Simple pictures, tough problems.

[Peter J.B. Hancock]

A comment on Steve Lehar's comment: just how hierarchical is the
visual system? I'm told (sorry, I don't know a reference and my source
is away on holiday) of evidence that some complex cells respond {\em before}
 simple cells.  I understand there is some debate about the
simple/complex dichotomy anyway, but such a result does challenge the
traditional story of simple cells feeding into complex ones.

More remarkably, recent work from Dave Perrett's group at St. Andrew's
is showing that face-sensitive cells in monkeys not only respond
within 90mS (yes, ninety milliseconds) of stimulus presentation, but
are highly stimulus-specific at that time (I don't know if this is yet
published).  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 {\em always} requires many cycles to settle down is wrong.

This begs the question of what all the feedback loops in the brain are
doing.  It may be that not all stimuli are as privileged as faces and
that more processing is required for less familiar things.  It may be
to do with the initial learning, rather like Grossberg's ART which may
cycle while learning, but is one-shot thereafter.  It may be to do
with tuning early systems for given visual tasks.  It certainly needs
more research...

Peter Hancock
Centre for Cognitive and Computational Neuroscience
University of Stirling.