Numerosity adaptation across size transformations

Abstract Numerosity adaptation occurs across sensory modalities and formats, pointing to an abstract and amodal numerosity coding. Recent findings, however, suggest that it may also be shaped by non-numerical visual properties and perceptual categorization. Here, we examined stimulus size because size variations can be interpreted either as feature differences between stimuli or as scale transformations of the same object viewed from different distances. Across two experiments, participants performed a numerosity discrimination task following adaptation to highly numerous dot arrays. In Experiment 1, adaptor and test stimuli consisted of either small or large dots, yielding congruent trials when their size was identical and incongruent trials when it differed. Numerosity adaptation was stronger in congruent than in incongruent trials, indicating that dot size modulates adaptation magnitude, similarly to other visual stimulus properties. In Experiment 2, adaptor size changed dynamically, mimicking the looming-receding motion in depth and promoting object continuity across size transformations. Dynamic size changes did not eliminate the size-dependent modulation of numerosity adaptation. Instead, salient changes in several non-numerical features significantly interact with numerosity processing, shaping aftereffect magnitude. Overall, these findings indicate that numerosity adaptation is a multifaceted phenomenon arising from interactions among numerical information, low-level visual properties, and visual temporal dynamics.