Visual crowding is a combination of an increase of positional uncertainty, source confusion, and featural averaging

William J Harrison,Peter J Bex

doi:10.1038/srep45551

Abstract

Although we perceive a richly detailed visual world, our ability to identify individual objects is severely limited in clutter, particularly in peripheral vision. Models of such “crowding” have generally been driven by the phenomenological misidentifications of crowded targets: using stimuli that do not easily combine to form a unique symbol (e.g. letters or objects), observers typically confuse the source of objects and report either the target or a distractor, but when continuous features are used (e.g. orientated gratings or line positions) observers report a feature somewhere between the target and distractor. To reconcile these accounts, we develop a hybrid method of adjustment that allows detailed analysis of these multiple error categories. Observers reported the orientation of a target, under several distractor conditions, by adjusting an identical foveal target. We apply new modelling to quantify whether perceptual reports show evidence of positional uncertainty, source confusion, and featural averaging on a trial-by-trial basis. Our results show that observers make a large proportion of source-confusion errors. However, our study also reveals the distribution of perceptual reports that underlie performance in this crowding task more generally: aggregate errors cannot be neatly labelled because they are heterogeneous and their structure depends on target-distractor distance.

Highlights

Target-distractor distance alone fails to predict target visibility
Feature substitutions – mistaking a distractor element for a target – are mostly found in paradigms in which the observer is required to report the categorical identity of target such as a letter; trials in which the observer reports a distractor identity instead of the target reveal source confusions[37,38,43], which may be independent of an increase in positional uncertainty[44,45,46]
Report error is defined as the difference between the reported orientation and the actual target orientation, with positive errors indicating a report that was more clockwise than the target

Summary

Introduction

Target-distractor distance alone fails to predict target visibility. For example, the extent of crowding varies according to: colour, shape, or polarity differences between target and flankers[31]; the duration of the crowded display[28]; and perceptual grouping of the flankers[32,33]. We term these phenomena: 1) positional uncertainty34,35, 2) feature averaging[36], and 3) source confusion[37,38]. In this figure we present the same target stimulus, a modified Landolt C, in a series of distractor conditions. The changes in target visibility while viewing the yellow, pink, and green stimuli demonstrate, in order, positional uncertainty, feature averaging, and source confusion. We use experiment and modelling to quantify changes in positional uncertainty, averaging, and source confusions in visual clutter

Methods

Results

Conclusion