Connecting visual objects reduces perceived numerosity and density for sparse but not dense patterns

How is numerosity encoded by the visual system? – directly, or derived indirectly from texture density? We recently suggested that thenumerosity of sparse patterns is encoded directly by dedicated mechanisms (which have been described as the “Approximate NumberSystem” ANS). However, at high dot densities, where items become “crowded” and difficult to segregate, “texture-density” mechanismscome into play. Here we tested the importance of item segmentation on numerosity and density perception at various stimulus densities, bymeasuring the effect of connecting visual objects with thin lines. The results confirmed many previous studies showing that connectingitems robustly reduces the apparent numerosity of patterns of moderate density. We further showed that the apparent density of moderate-density patterns is also reduced by connecting the dots. Crucially, we found that both these effects are strongly reduced at highernumerosities. Indeed for density judgments, the effect reverses, so connecting dots in dense patterns increases the apparent density (asexpected from the physical characteristics). The results provide clear support for the three-regime framework of number perception, andsuggest that for moderately sparse stimuli, numerosity – but not texture-density – is perceived directly