The Bouma law of crowding, revised: Critical spacing is equal across parts, not objects

Sarah Rosen, Ramakrishna Chakravarthi, D. G. Pelli

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42 Citations (Scopus)


Crowding is the inability to identify an object among flankers in the periphery. It is due to inappropriate incorporation of features from flanking objects in perception of the target. Crowding is characterized by measuring critical spacing, the minimum distance needed between a target and flankers to allow recognition. The existing Bouma law states that, at a given point and direction in the visual field, critical spacing, measured from the center of a target object to the center of a similar flanking object, is the same for all objects (Pelli & Tillman, 2008). Because flipping an object about its center preserves its center-to-center spacing to other objects, according to the Bouma law, crowding should be unaffected. However, because crowding is a result of feature combination, the location of features within an object might matter. In a series of experiments, we find that critical spacing is affected by the location of features within the flanker. For some flankers, a flip greatly reduces crowding even though it maintains target–flanker spacing and similarity. Our results suggest that the existing Bouma law applies to simple one-part objects, such as a single roman letter or a Gabor patch. Many objects consist of multiple parts; for example, a word is composed of multiple letters that crowd each other. To cope with such complex objects, we revise the Bouma law to say that critical spacing is equal across parts, rather than objects. This accounts for old and new findings.
Original languageEnglish
Article number10
Number of pages15
JournalJournal of Vision
Issue number6
Publication statusPublished - 4 Dec 2014

Bibliographical note

This research was supported by NIH grant EY04432 to Denis Pelli. We also thank Dennis Levi, Michael Landy, Athena Vouloumanos, Jacob Feldman, Katharine Tillman, and Nate Blanco for helpful comments. This research appeared as a chapter in Rosen's unpublished Ph.D. thesis (Psychology Department, New York University, 2012). This paper is one of a set of papers using crowding to characterize object recognition (Rosen, Chakravarthi, & Pelli, 2014; Rosen & Pelli, 2014a, 2014b).


  • crowding
  • peripheral vision
  • critical spacing
  • feature combination
  • Bouma Law


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