December 2002
Volume 43, Issue 13
ARVO Annual Meeting Abstract  |   December 2002
Detection- and discrimination thresholds for Landolt C in contour interaction
Author Affiliations & Notes
  • O Ehrt
    Ophthalmology & Vis Sci McGill University Montréal PQ Canada
  • CB Williams
    Ophthalmology & Vis Sci McGill University Montréal PQ Canada
  • RF Hess
    Ophthalmology & Vis Sci McGill University Montréal PQ Canada
  • Footnotes
    Commercial Relationships   O. Ehrt, None; C.B. Williams, None; R.F. Hess, None. Grant Identification: Canadian Institutes of Health Research: MOP 10818
Investigative Ophthalmology & Visual Science December 2002, Vol.43, 2837. doi:
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      O Ehrt, CB Williams, RF Hess; Detection- and discrimination thresholds for Landolt C in contour interaction . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2837.

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

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Abstract: : Purpose: Crowding and contour interaction are usually measured at the visual acuity limit as percent correct. We used a larger stimulus to see if these results generalize to stimuli closer to reading tasks and to be able to investigate the spatial frequency properties of contour interaction. Comparing contrast threshold for detecting and discriminating the stimulus could further help understand the mechanisms of contour interaction. Methods: The contrast thresholds for detecting the presence of a dark Landolt C (visual angle 0,5°) surrounded by high contrast bars at 0 - 2 gap width separation were determined on 2 normal subjects in a 2-interval AFC procedure with constant stimuli for foveal and 10° eccentric presentation (spatially scaled stimulus). Thresholds for discriminating the location of the gap were measured in the same conditions with a 4 - AFC procedure. We repeated these measurements with a filtered C (mean SF 1.9 cycles/letter) surrounded by filtered bars (mean SF 0.5 - 8 c/letter) at 0.5 bar width separation. Results: Mean contrast threshold increased from 0.07 (without bars) to 0.085 (bars abutting) for detection and from 0.11 to 0.17 for discrimination. The ratio c disc / c detect increased from 1.5 to 2.1 for dark bars and to 2.5 for white bars. Both contrast thresholds and their ratio did show a marked tuning function with regard to the spatial frequency of the bars. No qualitative difference was found between central and peripheral presentation. Conclusion: Discrimination and detection show different results. Discrimination is more affected by flanking bars than detection. Thus masking does only account for part of the - weak - contour interaction that is present in normal subjects.

Keywords: 368 contrast sensitivity • 586 spatial vision 

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