I have been arguing privately with some researchers about a
control theoretic approach to learning in the brain. I
thought it would be beneficial to bring up this topic for
some open discussion among scientists in all of the
brain-related fields. I am not a neurobiologist or a
cognitive scientist or a computational neuroscientist, so I
would love to get some inputs from them on this topic. Our
ability to develop autonomous robots that can learn on
their own depends on getting our basic learning framework
right. Otherwise, this science will be stalled for years to
come. I present my arguments below for a control theoretic
framework.
This note is being sent to many lists. I will collect
all of the responses from different sources and
redistribute them to all of the participating lists.
The last few discussions have been very productive. It has
led to the realization by many researchers in the
connectionist field that this science needs a "rebirth." So
I hope there would be a vigorous debate on this issue.
Asim Roy
Arizona State University
[Note from the moderator: If a short summary is sent to comp-neuro, and it
apears to have sufficient relevance to neuroscience, I will post it to this
mailing list. Otherwise, please look for followup discussion on connectionists
or one of the other lists to which this posting was sent.]
==========================================
BRAINS INTERNAL MECHANISMS - THE NEED FOR A NEW PARADIGM
Perhaps a more careful observation of living objects
as a physical system is needed to get a deeper insight on
how the brain actually works. Unlike other physical
systems, like a hurricane or an ocean or a volcano, some
living objects are quite unique in the sense that they
possess a physical entity called the "brain" which no other
systems have. And one of the most unique characteristics of
these systems is that the brain "controls" the behavior of
other entities in the system, such as the movement of its
various limbs, the eye movements and so on. Some scientists
might argue that it is equally valid to say that the limbs
inform and therefore control the brain. But saying that the
limbs inform and therefore control the brain would be the
same as saying that a car or a train or an airplane
"controls" its driver or pilot instead of the other way
round. Or that a nuclear power plant "controls" its central
operating station and its operators, and not the other way
round. Or that a country's central bank does not "control"
its economy by setting the interest rates (just tell Alan
Greenspan that he is not in charge and you will have no
place to hide). This is not to argue that there is no
substance to this viewpoint - one is fully justified in
viewing all feedback control systems in this manner, since
the control signals are essentially a function of the state
of the system. However, this viewpoint is not legitimate
for understanding the internal mechanisms of any such
system, because it overlooks the fact that the "control
signals" are actually determined by and originate from a
certain subsystem (a driver, a pilot, a control station, a
central bank) of the overall system and that these are the
signals that cause the state of the system to change. Such
a viewpoint also overlooks the fact that there are no other
entities in the system capable of computing and generating
those particular "control" signals.
The main argument above is that certain living objects, and
humans in particular, are physical systems very unlike a
hurricane or an ocean or a volcano. When these living
objects are observed from the point of view of their
various subsystems, the "controlling" function of the brain
is obvious; there exists controlling subsystems within
these system. However, there are no such "controlling"
subsystems in a hurricane or an ocean or a volcano or
similar physical systems. The statements made here do not
constitute any profound new scientific discovery by any
standard. They are a simple restatement of some ordinary
facts of biology and nature.
Some very logical questions that arise naturally are as
follows: Could there exist similar "controlling"
substructures within the brain? In other words, could there
be parts of the brain that in some sense guide, govern and
control other parts of the brain? Could it be that synaptic
strengths of cells are actually controlled by other
entities within the brain than the cells themselves? If a
"controlling" subsystem can be observed at a higher macro
level in these living objects, why couldn't they exist at a
lower micro level, in the brain? If one were to propose
such a possibility, it should not be an illogical one; it
would be very consistent with verifiable external
characteristics of these living objects at the macro level.
And I think it is also supported by growing evidence from
neuroscience that extrasynaptic neuromodulators
(extracellular signals from sources external to the cells)
influence synapses, that they can effect permanent changes
in the synaptic strengths and that they originate from
certain fixed sources in the brain (I would love to get
some more insight and recent references from
neuroscientists on this).
Overall, it appears that a very convincing argument can be
made that there are subsystems within the brain that
control other subsystems. This "control theoretic" notion
would thus allow external sources to directly control a
cell's behavior. It would not be fair if it is not
acknowledged here that such control theoretic notions are
already in use, in one form or another, in almost all
connectionist learning systems. For example, the various
constructive learning algorithms, such as adaptive
resonance theory (ART) and radial basis function (RBF)
networks, use non-local means to "decide" when to expand the
size of the network. And the back-propagation algorithm
itself depends on a non-local, external source to provide
it with the design of a network in which to learn. So
connectionist systems inadvertently acknowledge this
"control theoretic" idea, by using a "master or controlling
subsystem" that designs networks and sets learning
parameters for them. In other words, as baffling as
it may sound, the control theoretic ideas have been in use
all along; they are nothing new. However, although these
control theoretic ideas have been in use all along in
connectionism, muddled in with other ideas, only recently
has there been some explicit acknowledgment of the need for
it. For example, Kohonen, in a 1993 paper in Neural
Networks, explicitly mentions "chemical agents, which
are formed or released extracellularly at or in the
neighborhood of highly active cells" in order to provide
the physiological justification for his self-organizing map
(SOM) algorithm.
In addition, a control theoretic framework resolves many of
the problems and dilemmas of classical connectionism. Under
such a framework, learning need no longer be instantaneous
and can wait until some information is collected about the
problem at hand. Learning can always be invoked by a
controlling subsystem at a later point in time. This would
also facilitate understanding the complexity of the problem
from the information that has been collected and stored
already. Such a framework would also resolve the network
design dilemma and the problems of algorithmic efficiency
that have plagued the field for so long. So one can
argue very strongly for such a theory of the brain from
both a computational point of view and from the point of
view of being consistent with externally observed human
learning behavior.
All comments and criticisms of these ideas are welcome.