Distinct serial dependence between small and large numerosity processing

Abstract The serial dependence effect (SDE) is a bias in which the representation of current stimuli is attracted to previous stimuli. Although SDE has been observed in many substrates of visual processing, it remains unsolved whether humans adopt a similar SDE to achieve continuity and stability of numerosity processing in two distinct numerical ranges: subitizing (i.e., small numerosity processing, likely related to early object recognition) and estimation (i.e., large numerosity processing, likely related to ensemble numerosity extraction). Here, we demonstrate that subitizing and estimation are characterized by distinct SDEs, underpinned by different mechanisms: a temporal hysteresis-based perceptual mechanism for subitizing and a combination of perceptual and post-perceptual mechanisms for estimation. Critically, a novel type of SDE characterized by reduced processing precision is found in subitizing only, implying that the continuity and stability of numerical processing can be dissociable in dynamic situations where numerical information is integrated over time. Our findings reveal the multifaceted nature of SDE mechanisms and suggest their engagement with cognitive modules likely subserving different functionalities.