Abstract

Small and large numbers are processed more quickly and accurately when responses are executed in the left and right side of space, respectively (SNARC effect: Spatial-Numerical Association of Response Codes; Dehaene et al., 1993). By means of the SNARC effect, we investigated the contribution of non-numerical perceptual factors to the processing of numerosity of visual stimuli (circles). Eighty neurologically healthy participants took part in the study. The participants’ task was to decide, by means of lateralized responses (left vs. right), whether the numerosity of the circles was smaller or larger than 20. In addition to spatial–numerical compatibility (i.e., the SNARC effect), the following non-numerical perceptual factors were manipulated: luminance, mean density, total surface area, and total perimeter). The results showed the presence of the SNARC effect, suggesting its independence from non-numerical perceptual factors. Nevertheless, an interaction also emerged between the SNARC effect and some non-numerical perceptual factors (i.e., total surface area, and total perimeter). In conclusion, we argue that numerosity processing largely relies on mechanisms that are independent from non-numerical perceptual ones. Nevertheless, numerical (numerosity) and non-numerical perceptual information could converge in modulating the magnitude of the SNARC effect.