An undeniable interplay: Both numerosity and visual features affect estimation of non-symbolic stimuli

Converging lines of evidence suggest that the numerical abilities in Humans are rooted in the approximate number system (ANS): an innate, non-verbal mechanism that enables to estimate the numerosity of a set of items with little effort. Nevertheless, the high correlation between visual features and numerosity in the natural environment always constituted a relevant methodological problem that gathered growing concern throughout the years. This issue led some researchers to cast doubts on the existence of a system able to process numerical information independently from the influence of visual features. In the present study, we sought to shed light on the interplay between numerosity and visual features. To this aim, we implemented a non-symbolic estimation task which included a calibration phase. After performing a pre-calibration block, participants were presented with the calibration image for 20 s, and they were divided in three groups, according to the calibration stimulus they att...