Re: poly: Software for superintelligence

carl feynman <carlf@atg.com> writes:

> Your (straw man) model seems to be something like "Prenatal development
> causes the creation of a complex architecture of various cell types
> organized into subnetworks, each of which turns into a useful functional
> area when exposed to stimuli after birth." Let's call this model A. In
> another model, which I will call model B, the large-scale structure of the
> cortex is fairly homogenous, but there are lots of possible types each cell
> can be recruited into depending on signals it recieves from its
> environment. The cell types are regulated by mutually stimulating and
> inhibiting sets of regulatory proteins, much like the reguatory cascades
> that decide whether a fetal cell will be a liver cell or a spleen cell.
> Depending on type, a particular cortical cell will have different
> thresholds and electrical behavior, grow synapses onto other cells in
> different patterns, display different recognition proteins on its surface,
> and perhaps even release and respond to different neurotransmitters. In
> order to make this model falsifiable (:-), here's two things neurons can't
> do: grow new long-range (more than 1 mm) connections, and change from one
> cell type to another, except along a one-way branching path.

It should be noted that neurons do sometimes grow new long-range
connections. But other than that model B sounds closest to what is
accepted among neuroscientists.
 
> Let's look at your three points:
>
> >First, consider the plasticity of the neocortex, especially in
> >infants. It is known that cortical lesions, even sizeable ones, can
> >often be compensated for if they occur at an early age. Other cortical
> >areas take over the functions that would normally have been developed
> >in the destroyed region.
>
> Model B explains this by saying that recruitment to various cell types
> happens largely during infant development. Cells are recruited from a
> less-differentiated pool into more differentiated states.

I think a more likely answer is that there is also a certain lee-way
in the duties of a certain cell; by adjustning its synaptic weights it
can take up some slack left by the damaged part and do additional
functions. In some sense the cells are only differentiated as to basic
type and shape, not purpose.

> >"Laminations and vertical connections between lamina are hallmarks of
> >all cortical systems, the morphological and physiological
> >characteristics of cortical neurons are equivalent in different
> >species, as are the kinds of synaptic interactions involving cortical
> >neurons. This similarity in the organization of the cerebral cortex
> >extends even to the specific details of cortical circuitry. (White
> >1989, p. 179)."
>
> I'd like to know what the author considers the "details of cortical
> circuitry". As far as I know, the wiring diagram of any particular
> cortical column is not known to any degree of detail. If, for example, one
> column contained rings of six neurons, each neuron of which inhibited its
> neighbors, and another contained similar rings of five neurons, we would
> not yet be able to tell the difference. However, these rings are
> functionally immensely different: one is a flip-flop and the other is an
> oscillator. I may be wrong about this; perhaps someone who has advanced
> professional Swedish knowledge of neurophysiology can comment.

How could I resist commenting on this :-)

I think you are right in that we do not know how a cortical column
really is wired, although we know a few general principles. Nick is
right in that the cortex is fairly similar on the macro- and
meso-scale, but there seems to be noticeable differences in "style" in
small regions (smaller than the classical Brodmann areas) and the
detailled structure is likely semi-random. What happens is that the
cells likely "format" their connections to do whatever they are
supposed to do, there is some evidence that this occurs in the
neonatal hippocampus.

At present nobody knows if the cortex is regular (every column does
the same) or complex (each column is unique). My guess is that it is a
bit of both; experience formats originally fairly random circuits into
usable structures. These circuits have different "styles" in different
parts of the brain (a result of evolution), making them more
efficient.

At the workshop on hippocampal modelling this weekend I got into a
discussion with Per Andersen (big name in LTP research) and Michael
Hasselmo (Harvard) about this. Hasselmo thought that in a few years,
we would have a breakthrough in cortex understanding equal to the atom
model of Niels Bohr, Andersen was more sceptical and I suggested that
the basic rule may be developmental rather than structural. We seemed
to reach some kind of agreeement that it was a combination of
self-organizing development and later self-organization based on
experiences that created cortical structures. But the question is
still unsettled, and even the Big Guys disagree.

> I would argue that there are no 'basic paradigms', just an annoyingly vast
> pile of details.

Maybe. It depends on if biology contains firm level-boundaries or not,
and if we can find them.

-- 
-----------------------------------------------------------------------
Anders Sandberg                                      Towards Ascension!
asa@nada.kth.se                            http://www.nada.kth.se/~asa/
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