June 2006
Volume 6, Issue 6
Free
Vision Sciences Society Annual Meeting Abstract  |   June 2006
Neural correlates of letter crowding in the periphery
Author Affiliations
  • A. Cyrus Arman
    Neuroscience Graduate Program, University of Southern California
  • Susana T. L. Chung
    College of Optometry, University of Houston
  • Bosco S. Tjan
    Neuroscience Graduate Program, University of Southern California, and Deparment of Psychology, University of Southern California
Journal of Vision June 2006, Vol.6, 804. doi:10.1167/6.6.804
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      A. Cyrus Arman, Susana T. L. Chung, Bosco S. Tjan; Neural correlates of letter crowding in the periphery. Journal of Vision 2006;6(6):804. doi: 10.1167/6.6.804.

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

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Abstract

PURPOSE: In the periphery, a letter target larger than the size of the observer's acuity can become unrecognizable when flanked by other letters. This “crowding” phenomenon is a vivid demonstration of the nonlinear nature of the deficits in form vision in the periphery. We used a combination of psychophysics and functional-imaging to determine the neural loci of crowding. METHODS: Subjects performed a four-alternative-forced-choice letter identification task in the lower-right periphery at 5° eccentricity. The target-letter (Sloan Letters K, V, N, S) appeared for 100ms in four conditions: target-alone, or with letter flankers at 1.25x, 2x, and 3x letter height center-to-center separation.

RESULTS: Subject's psychophysical performance ranged from 50% in the crowded condition (1.25x separations) to near perfect in the non-crowded conditions (3x separation and ‘target-alone’). In early visual areas (V1, V2, V3) there was little or no difference in the peak amplitude of the BOLD response across conditions. However, in V4 there was a suppression in the crowded condition compared to the non-crowded conditions. This pattern of activity was reversed in anterior regions of LO and in the vicinity of IPS.

CONCLUSION: Our findings suggest a bottom-up origin of crowding: crowding impedes the formation of high-level features somewhere between V1 and V4, resulting in impoverished inputs to LO, which then recruits more spatial-attention to enhance and process the input. In essence, it does not appear to be a lack of attention that causes crowding; instead, crowding impedes bottom-up feature integration and increases attentional demands.

Arman, A. C. Chung, S. T. L. Tjan, B. S. (2006). Neural correlates of letter crowding in the periphery [Abstract]. Journal of Vision, 6(6):804, 804a, http://journalofvision.org/6/6/804/, doi:10.1167/6.6.804. [CrossRef]
Footnotes
 Supported by: NIH/NEI R03-EY016391 (BST) and R01-EY12810 (STLC)
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