Michelle T Glickman mglickma at mail2.sas.upenn.edu
Wed Oct 28 19:11:49 EST 1998


	I must say that I honestly haven't been following this thread 
religiously, but as a neuro person who truly believes that both genetics 
and environment play major roles in the neuronal connections established 
(particularly during certain critical periods, i.e. vision, hearing, 
etc.), I ask only one thing:  please refrain from insulting one another.  
Discussing various aspects of cognitive neuroscience is one thing -- 
insulting one another is quite another.

	--Michelle :)

Ray Scanlon (rscanlon at wsg.net) wrote:

: Neil Rickert wrote in message <715iua$8ur at ux.cs.niu.edu>...
: >"Ray Scanlon" <rscanlon at wsg.net> writes:
: >
: >If N neurons have to meet up accurately with N locations, there are
: >N! (that's a factorial) ways of doing this.  Given that N is very
: >large, N! exceeds the total storage capacity of the known universe.
: >Therefore the DNA could not possibly encode such a high degree of
: >specificity.

: I don't know how this "coding capacity" argument go started but I have seen
: it again and again. It is a strawman. It is allied to the view of the brain
: as a bowl of porridge with all the structure of oatmeal.

: We are not talking about the possible permutations of n things taken n at a
: time. We are talking about the human (as a vertebrate) brain with between
: 100 and 1000 billion neurons all precisely connected according to rules.

: Rule number one: Remember thy neighbor.

: Again and again and again in the brain, topology is preserved. There are
: about one million ganglion cells in the retina, their axons make up the
: optic nerve. The axons from one eye are joined at the optic chiasm by the
: axons from the other eye. The axons then split with the axons from the left
: half of both retinas continuing to the left brain and the right halves going
: to the right brain. the axons are completely scrambled in the nerve but upon
: arriving at the lateral geniculate nucleus (LGN), rule number one takes
: over. The axons split into six groups each having its destination in a layer
: of the LGN, three for the left eye and three for the right eye. The axons
: sort themselves out and restore the topology, neighbor by neighbor. The
: axons terminate on cells in the LGN.  Each layer contains a topological
: mapping of the retina.

: The axons from the cells of the LGN proceed (with laterals to the reticular
: nucleus of the thalamus) to area seventeen of the cortex where the axons of
: the four upper layers terminate in layer 4Cbeta and the two lower layers go
: to layer 4Calpha of the cortex. Again rule number one determines that
: topology is preserved.

: How is the rule implemented? There are three major possibilities: mechanical
: guidance, following chemical gradients, or homing in on some complementary
: molecule. This is an active area of research with results pointing toward
: more than one mechanism.

: The important thing is that this beautifully precise wiring is accomplished
: with only a few rules. One to preserve neighborhood, and several to
: determine destination. This is NOT n factorial. Mathematical arguments from
: the "bowl of porridge" are hopelessly naive.

: Other classes of sensory input (somatic, auditory) similarly follow rule
: number one.

: When the axons that originate in the cortex proceed to association areas,
: again topology is preserved.

: >>My disagreement is with your statement that the brain is not limited to
: >>signal energy flowing through the sensory neurons and information
: >>encoded in DNA. What is this other source?
: >
: >For example, the chemical constituents of the air, the food, other
: >parts of the environment.

: Any molecule from the environment that desires to get to the brain must pass
: the blood-brain barrier. This is not easy and is a basic pharmacological
: problem when attempting to get medication to the brain.

: Don't forget that I have never downplayed the environment. The DNA
: constructs the brain, the milieu impinges on it. The effect of the milieu on
: the brain is called "learning". The important thing is that the milieu
: affects the brain through the sensory neurons and, from the past, through
: the DNA.

: You say I am preaching dogma but I say I am merely reciting basic
: neuroscience.

: Ray
: Those interested in how the brain works might look at
: www.wsg.net/~rscanlon/brain.html

* Michelle Glickman :)					     *
* mglickma at sas.upenn.edu  417-0105			     *
* University of Pennsylvania                                 *	
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*	-Garth Brooks, 'The River'			     *

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