Adaptation acts directly on the sensory representation of numerosity.

Like most perceptual systems, the perception of numerosity is susceptible to adaptation. However, while adaptation to large numerosities is large and spectacular, obvious on demonstration, adapting to sparse fields (reverse adaptation) causes smaller effects, not readily visible on demonstration, raising the possibility that it may be due to response biases rather than genuine changes to perceptual mechanisms. To investigate this further, we measured adaptation to both dense and sparse dot patterns while simultaneously measuring confidence and reaction times. Both forms of adaptation distorted numerosity estimates in the expected direction, robustly and highly significantly. Importantly, the shifts in perceived numerosity were accompanied by similar shifts in confidence and reaction-time distributions. After adaptation, maximum uncertainty and minima in response-times occurred at the point of subjective (rather than physical) equality of the matching task, suggesting that adaptation acts directly on the sensory representation of numerosity, before the decisional processes. These results reinforce existing evidence for adaption of numerosity, consistent with it being a primary perceptual attribute.