H. R. Wilson and F. Wilkinson.
  <b>Evolving Concepts of Spatial Channels in Vision: from
  Independence to nonlinear Interactions</b>.
  <i>Perception and Psychopysics</i>, 26:939-960, 1997.
  <A HREF="http://vis.cs.brown.edu/results/bibtex/Wilson-1997-ECS.bib">(bibtex:
  Wilson-1997-ECS)</A>.<br />