May 2005
Volume 46, Issue 13
Free
ARVO Annual Meeting Abstract  |   May 2005
Negative Contrasts Predominate in Natural Images
Author Affiliations & Notes
  • C. Ratliff
    Physics,
    University Pennsylvania, Philadelphia, PA
  • P. Sterling
    Neuroscience,
    University Pennsylvania, Philadelphia, PA
  • V. Balasubramanian
    Physics,
    University Pennsylvania, Philadelphia, PA
  • Footnotes
    Commercial Relationships  C. Ratliff, None; P. Sterling, None; V. Balasubramanian, None.
  • Footnotes
    Support  NIH Grant EY08124, NSF IBN–0344678, Research Foundation of the University of Pennsylvania
Investigative Ophthalmology & Visual Science May 2005, Vol.46, 4685. doi:
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      C. Ratliff, P. Sterling, V. Balasubramanian; Negative Contrasts Predominate in Natural Images . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4685.

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

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Abstract

Abstract: : Purpose: OFF ganglion cells have smaller dendritic fields and denser sampling arrays than the corresponding ON cells. We tested whether this corresponds to a naturally asymmetric distribution of negative and positive contrasts. Methods: Natural images (100 from van Hateren's library) were convolved with divisively normalized, center–surround filters at a range of spatial scales. The resulting contrasts were rectified into positive and negative channels; then the contrasts for each channel were quantized into ten equally probable response levels. We calculated for small arrays of rectified filters the mutual information that they convey about their intensity inputs. This calculation was repeated for various filter sizes and spacings. Results: For natural scenes, narrower filters were more likely to report contrasts near zero, and broader filters to report a more uniform distribution of contrasts. Regardless of filter size, negative contrasts outnumbered positive contrasts by about 40%. As controls we convolved these filters with two kinds of artificial images: (i) gaussian distribution of intensities with 1/f2 power spectra identical to natural images ('pink noise'); (ii) natural distribution of intensities with 1/f2 power spectra ('natural pink noise'). For pink noise narrow and broad filters reported the same distribution of contrasts with equal proportions of negative and positive contrasts. For natural pink noise, narrow and broad filters reported the same distribution of contrasts, but negative contrasts outnumbered positive contrasts by about 50%. The difference between natural images and natural pink noise indicates that natural scenes contain spatial correlations beyond those that are scale–invariant. These extra correlations account for about half of the information due to local correlations in natural scenes. Conclusions: Natural images comprise asymmetric distributions of contrast, with dark predominating over bright. The magnitude of this asymmetry roughly matches the asymmetric structure of OFF and ON ganglion cell mosaics. The power spectra and intensity distributions of natural images alone explain the predominance of dark over bright, but not the differences observed between the narrow and broad filters.

Keywords: retina • ganglion cells • image processing 
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