Nichole Suero Gonzalez¹, Frank Durgin¹, Alex Huk²; ¹Swarthmore College, ²University of California, Los Angeles

Density is a visual property that seems relevant to the estimation of visual number. Some argue that number is encoded directly; others that density and number are entangled: Both density and number are strongly biased by patch size. One reason patch size may affect perceived density is that size and density, rather than number, are distance cues, so a larger patch might be a nearer patch that is higher in density than it appears. To study effects of patch size on density and number in a more naturalistic situation in which depth matters, we used virtual environments, rendered in a high-resolution head-mounted display (HMD). The basic task consisted of a brief presentation of a circular dot texture (~ 30 dots) at 1m viewing distance, followed by a second dot texture at another viewing distance (0.5-2m). The second texture varied in size, number, and density. Participants (14) made either number comparisons (was the second patch more or fewer dots than the first?) or density comparisons (was the second patch denser or less dense than the first?) in separate sessions. An additional between-session manipulation involved using either retinally matched textures (the simulated dots and patches of dots were enlarged proportionally with distance to project the same retinal sizes), or real-world image variations in another, such that the dots and patches were retinally smaller at far distances and retinally bigger at near distances. We consistently found that both number and density judgments were strongly and equally affected by the relative sizes of the two patches. This shows that, even in a simulated real-world context, participants' judgments of relative number and of relative density remain entangled. Because number is (in principle) invariant with distance, effects of patch size on number contradict direct encoding, and support the idea that number is entangled with density perception.

Acknowledgements: Swarthmore College