# Optimal Coding

### o Temporal decorrelation: a theory of the lateral geniculate nucleus


Natural time-varying images possess significant temporal correlations
when  sampled  frame   by   frame   by   the  photoreceptors.   These
correlations persist even after retinal  processing  and hence, under
natural  activation  conditions,  the  signal  sent  to  the  lateral
geniculate  nucleus  is  temporally  redundant  or  inefficient.   We
explore the hypothesis that the LGN is concerned, among other things,
with improving  efficiency  of  visual  representation through active
temporal decorrelation of the retinal  signal  much  in  the same way
that  the  retina  improves  efficiency  by  spatially  decorrelating
incoming images.  Using some recently measured statistical properties
of  time-varying  images,  we  predict  the spatio-temporal receptive
fields that achieve this decorrelation.  It is shown that, because of
neuronal nonlinearities, temporal decorrelation requires two response
types,  the  {\it  lagged}  and  {\it  nonlagged},  just  as  spatial
decorrelation requires {\it on} and  {\it  off}  response types.  The
tuning and response properties  of  the  predicted  LGN cells compare
quantitatively well with  what  is  observed  in recent physiological
experiments.

^{Dong D W and Atick J J 1995}
{Temporal decorrelation:  a theory of lagged and nonlagged
responses in the lateral geniculate nucleus}
{Network: Computation in Neural Systems}{ Vol~6(2) pp~159-178}


### o Spatiotemporal inseparability of natural images and visual sensitivities

   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.

^{Dong D W 2001}
{In: Computational, neural & ecological constraints of visual motion
processing (Zanker JM, Zeil J, eds)} {pp~371-380}


### ( Papers' Index of Dawei Dong )

Send comments to Dawei Dong: dawei@ccs.fau.edu