Abstract
Introduction: Human beings are usually faster and more accurate when comparing the magnitude of two numbers with greater numerical distance between them. These judgments suggest that numbers are represented spatially along a continuum. By combining a choice-reaching task with a numeric comparison between Arabic numbers, previous studies have shown that adults and children associate numerical magnitudes with spatial response codes (Erb et al., 2018; Song & Nakayama, 2008). Human beings also subitize objects - make judgments about numerical quantities for small sets of objects up to three, rapidly and without counting. Because most studies on subitization only used discrete responses, knowledge is still scant about how subitization evolves and interacts with spatial number representations. Methods: We combined a choice-reaching task and a numeric comparison task. Participants were asked to compare an Arabic number or a number of dots (from 1 to 7) presented on the center square among three to the number four (the standard). They reached to touch the corresponding square: left (less than), right (greater than), or center (equal to). Results: In the Arabic condition, we replicated the numerical distance effect in reach trajectories; the greater the numeric distance between the target and standard, the greater the deviation of the trajectory from the standard. In the dot condition, of interest was whether and how subitizing (judging quantities 1-3) differed from judging larger quantities when making a numerical comparison. We observed that participants initiated their reach movements faster within the subitizing range (less than four dots). We also showed asymmetric trajectory deviation led by subitization within the same numerical distance: a greater deviation from the standard in five- compared to three-dot trials. Conclusion: These results suggest that while subitization lowers the response threshold to initiate movement, it increases local competition for response selection captured by trajectory deviation.