Re[?]: What is a "hybrid" model?

[Jonathan_Stein@hub1.comverse.com]

 I have been following this thread and can't resist a few comments
 regarding the basic differences between "symbolic" and "neural" processes,
 and the interplay between them.
 
 First, it can be easily demonstrated that the two types of processes both
 exist and are differentiable. Consider, for example, the task of
 determining whether three specific dots among many others form an
 equilateral triangle. When the three dots are red and all the others
 black, this task can be performed quickly, while if the three points are
 marked by different shape (e.g.. small squares) than the others (e.g.
 misc. circles, triangles, etc. of roughly the same size) we resort to
 exhaustive search. The first problem is solved using a connectionist
 technique, while for the second we resort to good old fashioned AI.
 
 Next, it has been demonstrated in psychophysical experiments that there
 are two types of learning. The first type is gradual, with slowly 
 improving performance, while in primates there is also "sudden" learning, 
 where the subject (EUREKA!) discovers a symbolic representation
 simplifying the task. Thus not only is the basic hardware different for
 the two processes, different learning algorithms are used as well.
 
 Finally, regarding the interplay between the two. Biology does not cleanly
 separate the task with defined interfaces (as people typically try to
 do) but employs level mixing. In both speech recognition and reading
 problems it has been demonstrated that the lower (neural) levels provide
 initial best hypotheses, which can be rejected by higher (syntactic, 
 semantic or pragmatic) levels. A nice example is to quickly say
  "How do you wreck a nice beach?"
 in the context of a conversation about speech recognition. Most people
 will hear
  "How do you recognize speech?"
 Another interesting aspect surfaces when there are several different lower
 levels feeding higher ones. In the famous BAGADA experiment a subject
 listens to one phoneme while seeing a film of someone saying a different
 one, and reports hearing a third!
 
 Thus the idea behind "hybrid" systems, composed of decision theoretic and
 symbolic layers is neither  1) trivial and ugly,  2) a hack - wiring 
 together two unrelated layers, nor  3) a matter of semantics and of no 
 interest. Calling them symbiotic rather than hybrid IS a matter of 
 semantics.
 
 Jonathan Stein