June 2007
Volume 7, Issue 9
Free
Vision Sciences Society Annual Meeting Abstract  |   June 2007
Equivalent noise reveals that visual crowding is not an attentional effect
Author Affiliations
  • John Cass
    UCL Institute of Ophthalmology, London, United Kingdom
  • Peter Bex
    UCL Institute of Ophthalmology, London, United Kingdom
  • Roger Watt
    Department of Psychology, University of Stirling, United Kingdom
  • Steven Dakin
    UCL Institute of Ophthalmology, London, United Kingdom
Journal of Vision June 2007, Vol.7, 339. doi:10.1167/7.9.339
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      John Cass, Peter Bex, Roger Watt, Steven Dakin; Equivalent noise reveals that visual crowding is not an attentional effect. Journal of Vision 2007;7(9):339. doi: 10.1167/7.9.339.

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

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Abstract

Several theories propose that the phenomenon of visual crowding - the propensity for neighbouring objects to interfere with target recognition - is caused by a reduction in the spatial resolution of visual attention. We tested this hypothesis by examining the relative effects of visual crowding and attentional load on orientation discrimination. The task involved identifying the mean orientation (clockwise vs. anti-clockwise relative to vertical) of six spatially distributed orientation-filtered noise targets. These were arranged iso-eccentrically, either in isolation or in the presence of distractor objects (randomly oriented filtered noise), located at a constant distance between fixation and the targets. By measuring orientation-discrimination performance at various levels of target-orientation variability, equivalent noise analysis yields (a) the precision with which subjects can estimate the orientation of any one patch (local noise) and (b) the number of samples over which they are averaging (global sampling). In some conditions subjects also performed an attentionally demanding foveal “dual-task”. This was to report the orientation of a white ‘T’ amongst a dynamic stream of randomly oriented black ‘T’s embedded in noise, the proportion of which was varied to maintain 75% correct foveal performance. We report a dissociation between the effects of crowding and attention: whereas crowding induces increases in local noise, attention produces global undersampling. A dual-pass experiment (ie. Two runs using identical stimuli) reveals that crowding does not disrupt the relationship between stimulus-specific-agreement (across runs) and percent-correct performance, indicating that crowding is stimulus-driven rather than an effect of attentional localisation. This psychophysical dissociation between crowding and attention suggests that they are mediated by distinct neural mechanisms.

Cass, J. Bex, P. Watt, R. Dakin, S. (2007). Equivalent noise reveals that visual crowding is not an attentional effect [Abstract]. Journal of Vision, 7(9):339, 339a, http://journalofvision.org/7/9/339/, doi:10.1167/7.9.339. [CrossRef]
Footnotes
 Funded by the Wellcome Trust
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