The visual system is concerned with the  perception  of  objects in a
   dynamic world.  A significant fact about  natural time-varying images
   is that they do not change  randomly  over  space-time; instead image
   intensities at different times  and/or  spatial  positions are highly
   correlated.  We measured the  spatiotemporal  correlation function --
   equivalently the power spectrum -- of natural images and we find that
   it is non-separable, i.e., coupled in space and time,  and exhibits a
   very interesting scaling behaviour.  This  behaviour  is  shown to be
   related to  the  motion  in  the  images  and  the  power spectrum is
   naturally separable into a spatial  term  and  a  velocity term.  The
   same kind of spatiotemporal  coupling  and  scaling  exists in visual
   sensitivity measured in physiological and psychophysical experiments.
   By  assuming  that  the  visual  system   is   optimized  to  process
   information of natural  images,  a  quantitative  relationship can be
   derived between the power spectrum of natural  images  and the visual
   sensitivity, This reveals some interesting aspects of motion vision.

   (In: Computational, neural & ecological constraints of visual motion
    processing, Zanker JM, Zeil J, eds, page 371-380. 2001)

   (Papers' Index of Dawei Dong)