September 2005
Volume 5, Issue 8
Vision Sciences Society Annual Meeting Abstract  |   September 2005
Contrast gradients increase apparent egospeed while moving through simulated fog
Author Affiliations
  • Brian P. Dyre
    University of Idaho
  • William A. Schaudt
    Virginia Tech Transportation Institute
  • Roger T. Lew
    University of Idaho
Journal of Vision September 2005, Vol.5, 335. doi:10.1167/5.8.335
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      Brian P. Dyre, William A. Schaudt, Roger T. Lew; Contrast gradients increase apparent egospeed while moving through simulated fog. Journal of Vision 2005;5(8):335. doi: 10.1167/5.8.335.

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

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When fog is simulated as a global reduction in contrast, apparent egospeed decreases as fog becomes denser (Snowden, Stimpson, and Ruddle, 1998, Nature). However, fog is more realistically modeled as Mei scattering of ambient light, which reduces contrast exponentially as distance increases. In addition to reducing global contrast, this exponential fog model introduces a contrast gradient in depth, which may change the sampling of optical flow to emphasize nearer objects, thereby increasing the rate of global optical flow, which may result in increases in apparent egospeed (Larish & Flach, 1990, JEP:HPP). We examined whether apparent egospeed is affected by this contrast gradient when global contrast is held constant and fog is modeled exponentially. Observers sequentially viewed pairs of 1-3 s computer simulations of observer translation over a textured groundplane. The display pairs consisted of a standard, for which the simulated translational speed and fog density remained fixed throughout the experiment, and a comparison, for which the speed and density each varied independently over five levels. Observers indicated which display produced greater apparent egospeed. For each level of fog density, apparent egospeed and Weber fractions for egospeed discrimination were estimated by fitting 2-parameter sigmoid functions to the proportion of “faster” judgments as functions of translational speed. Results showed that apparent egospeed increased linearly by approximately 5% as the exponential fog density parameter increased 67%. Weber fractions were unaffected (µ=.069). While moving through real fog, this increase in apparent egospeed due to the contrast gradient opposes the decrease in apparent egospeed due to the global reduction in contrast. Hence, a more accurate understanding of how fog density affects apparent egospeed must account for changes in both the contrast gradient and global contrast. Further experiments examining these variables simultaneously will be discussed.

Dyre, B. P. Schaudt, W. A. Lew, R. T. (2005). Contrast gradients increase apparent egospeed while moving through simulated fog [Abstract]. Journal of Vision, 5(8):335, 335a,, doi:10.1167/5.8.335. [CrossRef]

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