July 2013
Volume 13, Issue 9
Free
Vision Sciences Society Annual Meeting Abstract  |   July 2013
THE FOUR HUMAN VISUAL MECHANISMS SENSITIVE TO GRAY SCALE SCRAMBLES.
Author Affiliations
  • Charles Chubb
    Department of Cognitive Sciences, University of California at Irvine
  • Andrew E. Silva
    Department of Psychology, University of California at Los Angeles
Journal of Vision July 2013, Vol.13, 293. doi:10.1167/13.9.293
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      Charles Chubb, Andrew E. Silva; THE FOUR HUMAN VISUAL MECHANISMS SENSITIVE TO GRAY SCALE SCRAMBLES.. Journal of Vision 2013;13(9):293. doi: 10.1167/13.9.293.

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

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Abstract

Previous research has shown that human vision has three dimensions of sensitivity to grayscale scrambles (spatially random mixtures of different grayscales). However, the mechanisms that confer this sensitivity remain obscure. The current experiments resolve this issue by focusing on search asymmetries revealed when the participant is required to localize a small, circular target patch of scramble with one histogram in a large annular background of scramble with a different histogram. For each of three orthogonal histogram perturbations λk, k=1,2,3, the participant was tested in two (separately blocked) complementary attention conditions: in the first attention condition, the target histogram was dominated by λk and the background was dominated by -λk; in the second, the target histogram was dominated by -λk, the background by λk. Results are modeled by assuming that (1) the participant has a fixed set of mechanisms, each of which is activated nonnegatively by different grayscales, and (2) in a given attention condition the participant uses an optimal linear combination (with weights constrained to sum to 1) of only those mechanisms more strongly activated by the target than by the background. Each of three participants yields the same pattern of results. Four mechanisms are implicated: (1) one mechanism whose sensitivity is zero for black, increases linearly with increasing grayscale and saturates near white; (2) another mechanism whose sensitivity is maximal for black, decreases linearly and flattens to 0 near white; (3) the (previously characterized) "blackshot" mechanism, sharply tuned to the blackest grayscales; (4) a (previously unknown) mechanism whose sensitivity is zero for black, rises sharply to its maximum for grayscales slightly darker than mid-gray, then falls to uniform half-height for all higher grayscales. Because the sensitivity functions of mechanisms (1) and (2) are nearly linearly dependent, these mechanisms collectively confer sensitivity to a 3-dimensional space of histogram variations.

Meeting abstract presented at VSS 2013

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