# The radial–tangential anisotropy of numerosity perception

## TL;DR

This study shows that people perceive fewer items when they are arranged radially in their peripheral vision compared to when they are arranged tangentially.

## Contribution

The paper introduces evidence that numerosity perception is affected by radial–tangential anisotropy in peripheral vision.

## Key findings

- Radial displays were consistently reported as less numerous than tangential displays.
- The effect was observed across weak and strong interference conditions and with mixed contrast polarity.
- The anisotropy is attributed to local spatial interactions between visual items.

## Abstract

Humans can estimate the number of visually presented items without counting. In most studies on numerosity perception, items are uniformly distributed across displays, with identical distributions in central and eccentric parts. However, the neural and perceptual representation of the human visual field differs between the fovea and the periphery. For example, in peripheral vision, there are strong asymmetries with regard to perceptual interferences between visual items. In particular, items arranged radially usually interfere more strongly with each other than items arranged tangentially (the radial–tangential anisotropy). This has been shown for crowding (the deleterious effect of clutter on target identification) and redundancy masking (the reduction of the number of perceived items in repeating patterns). In the present study, we tested how the radial–tangential anisotropy of peripheral vision impacts numerosity perception. In four experiments, we presented displays with varying numbers of discs that were predominantly arranged radially or tangentially, forming strong and weak interference conditions, respectively. Participants were asked to report the number of discs. We found that radial displays were reported as less numerous than tangential displays for all radial and tangential manipulations: weak (Experiment 1), strong (Experiment 2), and when using displays with mixed contrast polarity discs (Experiments 3 and 4). We propose that numerosity perception exhibits a significant radial–tangential anisotropy, resulting from local spatial interactions between items.