Grouping and crowding affect target appearance over different spatial scales.

Bilge Sayim,Patrick Cavanagh

doi:10.1371/journal.pone.0071188

Abstract

Crowding is the impairment of peripheral target perception by nearby flankers. A number of recent studies have shown that crowding shares many features with grouping. Here, we investigate whether effects of crowding and grouping on target perception are related by asking whether they operate over the same spatial scale. A target letter T had two sets of flanking Ts of varying orientations. The first set was presented close to the target, yielding strong crowding. The second set was either close enough to cause crowding on their own or too far to cause crowding on their own. The Ts of the second set had the same orientation that either matched the target’s orientation (Grouped condition) or not (Ungrouped condition). In Experiment 1, the Grouped flankers reduced crowding independently of their distance from the target, suggesting that grouping operated over larger distances than crowding. In Experiments 2 and 3 we found that grouping did not affect sensitivity but produced a strong bias to report that the grouped orientation was present at the target location whether or not it was. Finally, we investigated whether this bias was a response or perceptual bias, rejecting the former in favor of a perceptual grouping explanation. We suggest that the effect of grouping is to assimilate the target to the identity of surrounding flankers when they are all the same, and that this shape assimilation effect differs in its spatial scale from the integration effect of crowding.

Highlights

Target perception in the periphery is impaired by nearby flankers, an interference effect that is called crowding, e.g., [1,2,3], see [4]; for recent reviews see [5,6,7]
Crowding has been explained by several - not necessarily mutually exclusive - mechanisms, such as pooling [11,12], excessive feature integration, e.g., [10], substitution [13,14] and limits of attentional resolution [15,16]
We presented a target letter T of varying orientations flanked by two sets of Ts, making a crossshape with the target in the center (Figure 3)

Summary

Introduction

Target perception in the periphery is impaired by nearby flankers, an interference effect that is called crowding, e.g., [1,2,3], see [4]; for recent reviews see [5,6,7]. Most accounts of crowding agree that crowding is a form of integration over space: target features are spuriously combined with flanker features, e.g., [10,11,12], but see [14]. It is often proposed that the region in which flankers cause crowding, the critical spacing, is around 0.5 times the eccentricity of the target in radial direction ( referred to as Bouma’s law; [3]) and about half that size in tangential direction [8]. Several studies have shown that crowding is reduced when the target differs from the flankers in basic features, such as color [18,21,23,24], see [20], contrast polarity [18], or orientation [2,25,26,27]

Methods

Results

Conclusion