Abstract
Numerosity adaptation, the phenomenon where prolonged exposure to a stimulus of greater numerosity makes the subsequent stimulus appear less numerous, and conversely, has been confined to moderated numerosities. This study investigated whether the estimation of small numerosities (1–4), which is performed rapidly and accurately due to the mechanism of subitizing, is susceptible to adaptation. After adapting to a 50-dot stimulus, participants were presented with stimuli consisting of 1–5 color sets. In some trials, participants were informed of the target color set before the presentation of the stimulus, while in others, they were instructed afterwards. When estimating dots in the single-color set or superset, no adaptation aftereffect was observed. The coefficient of variation (CV) was below 0.05, indicating the effective function of subitizing. However, when enumerating subsets in parallel, adaptation biased the estimation. The CV in estimating subitized numerosities was comparable to and correlated with that of estimating moderate numerosities, suggesting that subitizing was superseded by numerosity estimation. Greater aftereffects occur in the probe-after conditions, accompanied by higher perceptual uncertainty. The function of numerosity adaptation can be demonstrated within a Bayesian framework, where the prior adaptor is more weighted to optimize the detection of deviation under high uncertainty.