March 2012
Volume 53, Issue 14
ARVO Annual Meeting Abstract  |   March 2012
Crowding of Low-contrast Acuity Targets
Author Affiliations & Notes
  • Harold E. Bedell
    College of Optometry, University of Houston, Houston, Texas
  • John Siderov
    Vision & Hearing Sciences, Anglia Ruskin University, Cambridge, United Kingdom
  • Footnotes
    Commercial Relationships  Harold E. Bedell, None; John Siderov, None
  • Footnotes
    Support  Supported in part by a grant from the Evelyn Trust, Cambridge, UK
Investigative Ophthalmology & Visual Science March 2012, Vol.53, 4798. doi:
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      Harold E. Bedell, John Siderov; Crowding of Low-contrast Acuity Targets. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4798.

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      © ARVO (1962-2015); The Authors (2016-present)

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Purpose: : Previous investigators reported the impairment of foveal visual acuity that is produced by nearby flanking targets (crowding), is reduced or eliminated when acuity is measured for low-contrast targets. Unlike earlier studies, we compared crowding for high- and low-contrast acuity targets using flankers at fixed angular separations, rather than at specific multiples of the acuity target’s stroke width.

Methods: : Percent correct letter identification was determined in 4 adult observers for computer-generated, high-contrast dark Sloan letters (89.5% contrast, 20/11 - 20/15 equivalent) surrounded by 4 flanking bars separated by 0.5 to 5 stroke widths. Similar measures were obtained also for Sloan letters of lower contrast (range: 2.5% to 7.9% contrast across observers), selected to reduce each observer’s visual acuity by 0.2 and 0.4 logMAR. For the low-contrast letters, flanking bar separations ranged between 0.15 and 3.2 stroke widths for the different observers and letter sizes, corresponding to the same angular separations used for the high-contrast letters (range: 0.3 to 3.6 min arc). In all conditions, Sloan letters and flanking bars had the same contrast.

Results: : As reported previously, percent correct letter identification for high-contrast acuity letters is reduced when flanking bars are separated by less than 5 stroke widths. Very similar magnitudes of crowding (~1.3 z scores) occur for low-contrast letters, but for a range of flanking-bar separations that corresponds to a smaller number of stroke widths. The crowding functions obtained for high- and low-contrast letters are very similar when percent correct letter identification is plotted against the flanker separation in min arc.

Conclusions: : Our data for low-contrast letters are consistent with those of previous investigators, who only examined crowding effects for flankers at specific multiples of the letter stroke width. The results indicate that crowding of foveal acuity targets occurs within a fixed angular zone of a few min arc, regardless of the size or contrast of the acuity target.

Keywords: visual acuity • discrimination • pattern vision 

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