Simple pictures, tough problems.

[Steve Lehar]

"Peter J.B. Hancock" <pjh at compsci.stirling.ac.uk> argues...

> just how hierarchical is the visual system?  I'm told of evidence that
> some complex cells respond BEFORE simple cells.

The  fact that a  complex cell fires  BEFORE a  simple  cell does  not
preclude the possibility that  the signal  was  provided originally by
the  simple cells.  What   we are  talking  about  here is  a resonant
process- like a bow  running over  a violin  string.  The bow produces
random vibration on  the string, but  the string only responds to  one
frequency  component of that vibration.   The resultant tone  does not
"start" at the bow and spread outward towards the bridge and neck, but
rather,  a  resonant    vibration emerges  from    the  entire  system
simultaneously.  In the same way, simple cells of all orientations are
constantly firing to greater or lesser degree to any visual input, but
the  complex   cells (by  simultaneous  cooperative  and   competitive
interactions) will resonate  to a larger global  pattern hidden in all
the confusion  of the simple cells.   The fact that  the  complex cell
fires first simply reflects the fact that the visual  system puts more
faith in  a global harmony of  coherent edges than in  a  cacophany of
individual local responses, and indeed the local edges may not even be
perceived until the global coherence is established. This same kind of
interaction would dictate that even highe level  patterns, as detected
by   hypercomplex  or higher  order cells,  would  register before the
complex  cells, so that although  the  input signal arrives bottom-up,
recognition resonance is established top-down,   thus ensuring  global
consistancy of the entire scene.

According  to  the   Boundary Contour   System  (BCS)   model,   local
competitive  and  cooperative interactions occur  between  the  lowest
level detectors  to enhance  those that   are  compatible with  global
segmentations while suppressing those that are  incompatible.   If the
lowest level  interactions could  settle   into  stable configurations
before the   global ones could exert  their  influence, the perception
would be dominated by  local, not global  consistancies, like a damped
violin string which produces a scratchy random noise when bowed.

Peter Hancock continues...

> face-sensitive cells in  monkeys not only  respond within 90mS  ... of
> stimulus presentation, but  are highly stimulus-specific at  that time
> ... This does not leave very  much time for any  pretty hierachies and
> feedback loops.  It implies that  recognition of such highly  familiar
> objects is extremely   feed-forward and  that  any  model  that ALWAYS
> requires many cycles to settle down is wrong.

Well, it depends on what kind of cycles we are talking  about.  If you
mean iterations of a sequential algorithm then I would  have to agree,
and current  implementations of the BCS  and  MRBCS  are  of necessity
performed iteratively  on  sequential binary machines.  But the visual
architecture that   they presume   to  emulate  is a   parallel analog
resonant  system,  like  a  violin string  (but  of course   much more
complex) so that it does not take any number of "cycles" as such for a
resonance to set in.  Also, in  considering recognition time, top-down
expectation plays a  very large role-  it takes considerably longer to
recognise an unexpected  or out-of-context  pattern   than an expected
one.