IUBio

Cross - Wired Eyes

George Hammond ghammond at mediaone.net
Tue May 15 22:00:03 EST 2001


Filip van den Bergh wrote:
> 
> Karl Self <karl.self at gmx.net> wrote in message
> news:9dkgbm$j0icu$3 at ID-34153.news.dfncis.de...
> > -  why is this so?
> 
> Well, I don't think anybody knows for sure, but there are some nice
> hypotheses on that question. One explanation I've heard more than once (e.g.
> Braitenberg: Vehicles) is that it's a matter of very early evolution.
> Imagine a very simple organism that has only two sensory units and two motor
> units. Now suppose those sensory units are wired to the motor units (like in
> my very bad figure A, at the bottom of this post). That would mean that if
> the sensors sensed something, the organism would move away from whatever the
> sensory unit noticed. If the sensory units would be crossed (very bad figure
> B), then the organism would move towards the distal source of excitation. I
> suppose you'd say there is as much reason to move away from stuff (e.g.
> predators or too much heat) as there is to move towards other stuff (food,
> warmth). I think that's true, but I'd start with getting some food for my
> very simple organism. Later, different (in this case uncrossed sensors)
> could detect danger and avoid it.
> It's but a theory (Braitenberg would call it an experiment in synthetic
> psychology), but a very likeable one at that.

[Hammond]
This is a very powerful line of analysis.  The situation
certainly is complex.  For instance, there are two major
decussations in optical-motor response.  One in the decussation
of the optic tract, and the other in the decussation of the
pyramidal tracts.  I would suppose they have different origins.
the optic decussation is probably to nullify the lens inversion,
but the (more massive) pyramidal decussation may be something along
the lines you describe.  Also, the fornix has a suspicious
congruence with the optic chiasma which would have to be analyzed.

> 
> Filip van den Bergh
> 
> Badly drawn figures (best viewed in courier font)
> S = sensory unit
> M = motor unit
> | = straight connection
> X = crossed connection
> 
> Figure A
> 
> S1   S2
> |     |
> M1   M2
> 
> Figure B
> 
> S1   S2
>    X
> M1   M2

-- 
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George Hammond, M.S. Physics
Email:    ghammond at mediaone.net
Website:  http://people.ne.mediaone.net/ghammond/index.html
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