Re: Finding useful functions- part 1

From: patty (pattyNO_at_SPAMicyberspace.net)
Date: 10/25/04 

Date: Mon, 25 Oct 2004 15:38:12 GMT

Bill Modlin wrote:
> Our brains have innate structure tailored by evolutionary processes
> over a long period of time. This structure performs functions that
> contribute to our behavior in ways that somewhere along the line
> probably helped individuals to survive, or at least didn't hurt.
>
> Many of those functions are not fully determined by genetics alone.
> There is an innate framework, but details are filled in by processes
> of conditioning and association, and to some degree the framework
> itself is mutable if environmental conditions differ sufficiently
> from those for which it evolved. There are few sharp lines between
> innate and acquired neural function.
>
> Feature discrimination in the early visual system is sometimes
> called innate. Certainly it is innate that the cells grow into
> layers of tissue appropriate for performing useful feature
> discriminations. However, it seems the specific connections and
> weights to implement particular discriminations get filled in by
> adaptation to correlations in the ensemble of signals flowing from
> the retina. For example, we can change the distribution of
> particular detectors dramatically by raising a cat in an abnormal
> visual environment. It seems cells are not so much genetically
> determined to perform specific discriminations, as that they acquire
> discrimination functions appropriate to the signals they encounter
> in their genetically determined position in the network.
>
> There are places where neural projections bring together signals
> originating from corresponding points in the left and right eyes.
> This allows merging both images to fill in details missing from one
> or the other, estimating depth from discrepancies in the two images,
> and so on. There is genetic direction to cause axonal projections
> carrying signals from one eye to grow toward the normally expected
> locations of the corresponding signal paths from the other. But
> (from experiments on Xenopus frogs) if one eye is surgically rotated
> before the connections are formed, so that the locations of
> correlated signals are altered, we see the projections grow first
> toward the normal target location, then veer off sharply to connect
> with the very different cells now in position to be correlated.
>
> Many topographic maps can be found in the brain, so that for example
> neigboring sections of neural tissue are excited by stimulii from
> adjacent sections of skin. One might imagine a fixed wiring scheme
> under genetic control to hook up these maps, but when we surgically
> swap small patches of skin the connections change to preserve the
> mapping. It takes some time, but after a while we find that the
> moved sensors now activate sections of the remote neural map that
> correspond to their new positions.
>
> A reasonable interpretation is that the "wiring" of neural circuitry
> is only loosely determined by a genetic blueprint. Most of the
> actual connections (and therefore the functions performed) are
> established as a result of correlations between the activities of
> potentially connected cells. Not only are the initial connections
> determined by correlations, but even after a stable connection
> pattern is established, the connections will change if the
> correlations change.
>
> From the viewpoint of a single cell, it strengthens connections to
> others correlated with its own activity and weakens others, much as
> postulated by Hebb so many years ago. While direct observation of
> such changes in individual active synapses is still difficult, we
> can observe at least one related mechanism in widespread use. Cells
> in a child's brain sprout huge dendritic trees and eventually make
> something like 200,000 synaptic connections. By adulthood these are
> trimmed back to an average of 10 to 20 thousand. The only plausible
> explanation for this of which I am aware is that the surviving
> connections are those that showed correlation with the activity of
> the cell. Uncorrelated connections simply drop out of the picture.
>
> Overall, the point is that the functions computed by cells in the
> brain are largely determined by the correlations encountered in the
> signals accessible to the cell, rather than by genetic control.
>
> This is learning or conditioning, but it is not the kind of
> feedback-driven learning that is usually intended when one speaks of
> operant conditioning. This sort of learning does not depend on
> consequences of the output of the function, and would occur even if
> the output were not connected to anything else and could therefore
> have no consequences extending beyond the cell doing the learning.
>

What evidence do you have that this happens at all ?

patty

> From an evolutionary perspective, such learning mechanisms exist
> because they do indeed often have useful behavioral consequences.
> But the evolutionary connection is between the learning mechanisms
> and ensembles of behavior, not between the individual functions
> learned and specific contingencies associated with those functions.
>
> --------
>
> None of the above should be taken as suggesting that other sorts of
> learning can be ignored. To implement AI we will require an
> understanding of many facets of adaptive behavior, including the
> operant conditioning or reinforcement learning that has been the
> sole focus of certain vocal participants in CAP.
>
> But I do suggest that these correlation-driven "unsupervised"
> mechanisms provide a critically important underpinning for other
> learning paradigms, that they are necessary parts of an explanation
> of how all our behavior-generating mechanisms actually work.
>
> <to be continued in further posts>
>
> Bill Modlin
>
>
>
>
>

Relevant Pages

  • Re: Finding useful functions- part 1
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  • Finding useful functions- part 1
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