Ebbinghaus illusion changes numerosity perception independent of density perception

It has been posited that number and space are processed with shared mechanisms. We examined whether the stimulus configuration that leads to the Ebbinghaus size illusion would change numerosity perception. We first prepared visual stimuli with which perceived densities were equated for given dot number and area size (Experiment 1). In the subsequent experiments using the prepared stimuli, two sets of dots were surrounded by two sets of inducers (one consisted of small circles and the other of large circles). Participants reported which area contained the larger number of dots (Experiment 2) and which area where the dots appeared was larger (Experiment 3). The results showed that dots surrounded by the small (large) inducers appeared more (less) numerous and occupied a larger (smaller) spatial extent. These results suggest that visual processes of numerosity interact with those of size perception independent of visual processes for perceived density.