AI, NN, CNS (central nervous system)

[Terry Sejnowski]

In 1957 Hartline and Ratliff published a paper on "Inhibitory interaction
of receptor units in the eye of Limulus" (J. Gen Physiol, 40: 357-376).
They described a set of elegant experiments on lateral interactions
between neighboring ommatidia and summarized their results in a model
of an effective network of lateral inhibitory interactions.  
Their model would today be called a connectionist model -- in fact it
was linear.  Not only were they able to accurately summarize a lot
of data, but they were able to use the model to explore the idea
of contrast enhancement through lateral inhibition, something we take
for granted today. This work led to a Nobel Prize for Hartline.  In the
intevening years this model has been elaborated in many interesting
ways, including generalizations to time-dependent patterns of light,
nonlinear interactions at extreme light levels, and the biophysical 
properties of noise in the photoreceptors.  The essence of the orignal
model, however, still stands even though these more elaborate and 
realistic models are more accurate and more complete.  Bob Barlow at
Syracuse has implemented a version of the model for the whole retina
on their Connection Machine and is passing movies of the real world
seen by the Limulus underwater through the simulated retina.  The
original Hartline-Ratliff model, however, is still a useful reference
landmark toward which all these elaborations point.  There is value
in having an abstract, simplifying model to anchor the elaborations.  

	This single example should be enough to 1) illustrate
the utility of simplifying models in studying real biological
problems and 2) underline the importance of paying careful 
attention to biological data when attempting to apply
such models to other biological systems.  It should also be
noted that Hartline and Ratliff would not have been able to
develop their model if the mathematics of linear networks had
not already been established by mathematicians, physicists, and
engineers, most of whom were not interested in biological problems.
Without the development of a mathematics of nonlinear dynamical
systems there will be no future models for future Hartlines
and Ratliffs to apply to future biological problems.  I find
it encouraging that so many good scientists who are confronting
so many difficult problems in psychology, biology and computation
are begining to at least speak the same mathematics.

	I do not think that anything is going to be settled by
debating ideologies, except who is the better debater.  Precious
bandwidth is better spent discussing specific problems.

Terry

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