left-right reversal of CNS: Why?

kenneth paul collins KPCollins at postoffice.worldnet.att.net
Sun Sep 15 22:28:39 EST 1996


Jerry Larson wrote:
 
> > > If you could take and
> > > twist it around through space, it would still work; in fact just that has
> > > happened.
> >
> > ...not if neighborhood relationships are not preserved within the global
> > Geometry... frogs whose optical tecta are surgically relocated, for instance,
> > strike with their tongues, at insects, in the wrong direction... missing the
> > insects... (technical point... beyond normal joint movement, the nervous
> > system doesn't respond well to being subjected to "twisting" manipulation)...
> 
> I said IF, didn't I?  Again there are two levels that have to be
> distinguished. When I said it had happened, I meant, of course, in the
> course of evolution.   If you could take a circuit and twist it all around
> through space without distorting the connectivity, or creating problems
> with heat, blood flow, or whatever, it would still work.  I pointed this
> out to show that whether the "image" on the cortex was right side up or
> not doesn't matter.  If I understand the above paragraph, you're saying,
> yes it does, if you could twist something around it would get all confused
> and work backwards.  Unproven.

...if the abstract topology is "twisted" without altering any of the neighborhood 
relationships, the image orientation, within the neural topology, remains 
invariant...

> I know about the shufflebrain salamanders and stuff.  That's not the
> hypothetical case I was talking about, since the connective  relationships
> are NOT preserved. If it affects the case, it goes to show that the
> cross-connectedness is at lower, more primitive levels.  There are
> experiments with human beings who learned to function just fine with
> inverting glasses. 

...set up a competition between them and "normals"... hand-eye coordination, 
foot-eye coordination... static- and moving-obstacle avoidance... handwriting... 
typing... pie-eating-contests... catching beachballs (can't even use baseballs 
:-)... whatever...

...organically-intact "normals" will out-perform the inverted-vision folks 
robustly...

> A frog doesn't have the plasticity to learn something
> like that, is all.  There is some plasticity in human somatomotor cortex;
> a lot of functions can expand or contract in the homunculus (after loss of
> a body part, or through intensive use of certain skills such as playing an
> instrument), though they can't switch places, of course, and if the hand
> area gets injured, that function can't come back.

...depending on developmental period (earlier is more-robust), cortical areas can 
be completely rewired...

> 
> Suppose you took a mammal of some sort, say a chimp, and disconnected  the
> face area of cortex from the hand area, with some nice small subpial
> cuts.  I suggest it wouldn't have a lot of effect.  Certainly the chimp
> isn't going to start reaching backward for bananas in front of him!  The
> arm and hand cortex is still connected to the contra arm, and still in
> communication with the visual system.  Now do the experiment under local
> anesthesia, and retract the isolated gyri away from each other, changing
> the spatial relationship of the two motor control areas.  As long as those
> two areas of cortex are connected to what they're supposed to connect to,
> and have blood supply and so on, things will continue to work properly.

...Karl Lashley got what you're talking about sorted out 50 years ago...

> In fact, as I pointed out, the "homunculi" in the cerebellum do NOT
> maintain the topological relationships, and they get rather distorted in
> thalamus too.

...you are misinformed... 

> > ...the topology that was described =permeates= the nervous system... it's
> > functionality has only become augmented as evolutionary development
> > occurred...
> 
> I'm inclined to agree with that.  The point is, the original questioner
> asked  why is there crossed control?  I suggest it isn't so the contra
> somatomotor cortex can be a few centimeters closer to the ipsi
> cerebellum--even if that's true,

...that to which you refer is only a small portion of what I discussed... has it 
scrolled? 

...the main thing is that the "crossed" topology alligns things so that the 
infinitely-large pain-avoidance problem can be resolved in millisecond time frame 
by simply minimizing TD E/I minimization... and since the problem-solving power of 
this neuroanatomical arangement is so great, and since any "higher-level" nervous 
system function that totally-eliminated the functionality of the pain-avoidance 
architecture, that architecture permeates CNS architecture all the way up to 
"highest levels"...

> it's a later development--but because of
> the way nervous systems evolved in critters with bilateral symmetry.  Your
> ideas about the cerebellum are interesting, but remain speculative, imo.

...it's all =verified=...

> It's quite clear that you believe very strongly in them, and that I'm not
> convinced, but let's not keep arguing back and forth, let's try to bring
> in new ideas or deepen the understanding.

... no thanks... I've been at all of this so intensely, for so long, that I'll 
throw up if I don't turn to other things for a while... :-) 

...besides, I've lost my employment and have to give up my internet account...

...and besides, forgive me, you are "botching" the most-beautiful physical stuff of 
which I'm aware... 

> > ...as has been discussed, the antecedants of mammalian decussation can be
> > traced all the way back to organisms having radial symmetry... ken collins
> 
> Well, we did talk about that before, or you did, but I don't think you
> proved your case at all.  You said something to the effect that nervous
> impulses diffuse around the nervous system of a radial organism in a way
> that's slow and not well organized, and that proves the NEED for
> decussation. Looks to me like they need a central nervous system all
> right, but lacking sides, the question of fibers crossing to the other
> side doesn't arise.

...go watch a jellyfish... 

> I was asking about where do you actually SEE EVIDENCE of crossed control?

...I stand on what I posted.

> Do snakes have crossed motor control?  Beetles?  Planaria?  Are there any
> animals that have bilateral symmetry and a rudimentary nervous system, but
> no crossed fibers? (Ken, I know you're going to say no.  I wonder if you,
> or anyone else, can offer some interesting examples, though).

...there is a tree schrew that's wired ipsilaterally... I've not studied it 
though..

> Anyway, I think this is an interesting point about radial vs. bilateral
> symmetry.  Radial organisms do look as if they could stand to have some
> more organization, i.e. a central nervous system, and I can imagine a
> radial critter, longer than it is wide, with a central nervous system,
> where?  At the center, where else, with a front and a back, putting mouth
> and eyes (5 of them, say, facing in 5 directions) at the front, grasping
> tentacles (5) at the front, swim fins (5, why not?) at the back.  But it
> didn't happen, not on this planet at any rate; invention of the front-back
> distinction, the left-right distinction, and the central nervous system,
> all came pretty much together in a package deal.  And I bet crossed
> control came in the same box.
> 
> Jerry

...I'm sorry, if you wish to do so, please continue without me... ken collins



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