Abstract
Human observers adopt two types of strategies for rapid enumeration -- subitizing and estimation for small and large sets, respectively. According to spatial and temporal presentations of numerical stimuli, subitizing can be divided into simultaneous subitizing and sequential subitizing. Previous studies had shown that simultaneous subitizing is related to object individuation (a process of separating each item in a set from other items, based on its temporal or/and spatial characteristics). However, there is so far little research to tackle the underlying mechanism of sequential subitizing and no study to explore directly whether these two types of subitizing share a common object individuation-based mechanism. The present study pits the two subitizing types against each other in a dual-task paradigm. Participants were required to complete a sequential enumeration task overlapped temporally with a simultaneous enumeration task. Our results demonstrated that the sequential enumeration significantly influenced the simultaneous subitizing, even after the effects of mere physical exposure were partialled out. Comparing to a dual-task composed of sequential counting and simultaneous subitizing, a significant "resource competition" was revealed for a dual-task composed of sequential subitizing and simultaneous subitizing, manifested by increased error rates and decreased accuracy and precision. In contrast, the sequential enumeration had no such a carry-over effect on the simultaneous estimation when object individuation is not involved. Taken together, our study demonstrated that simultaneous and sequential subitizing share a common object individuation-based mechanism. When these two types of subitizing were temporally close to each other, a "resource competition" can occur, likely due to the limited capacity of object individuation processing.