Characterizing the in-out asymmetry in visual crowding

Ramakrishna Chakravarthi* (Corresponding Author), Jirko Rubruck, Nikki Kipling, Alasdair D. F. Clarke

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

3 Citations (Scopus)
3 Downloads (Pure)


An object's processing is impaired by the presence of nearby clutter. Several distinct mechanisms, such as masking and visual crowding, are thought to contribute to such flanker-induced interference. It is therefore important to determine which mechanism is operational in any given situation. Previous studies have proposed that the in-out asymmetry (IOA), where a peripheral flanker interferes with the target more than a foveal flanker, is diagnostic of crowding. However, several studies have documented inconsistencies in the occurrence of this asymmetry, particularly at locations beyond the horizontal meridian, casting doubt on its ability to delineate crowding. In this study, to determine if IOA is diagnostic of crowding, we extensively charted its properties. We asked a relatively large set of participants (n = 38) to identify a briefly presented peripheral letter flanked by a single inward or outward letter at one of four locations. We also manipulated target location uncertainty and attentional allocation by blocking, randomizing or pre-cueing the target location. Using multilevel Bayesian regression analysis, we found robust IOA at all locations, although its strength was modulated by target location, location uncertainty, and attentional allocation. Our findings suggest that IOA can be an excellent marker of crowding, to the extent that it is not observed in other flanker-interference mechanisms, such as masking.
Original languageEnglish
Article number10
Number of pages14
JournalJournal of Vision
Issue number11
Early online date20 Oct 2021
Publication statusPublished - Oct 2021

Bibliographical note

We thank John Greenwood, Mauro Manassi and Will Harrison for valuable comments on the study design and particularly for the suggestion of scaling letters using QUEST to estimate the extent of crowding (John Greenwood). We also thank Marlene Poncet for several discussions about the results. RC was supported by grant BB/R009287/1 from UKRI BBSRC.

Data Availability Statement

All supplementary materials are available as an html file at the OSF site:


  • crowding
  • masking
  • inner-outer asymmetry (IOA)
  • attention
  • location uncertainty


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