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[Jim Bower]

	First, I apologize to those that are not interested in this debate
for the amount of traffic it has generated (especially from me).  This
will be my last posting on this subject, but the assumed nature of the 
relationship between AI, neural networks, and connectionism on the one 
hand and the structure of the brain, on the other, has been increasingly
troubling me. With respect to David Rumelhart's
comments, it was probably an error on my part to attempt to more
narrowly define computational neuroscience as modeling related to
the actual structure of the brain (incidentally Steve Lehar's interpretation 
of my definition is off by 180 degrees).  The field as a
whole would not accept this definition.  However, the point that I
was (and have been) trying to make, is that most connectionist modeling 
is related to more cognitive descriptions of brain function, not
to the actual structure of the nervous system.  Further, as my recent
interaction with S. Hanson/J. Pollack over the net should have made
clear, the mapping between cognitive descriptions of brain function,
and actual brain structure is not at all straight forward.
Accordingly, for those of us that are interested in understanding the
brain's structure, it is not clear to me how useful these connectionist
models will be just as it is not clear how useful cognitive approaches 
or AI will be.  Thus, while my previous use of the word irrelevant
was with respect to an historical argument about modeling that
Terry Sejnowski had made, it is really not yet clear what the relevancy 
of the majority of connectionism will be to neurobiology.
That is not to say that this work is irrelevant to its intended objective. 
Clearly connectionism is already making a substantial contribution 
in a number of different fields. It is also possible that useful
tools will be developed.  It is simply to say that if one is interested
in understanding how the brain works, I believe it is necessary to
address ones modeling efforts to the brain's detailed structure.
Understanding this very complex system will not simply fall out by
applying connectionist ideas to speech recognition problems.  
 	It is true that there is a growing effort to apply connectionist
modeling techniques to actual brain structures.  This network is not
the right place to discuss this still relatively minor component of
connectionism.  However, I will say that I fail to be convinced of the
usefulness of these models, and furthermore, I am concerned that
these efforts may actually serve to further obscure the distinctions
between brain organization and the organization of connectionist
models.   It seems to be precisely the association of connectionist
models with network implementations that has confused the question 
of biological plausibility to begin with.  The direct applications 
of connectionist tools to brain modeling makes these distinctions 
even tricker, especially in the larger connectionist/AI/ NN
field where most practitioners know very little about the structure
of the brain to begin with.  As a neurobiologist, however, I would assert 
that even a cursory look at the brain reveals a structure having
very little in common with connectionist models.  In my view this is
not simply a question of necessary modeling abstraction, it is a
question of the basic computational assumptions underlying network
construction (node structure, feed forward components, information
encoding, error detecting, learning, overall complexity). Further, if 
these things are changed substantially, then I would say one no longer
has a connectionist model.  
 	Finally, I would like to point out that I have spent much of the
last seven years communicating and cross-fertilizing with my connectionist, 
neural network, AI, engineering, physicist friends, colleagues, 
and students.  In fact, more than two thirds of the students
in my laboratory are from one or another of these disciplines and we
continue to learn a great deal from each other.  But for communication 
to be successful, and multidisciplinary efforts to be real, there
has to be a serious commitment to two way communication.  In my
view, within connectionism, there has been too much lip service paid
to biological plausibility and not enough commitment to finding out
what that really means.  
 

Jim Bower
jbower at smaug.cns.caltech.edu