found no difference in the number estimates between the two conditions, thus ruling out the role of local attention in numerosity perception. These results demonstrate that object co-occurrences cause numerical underestimation, suggesting that regularities serve as an implicit grouping cue that compresses numerical representations. Meeting abstract presented at VSS 2015.